Submitted

• Future intensification of northern hemisphere monsoons due to declining remote aerosol pollution. Sooraj, K. P., C. Dhara, D. C. Ayantika, K. Vishisth, K. M. Sumit, A. G. Turner and R. Krishnan (2025). Environmental Research Letters, submitted 8 April 2025.


• Systematic biases over the equatorial Indian Ocean and their influence on seasonal forecasts of the IOD. Gler, M., A. G. Turner, L. Hirons, C. Wainwright and C. Marzin (2025). Climate Dynamics, submitted 20 January 2025.


• Ecosystem-Atmosphere Exchanges of Carbon, Water, and Energy in India: A Synthesis of Insights from Eddy Covariance Measurements. Deb Burman, P., G. S. Bhat, Y. Tiwari, R. Morrison, S. Rodda, S. Mukherjee, V. K. Dadhwal, S. Launiainen, A. G. Turner, P. Das, G. Agarwal, D. Sarma, N. Gogoi, P. Gnanamoorthy, S. Paleri and D. Desai (2025). Global Change Biology, submitted 5 January 2025.


• The role of dry intrusions in breaks of the Indian summer monsoon. Deoras, A., Andrew G. Turner, A. Volonté, R. K. H. Schiemann, L. J. Wilcox and Arathy Menon (2025). Journal of Climate, submitted 2 January 2025.


• The role of soil moisture in the inland penetration of Indian monsoon low-pressure systems. Deoras, A., K. M. R. Hunt and A. G. Turner (2024). Quarterly Journal of the Royal Meteorological Society, submitted 22 November 2024.


• Strong inter-model differences and biases in CMIP6 simulations of PM_2.5, aerosol optical depth, and precipitation over Africa. Toolan, C. A., J. A. Amooli, L. J. Wilcox, B. H. Samset, A. G. Turner and D. M. Westervelt (2024). Atmospheric Chemistry and Physics, submitted 30 September 2024, preprint available online 23 October 2024.


• On the Decadal Variation of Southern Hemisphere Land Monsoon Precipitation. Wang, H., J. Cao, X. Lian, A. G. Turner, B. Wang and Z. Kong (2024). Journal of Climate, submitted 13 September 2024.


• High-resolution climate models improve simulation of monsoon rainfall changes in the Ganga-Brahmaputra-Meghna basin. Ali, H., A. G. Turner and H. J. Fowler (2024). Climate Dynamics, submitted 19 August 2024.


• Stronger influence of the tropical Atlantic on interannual variability of Northern Hemisphere land monsoon rainfall. Zhu, Z., Y. Jiang, J. Li and A. G. Turner (2024). Journal of Climate, submitted 12 August 2024.


• Interactions between subpolar and subtropical jet streams leads to extreme rainfall events over northern India in 2013 and 2023. Dey, D., L. Joseph, A. Ray, R. Marsh, N. Skliris, A. G. Turner, D. C. Ayantika, P. Mukhopadhyay, A. Chakraborty and S. Sil (2024). Climate Dynamics, submitted 7 August 2024, revised version submitted 22 January 2025.

Early-online & accepted refereed publications

122. Interactions between subpolar and subtropical jet streams lead to extreme rainfall events over the north Indian subcontinent in June 2013 and July 2023. Dey, D., L. Joseph, A. Ray, R. Marsh, N. Skliris, A. G. Turner, D. C. Ayantika, P. Mukhopadhyay, A. Chakraborty and S. Sil (2025). Climate Dynamics, submitted 7 August 2024, revised version submitted 22 January 2025, accepted 24 April 2024. |
     Abstract
     North Indian Subcontinent (NIS) is prone to disastrous and life-threatening floods during the summer monsoon period primarily due to its close proximity with the Himalayan foothills. Indian Meteorological Department (IMD) reported that during June 13-19, 2013, the Uttarakhand state in north India experienced a cumulative total of 322 mm rainfall, a 847% weekly departure against the long-term average rainfall (1971-2020) of 34 mm. After a decade, another state in the same region, Himachal Pradesh, received an unprecedented 223 mm of rainfall in just 4 days, viz 7-11 July 2023, a 436% deviation from the cumulative climatological rainfall of 41.6 mm for 4 days (July 7-11). It is shown that the atmospheric water vapor is transported towards NIS by two monsoon low pressure systems during the June 2013 event. In contrast, during the July 2023 flood, the monsoon trough shifted southward, resulting in the moisture transport pathway predominantly over the northern Indian Ocean. In conjunction, it was shown through upper-level air tracing that southward movement of the subpolar jet stream creates a trough in the subtropical jet stream, which intrudes along the western boundary of NIS, leading to upper-level divergence. This pattern was observed in both flood events. By leveraging the mass-conserving nature and unique capability of a Lagrangian tracing model to track atmospheric water backward in time, our novel analysis of the 2023 flood reveals that two evaporative sources near Madagascar and the western Indian Ocean were key contributors, and inland evaporation played a comparatively lesser role as compared to the 2013 case. The Bay of Bengal served primarily as a vapor transport pathway rather than a direct moisture source for both events. This novel Lagrangian approach, which exposes separate drivers of extreme monsoon rainfall, upstream and at lead times of days-weeks, has the potential to be used more extensively and operationally.
123. The skill of extended-range ERPAS and GloSea5 models in capturing extratropical-tropical interactions during the north Indian summer monsoon. Kalshetti, M., R. Chattopadhyay, A. G. Turner, R Phani, S. Joseph and A. K. Sahai (2025). International Journal of Climatology, submitted 8 December 2023, revised version submitted 15 August 2024, accepted 10 March 2025, published online 19 March 2025. |
     Abstract
     The transient eddies in the atmosphere are short-lived, moving disturbances prominent over mid-latitude. Transient eddy transport enables the exchange of mass, energy, and moisture between extratropical and tropical regions. Based on observations, about 40% of the rain that falls in northern India during the summer monsoons is influenced by transient eddy heat and momentum fluxes. During these rainfall cases, the four-stage cycle of transient eddy heat and a momentum feedback process exists. Global-scale circulation anomalies are generated due to their forcing on the mean flow. This impact of transient eddies on mean flow is referred to as eddy–eddy feedback, commencing in around 22 days. On a daily scale, the enhancement of rainfall over the Western Ghats and north-western India is linked with upper-tropospheric poleward transient eddy heat flux transport and equatorward transient eddy momentum flux transport. On a quasi-biweekly scale, however, the transport direction reverses. Additionally, this rainfall pattern is governed by the meridional passage of monsoon intraseasonal oscillation phases (MISO, tropical mode) and the zonal passage of wave number 7–8 patterns (Rossby wave, extratropical mode). Interactions of tropical-extratropical modes are associated with this eddy-eddy feedback that drives the hemispheric upper-tropospheric circulation patterns. This includes wave generation, propagation, and the dissipation of waves away from the source region. The hindcast skill analysis of subseasonal to seasonal scale models, namely Extended Range Prediction Application to Society (ERPAS) and Global Seasonal Forecast version 5 (GloSea5), shows that the models can predict northern Indian rainfall associated with eddy-mean flow interactions at 1-week lead times. After a week, the skill of both models diminishes under the influence of transient eddy transport. The monsoon circulation is more consistent and predictable in both models when transient eddies are absent. A theoretical understanding of the dynamical feedback of upper-tropospheric transient eddies is crucial for improving rainfall prediction.

Refereed publications

Articles are listed in order of appearance in published volumes.

1. Western disturbances and climate variability: a review of recent developments. Hunt, K. M. R., J.-P. Baudouin, A. G. Turner, A. P. Dimri, G. Jeelani, R. S. Pooja, R. Chattopadhyay, F. Cannon, T. Arulalan, M. S. Shekhar, T. P. Sabin and E. Palazzi (2025). Weather and Climate Dynamics, 6: 43-112. Submitted 19 March 2024, revised version submitted 15 August 2024, accepted 8 November 2024, published 9 January 2025. | Preprint discussions version, started 4 April 2024. |
   Abstract
   Western disturbances (WDs) are synoptic-scale weather systems embedded within the subtropical westerly jet. Manifesting as upper-level troughs often associated with a lower-tropospheric low over Western India, they share some dynamical features with extratropical cyclones. WDs are most common during the boreal winter (December to March), during which they bring the majority of precipitation—both rain and snow—to the Western Himalaya, as well as to surrounding areas of north India, Pakistan and the Tibetan Plateau. WDs are also associated with weather hazards such as heavy snowfall, hailstorms, fog, cloudbursts, avalanches, frost, and coldwaves. In this paper, we review the recent understanding and development on WDs. Recent studies have collectively made use of novel data, novel analysis techniques, and the increasing availability of high-resolution weather and climate models. This review is separated into six main sections—structure and thermodynamics, precipitation and impacts, teleconnections, modelling experiments, forecasting at a range of scales, and paleoclimate and climate change—each motivated with a brief discussion of the accomplishments and limitations of previous research. A number of step changes in understanding are synthesised. Use of new modelling frameworks and tracking algorithms has significantly improved knowledge of WD structure and variability, and a more frequentist approach can now be taken. Improved observation systems have helped quantification of water security over the Western Himalaya. Convection-permitting models have improved our understanding of how WDs interact with the Himalayas to trigger natural hazards. Improvements in paleoclimate and future climate modelling experiments have helped to explain how WDs and their impacts over the Himalaya respond to large-scale natural and anthropogenic forcings. We end by summarising unresolved questions and outlining key future WD research topics.


2. A Shorter Duration of the Indian Summer Monsoon in Constrained Projections. Y. Cheng, L. Wang, X. Chen, T. Zhou and A. G. Turner (2025). Geophysical Research Letters, submitted 1 October 2024, revised version submitted 29 November 2024, accepted 20 December 2024, published 4 January 2025. |
   Abstract
   A reliable projection of the future duration of the Indian summer monsoon (ISM) provides an important input for climate adaptation in the Indian subcontinent. Nevertheless, there is low confidence for projections of ISM duration, due to large inter-model uncertainty of onset and withdrawal changes. Here, we find that models with excessive sea surface temperature (SST) over the tropical western Pacific (WP) during spring and greater surface warming trends over the northern mid-high latitudes (NMHL) during autumn in the present day tend to overestimate future delays to ISM onset and withdrawal, respectively. This can be attributed to the influence of surface thermal conditions on upper-tropospheric warming patterns. Constrained by the observational WP SST and NMHL surface warming trends, projected ISM duration under a high-emission scenario is shortened by 6 days compared to the current climate, with a reduction of inter-model uncertainty by 46% relative to the unconstrained results.


3. Regional impacts poorly constrained by climate sensitivity. Swaminathan, R., J. Schewe, J. Walton, K. Zimmermann, C. Jones, R. A. Betts, C. Burton, C. D. Jones, M. Mengel, C. P.O. Reyer, A. G. Turner and K. Weigel (2024). Earth's Future, 12(12): e2024EF004901, submitted 13 May 2024, revised version submitted 3 October 2024, accepted 27 October 2024, published 5 December 2024. | arXiv version | UoR press release | Met Office press release |
   Abstract
   Climate risk assessments must account for a wide range of possible futures, so scientists often use simulations made by numerous global climate models to explore potential changes in regional climates and their impacts. Some of the latest-generation models have high effective climate sensitivities (EffCS). It has been argued these "hot" models are unrealistic and should therefore be excluded from analyses of climate change impacts. Whether this would improve regional impact assessments, or make them worse, is unclear. Here we show there is no universal relationship between EffCS and projected changes in a number of important climatic drivers of regional impacts. Analysing heavy rainfall events, meteorological drought, and fire weather in different regions, we find little or no significant correlation with EffCS for most regions and climatic drivers. Even when a correlation is found, internal variability and processes unrelated to EffCS have similar effects on projected changes in the climatic drivers as EffCS. Model selection based solely on EffCS appears to be unjustified and may neglect realistic impacts, leading to an underestimation of climate risks.


4. Constrained projections indicate less delay in onset of summer monsoon over the Bay of Bengal and South China Sea. Cheng, Y., L. Wang, X. Chen, T. Zhou, A. G. Turner and L. Wang (2024). Geophysical Research Letters, 15(21): e2024GL110994, submitted 23 June 2024, revised version submitted 9 September 2024, accepted 15 October 2024, published online 4 November 2024. |
   Abstract
   The summer monsoon onset over the Bay of Bengal and South China Sea signals the beginning of the Asian summer monsoon, critical for local fisheries, agriculture and livelihoods, so communities are concerned about its potential changes under global warming. Previous projections have suggested a delay, but the extent of this delay remains uncertain, undermining the reliability of the projections. Here, we show a significant correlation between the projected shift in Bay of Bengal/South China Sea monsoon onset and present-day sea surface temperature (SST) simulation over the western Pacific (WP). This emergent relationship arises from the spread of the precipitation response over the western-central Pacific to WP SST, as more precipitation induces stronger tropical upper-tropospheric warming, increasing westerly vertical shear near South Asia, and facilitating the onset delay. The rectified projections indicate that the delayed shift is almost halved compared to raw projections, and the intermodel uncertainty is reduced by 30%.


5. Tropical intraseasonal oscillations as key driver and source of predictability for the 2022 Pakistan record-breaking rainfall event. Xie, J., P.-C. Hsu, J.-Y. Lee, L. Wang and A. G. Turner (2024). npj Climate and Atmospheric Science, 7: 256, submitted 4 June 2024, revised version submitted 19 September 2024, accepted 15 October 2024, published 23 October 2024. |
   Abstract
   In August 2022, Pakistan experienced unprecedented monsoon rains, leading to devastating floods and landslides affecting millions. While previous research has mainly focused on the contributions of seasonal and synoptic anomalies, this study elucidates the dominant influences of tropical and extratropical intraseasonal oscillations on both the occurrence and subseasonal prediction of this extreme rainfall event. Our scale-decomposed moisture budget analysis revealed that intense rainfall in Pakistan was triggered and sustained by enhanced vertical moisture transport anomalies, primarily driven by interactions between intraseasonal circulation anomalies and the prevailing background moisture field when tropical and mid-latitude systems coincided over Pakistan. Evaluation of subseasonal-to-seasonal prediction models further highlighted the critical role of tropical intraseasonal modes in causing this extreme rainfall event in Pakistan. Models that accurately predicted northward-propagating intraseasonal convection with a forecast lead time of 8–22 days demonstrated good skill in predicting the extreme event over Pakistan.


6. The role of midlatitude dry air during the withdrawal of the Indian summer monsoon. Deoras, A., A. G. Turner, A. Volonté and A. Menon (2024). Quarterly Journal of the Royal Meteorological Society, 150(765): 5094-5112, submitted 2 February 2024, revised version submitted 15 May 2024, accepted 22 August 2024, published online 30 September 2024. |
   Abstract
   The Indian summer monsoon supplies over 75% of the country's annual precipitation, profoundly impacting over a billion people. Variability in the timing of its onset and withdrawal has a direct impact on the agricultural sector and other users of water resources. Previous studies have shown that a wedge of mid-tropospheric dry air emanating from the midlatitudes is present over India during the pre-monsoon, which gradually retreats toward the northwest as the onset progresses. The withdrawal of the monsoon is observed to progress in a southeast direction during September-October, but there is a lack of a conceptual model. In this study, we use observations and the ERA5 reanalysis to understand the dynamics and thermodynamics of the withdrawal. We find that the climatological mid-level dry intrusion appears over northwest India around mid-September. Vertical profiles associated with this dry air show how the most unfavourable environment for deep convection occurs in the northwest, where the withdrawal occurs first. As the withdrawal progresses, the wedge of dry air deepens throughout its horizontal extent and descends, as well as pushing further across the country. This stabilises the troposphere, suppressing deep convection and ultimately driving the withdrawal toward the southeast. By mid-October, the dry air engulfs most of India, causing the monsoon to withdraw from the entire country. Thus, the strengthening of the mid-level dry advection from the midlatitudes can explain the withdrawal of the monsoon and its direction, in a reversal of the processes at work during progression of the onset. This work establishes a new paradigm for the withdrawal of the Indian summer monsoon in terms of midlatitude interactions, which could be tested for other monsoon regions.


7. An uncertain future change in aridity over the tropics. Monerie, P.-A., R. Chadwick, L. J. Wilcox, A. G. Turner (2024). Environmental Research Letters, 19(5): 054048, submitted 25 September 2023, revised versions submitted February 2024 and 4 March 2024, accepted manuscript online 24 April 2024, published 3 May 2024. |
   Abstract
   An ensemble of climate models from phase six of the Coupled Model Intercomparison Project shows that temperature and potential evapotranspiration are projected to increase globally towards the end of the 21st century. However, climate models show a spatially heterogeneous change in precipitation over the tropics. Consequently, future changes in aridity (a measure of water availability) are complex and location-dependent. We assess future changes in aridity using three climate models and several single-forcing experiments. Near-term (2021-2040) changes in aridity are small, and we focus instead on its long-term (2081-2100) changes. We show that the increase in greenhouse gases primarily explains the spatial pattern, magnitude and ensemble spread of the long-term future changes in aridity. On this timescale, the effects of changes in emissions of anthropogenic aerosols are moderate compared to the effects of increases in atmospheric greenhouse gas concentrations. Model diversity in the responses to greenhouse gas concentration is large over northern Africa and North and South America. We suggest the large uncertainty is due to differences between models in simulating the effects of an increase in greenhouse gas concentrations on surface air temperature over the North Atlantic Ocean, on the interhemispheric temperature gradient, and on potential evapotranspiration over North and South America.


8. Using interpretable gradient-boosted decision-tree ensembles to uncover novel dynamical relationships governing monsoon low-pressure systems. Hunt, K. M. R. and A. G. Turner (2024). Quarterly Journal of the Royal Meteorological Society, 150(758): 1-24, submitted 6 February 2023, revised version submitted 4 July 2023, accepted 30 August 2023, first published online 27 September 2023. |
   Abstract
   Low-pressure systems (LPSs) are the primary rainbringers of the South Asian monsoon. Yet, their interactions with the large-scale monsoon circulation, as well as the highly variable land and sea surfaces they pass over, are complex and generally not well understood. In this paper, we present a novel, top-down approach to investigate these relationships and quantify their importance in describing LPS behaviour. We also show that if the approach is sufficiently well posed, it is productive at hypothesis generation. For each of five predictands — LPS intensification rate, propagation speed/direction, post-landfall survival, peak intensity, and precipitation rate — we train an additive decision-tree ensemble model using the XGBoost algorithm. Shapley value analysis is then applied to the models to determine which variables are important predictors and to establish their relationship with the predictand, with additional analysis following cases of interest. Novel relationships established using this technique include: that LPS vorticity intensifies preferentially in the early morning at the same time as the peak in the diurnal cycle of their convection occurs, that vertical wind shear suppresses continued growth of strong LPSs, that large-scale barotropic instability plays an important role in both the inland penetration and peak intensity of LPSs, and that LPS propagation depends on the depth of its vortex with shallower LPSs advected by low-level winds and taller LPSs advected by mid-level winds. We also use this framework to identify and discuss potential new avenues of research for monsoon LPSs.


9. Effect of global and regional SST biases on the East Asian Summer Monsoon in the MetUM GA7 and GC3 configurations. Franco-Díaz, A., N. P. Klingaman, A. G. Turner, B. Dong and L. Guo (2024). Climate Dynamics, 62: 1535-1553, submitted 23 December 2022, accepted 7 September 2023, published online 19 October 2023. |
   Abstract
   Climate-length experiments of the Met Office Unified Model Global Atmosphere 7.0 (GA7) and Global Coupled 3.0 (GC3) configurations are evaluated against observations and reanalyses for the simulation of the East Asian summer monsoon (EASM). The results show systematic model biases, such as overestimated rainfall over southern China and underestimated rainfall over northern China, suggesting a monsoon that does not penetrate northward enough. We evaluate the effects on the EASM of regional errors in sea-surface temperature (SST) conditions in three regions: the Pacific, the Indian, and the Atlantic Oceans. The global SST biases in GC3 configuration substantially shift the EASM seasonal cycle: a late northward progression of the EASM in the early/mid-monsoon season, and an early retreat of the monsoon that also reduces rainfall over most of northern China. The EASM seasonal rainfall bias in the EASM region is linked to changes in the locations and strength of the western North Pacific subtropical high, which is associated with biases in local evaporation and moisture transport towards South China. GC3 biases in the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) teleconnection pathways also influence the EASM biases. GC3 biases weaken the ENSO teleconnection to the EASM and cause a strong dry bias in southeast China during developing El Niño.


10. The Indian Easterly Jet during the Pre-monsoon Season in India. Croad, H. L., J. K. P. Shonk, A. Chevuturi, A. G. Turner and K. I. Hodges (2023). Geophysical Research Letters, 50(23): e2023GL105400, submitted 7 February 2022, revised version submitted 10 July 2023, second revision submitted 27 October 2023, accepted 9 November 2023, published 29 November 2023. |
    Abstract
    We identify for the first time an Indian Easterly Jet (IEJ) in the mid-troposphere during the pre-monsoon using reanalysis data. The IEJ is weaker and smaller than the African Easterly Jet over West Africa, with a climatological location of 10°N, 60-90°E, 700 hPa, and strength 6-7 m s^-1 during March-May. The IEJ is a thermal wind associated with low-level meridional gradients in temperature (positive) and moisture (negative), resulting from equatorward moist convection and poleward dry convection. The IEJ is associated with a negative meridional potential vorticity gradient, therefore satisfying the Charney-Stern necessary condition for instability. However, no wave activity is detected, suggesting that the potential for combined barotropic-baroclinic instability is not often realised. IEJ strong (weak) years feature increased (reduced) near-surface temperatures and drier (wetter) conditions over India. This study provides an introduction to the IEJ's role in pre-monsoon dynamics, and a platform for further research.


11. Unravelling the mechanism of summer monsoon rainfall modes over the west coast of India using model simulations. Phadtare, J., J. K. Fletcher, A. N. Ross, A. G. Turner, R. K. Schiemann and H. Burns (2023). Quarterly Journal of the Royal Meteorological Society, 149(757): 3164-3182, submitted 16 December 2022, revised version submitted 15 May 2023, accepted 22 July 2023, first published 4 August 2023. |
    Abstract
    A transition from a predominantly offshore to an onshore rainfall phase over the west coast of India was simulated using three one-way nested domains with 12-, 4-, and 1.33-km horizontal grid spacing in the Weather Research and Forecasting model. The mechanism of offshore-onshore rainfall oscillation and the orographic effects of the Western Ghats are studied. A convective parameterization scheme was employed only in the 12-km domain. A trough extending offshore from the west coast facilitates offshore rainfall. This trough is absent during the onshore phase, and rainfall occurs over the coast mainly via orographic uplift by the Western Ghats. The model overestimates rainfall over the Western Ghats at all resolutions as it consistently underestimates the boundary layer stratification along the coast. Weaker stratification weakens the blocking effect of the Western Ghats, resulting in anomalous deep convection and rainfall over its windward slopes. The 4- and 1.33-km domains simulate the offshore-to-onshore transition of rainfall but fail to capture a sufficient contrast in rainfall between land and sea compared to observations. The 12-km domain produces light rainfall, anchored along the coast, throughout the simulation period, and hence gravely underestimates the offshore rainfall. The offshore rainfall persisted in the 4- and 1.33-km domains in a sensitivity experiment in which the Western Ghats were flattened. This suggests that orographic effects do not significantly influence offshore rainfall. In another experiment, the convective parameterization scheme in the 12-km domain was turned off. This experiment simulated the offshore and onshore rainfall phases correctly to some extent but the rainfall intensity was unrealistically high. Thus, a model with a horizontal grid spacing of O(~ 1 km), in which convection evolves explicitly, is desired for simulating the west coast rainfall variations. However, improvements in the representation of boundary layer processes are needed to capture the land-sea contrast.


12. Validation of boreal summer tropical-extratropical causal links in seasonal forecasts. Di Capua, G., D. Coumou, B. van der Hurk, A. Weissheimer, A. G. Turner and R. V. Donner (2023). Weather and Climate Dynamics, 4: 701-723, submitted 11 August 2022, revised version submitted 24 April 2023, further revisions 6 July 2023, accepted 7 July 2023, published 14 August 2023. | Discussions version, started 24 August 2022. |
    Abstract
    Much of the forecast skill in the mid-latitudes on seasonal timescales originates from deep convection in the tropical belt. For boreal summer, such tropical-extratropical teleconnections are less well understood compared to winter. Here we validate the representation of boreal summer tropical-extratropical teleconnections in a general circulation model in comparison with observational data. To characterise variability between tropical convective activity and mid-latitude circulation, we identify the South Asian monsoon (SAM)-circumglobal teleconnection (CGT) pattern and the western North Pacific summer monsoon (WNPSM)-North Pacific high (NPH) pairs as the leading modes of tropical-extratropical coupled variability in both reanalysis (ERA5) and seasonal forecast (SEAS5) data. We calculate causal maps based on the Peter and Clark momentary conditional independence (PCMCI) causal discovery algorithm, which identifies causal links in a 2D field, to show the causal effect of each of these patterns on circulation and convection in the Northern Hemisphere. The spatial patterns and signs of the causal links in SEAS5 closely resemble those seen in ERA5, independent of the initialisation date of SEAS5. By performing a subsampling experiment (over time), we analyse the strengths of causal links in SEAS5 and show that they are qualitatively weaker than those in ERA5. We identify those regions for which SEAS5 data well reproduce ERA5 values, e.g. the southeastern USA, and highlight those where the bias is more prominent, e.g. North Africa and in general tropical regions. We demonstrate that different El Niño-Southern Oscillation phases have only a marginal effect on the strength of these links. Finally, we discuss the potential role of model mean-state biases in explaining differences between SEAS5 and ERA5 causal links.


13. CMIP6 skill at predicting interannual to multi-decadal summer monsoon precipitation variability. Monerie, P.-A., J. I. Robson, C. D. Ndiaye, C. Song and A. G. Turner (2023). Environmental Research Letters, 18: 094002, submitted 20 April 2023, accepted 20 July 2023, early online version 26 July 2023, published 7 August 2023. |
    Abstract
    Monsoons affect the economy, agriculture, and human health of two thirds of the world's population. Therefore, predicting variations in monsoon precipitation is societally important. We explore the ability of climate models from the 6th phase of the Climate Model Intercomparison Project (CMIP6) to predict summer monsoon precipitation variability by using hindcasts from the Decadal Climate Prediction Project (Component A). The multi-model ensemble-mean shows significant skill at predicting summer monsoon precipitation from one year to 6-9 years ahead. However, this skill is dependent on the model, monsoon domain, and lead-time. In general, the skill of the multi-model ensemble-mean prediction is low in year 1 but increases for longer lead times and is largely consistent with externally forced changes. The best captured region is northern Africa for the 2-5- and 6-9-year forecast lead times. In contrast, there is no significant skill using the ensemble-mean over East and South Asia and, furthermore, there is significant spread in skill among models for these domains. By sub-sampling the ensemble we show that the difference in skill between models is tied to the simulation of the externally forced response over East and South Asia, with models with a more skilful forced response capable of better predictions. A further contribution is from skilful prediction of Pacific Ocean temperatures for the South Asian summer monsoon at longer lead-times. Therefore, these results indicate that predictions of the East and South Asian monsoons could be significantly improved.


14. The influence of weather patterns and the Madden-Julian Oscillation on extreme precipitation over Sri Lanka. Deoras, A., A. G. Turner, K. M. R. Hunt and I. M. Shiromani Priyanthika Jayawardena (2023). Geophysical Research Letters, submitted 16 March 2023, accepted 22 June 2023, published online 27 July 2023. |
    Abstract
    Sri Lanka is affected by extreme precipitation events every year, which cause floods, land-slides and tremendous economic losses. We use the ERA5 reanalysis dataset to understand the association of extreme precipitation events with 30 weather patterns, which were originally derived to represent the variability of the Indian climate during January-December 1979-2016. We find that weather patterns that are most common during the northeast monsoon (December-February) and second intermonsoon (October-November) seasons produce the highest number of extreme precipitation events. Furthermore, extreme precipitation events occurring during these two seasons are more persistent than those during the southwest monsoon (May-September) and first intermonsoon (March-April) seasons. We analyse the modulation of extreme precipitation events by the Madden-Julian Oscillation, and find that their frequency is enhanced (suppressed) in phases 1-4 (5-8) for most weather patterns.


15. Uncertainty in simulating twentieth century West African precipitation trends: the role of anthropogenic aerosol emissions. Monerie, P.-A., A. Dittus, L. J. Wilcox and A. G. Turner (2023). Earth's Future, 11(2): e2022EF002995, submitted 22 June 2022, revised version submitted 18 October 2022, accepted 13 January 2023, first published 22 January 2023. |
    Abstract
    Anthropogenic aerosol emissions from North America and Europe have strong effects on the decadal variability of the West African monsoon. Anthropogenic aerosol effective radiative forcing is model dependent, but the impact of such uncertainty on the simulation of long-term West African monsoon variability is unknown. We use an ensemble of simulations with HadGEM3-GC3.1 that span the most recent estimates in simulated anthropogenic aerosol effective radiative forcing. We show that uncertainty in anthropogenic aerosol radiative forcing leads to significant uncertainty at simulating multi-decadal trends in West African precipitation. At the large scale, larger forcing leads to a larger decrease in the interhemispheric temperature gradients, in temperature over both the North Atlantic Ocean and northern Sahara. There are also differences in dynamic changes specific to the West African monsoon (locations of the Saharan heat low and African Easterly Jet, of the strength of the West African westerly jet, and of African Easterly Wave activity). We also assess effects on monsoon precipitation characteristics and temperature. We show that larger aerosol forcing results in a decrease of the number of rainy days and of heavy and extreme precipitation events and warm spells. However, simulated changes in onset and demise dates do not appear to be sensitive to the magnitude of aerosol forcing. Our results demonstrate the importance of reducing the uncertainty in anthropogenic aerosol forcing for understanding and predicting multi-decadal variability in the West African monsoon.


16. Intraseasonal oscillations of the Silk Road pattern lead to predictability in East Asian precipitation patterns and the Mei Yu front. Muetzelfeldt, M., R. K. Schiemann, A. G. Turner, P.-L. Vidale and A. Menon (2023). Environmental Research Communications, 5(1): 015003, submitted 23 September 2022, published 18 January 2023. |
    Abstract
    The Silk Road pattern (SRP) is analysed on intraseasonal timescales over summer using empirical orthogonal functions (EOFs) of the meridional wind at 200 hPa. The first two principal components explain almost equal amounts of variance, hence both are required to represent the intraseasonal SRP. The associated spatial loadings are 90° out of phase with each other, providing evidence that propagating oscillations are a natural mode of variability of the intraseasonal SRP. This is supported by Hovmöller diagrams of the meridional wind at 200 hPa and by phase diagrams of the first two EOFs, which both show a predominantly eastward-propagating oscillation. The oscillations are identified as plausibly being Rossby waves by means of waveguide theory. The subtropical westerly jet and East Asian rainfall patterns are found to be dependent on the phase of the oscillation: wet anomalies occur to the east of troughs in the jet, which are also regions where local jet entrances cause upper-level divergence via an ageostrophic circulation. Dry anomalies occur to the west of troughs, which are regions of upper-level convergence. The time-delayed location of the summer Mei Yu front relative to its climatology is dependent on the phase of the oscillation: when there is an upper-level trough located over the Korean Peninsula, the Mei Yu front is likely to be located further north than normal 3-8 days later, before returning to its climatological position. This suggests that the phase of the intraseasonal SRP acts as a potential source of predictability of the location of the Mei Yu front, which might allow for better prediction of the associated rains.


17. Nonlinear intensification of monsoon low pressure systems by the BSISO. Hunt, K. M. R. and A. G. Turner (2022). Weather and Climate Dynamics, 3: 1341-1358, submitted 9 June 2022, accepted 19 October 2022, published 18 November 2022. | Discussions version, started 14 June 2022. |
    Abstract
    More than half of the rainfall brought to the Indian subcontinent by the summer monsoon is associated with low-pressure systems (LPSs). Yet their relationship with the Boreal Summer Intraseasonal Oscillation (BSISO) - the dominant intraseasonal forcing on the monsoon - is only superficially understood. Using reanalysis data, we explore the relationship between the BSISO and LPS intensity, propagation, and precipitation, and associated underlying mechanisms. The BSISO has a large impact on mean monsoon vorticity and rainfall as it moves northward - maximising both in phases 2-3 over southern India and phases 5-6 over northern India - but a much weaker relationship with total column water vapour. We present evidence that LPS genesis also preferentially follows these phases of the BSISO. We identify significant relationships between BSISO phase and LPS precipitation and propagation: for example, during BSISO phase 5, LPSs over north India produce 51% heavier rainfall and propagate northwestward 20% more quickly. Using a combination of moisture flux linearisation and quasigeostrophic theory, we show that these relationships are driven by changes to the underlying dynamics, rather than the moisture content or thermodynamic structure, of the monsoon. Using the example of LPSs over northern India during BSISO phase 5, we show that the vertical structure of anomalous vorticity can be split into contributions from the BSISO and the nonlinear response of the LPS to anomalous BSISO circulation. Complementary hypotheses emerge about the source of this nonlinear vorticity response: nonlinear frictional convergence and secondary barotropic growth. We show that both are important. The BSISO imparts greater meridional shear on the background state, supporting LPS intensification. The BSISO background and nonlinear LPS response both contribute significantly to anomalous boundary layer convergence, and we show through vortex budget arguments that the former supports additional LPS intensification in boundary layer while the latter supports faster westward propagation. This work therefore yields important insights into the scale interactions controlling one of the dominant synoptic systems contributing to rainfall during the monsoon.


18. Froude number-based rainfall regimes over the Western Ghats mountains of India. Phadtare, J. A., J. K. Fletcher, A. N. Ross, A. G. Turner and R. K. H. Schiemann (2022). Quarterly Journal of the Royal Meteorological Society, 148(748): 3388-3405, submitted 27 December 2021, revised version submitted 7 June 2022, accepted 14 August 2022, published 7 October 2022. |
    Abstract
    The variations in the character of monsoonal rainfall over the Western Ghats region on the west coast of India are studied using radiosondes, satellite observations and reanalysis products. Summer monsoon rainfall over this region occurs in alternate offshore and onshore phases. It is shown that these phases are primarily controlled by the strength of the low-level westerly jet. Thus, a classification based on the Froude number, F = U/NH, of the onshore flow is proposed, where, H is the mountain height, U is the mean wind speed and N is the mean Brunt-Väisäla frequency over depth H. At low F (< 0.5), onshore winds are weak and the diurnal thermal fluctuation over the orography is strong; the land-sea and mountain-valley circulations are enhanced leading to a stronger diurnal control over the rainfall. A nocturnal offshore propagation of rainfall from the west coast is seen during this phase. Rainfall over the rainshadow region to the east of the Western Ghats also increases due to a weaker lee effect while it decreases over the Western Ghats due to a greater blocking effect. At high F (> 1), the orographic blocking of the low-level winds is weak. Thus, rainfall is enhanced over the Western Ghats and reduced over the rainshadow region due to a stronger lee effect. In this phase, the diurnal thermal fluctuation over the orography is weak. The bulk Richardson number is less than 1, suggesting a dominance of vertical wind shear over the buoyancy forces. The level of free convection and convective inhibition over the west coast are also very low. Hence, at high F, rainfall over the west coast mainly results from the mechanical uplifting of the westerly winds by the Western Ghats with no preference to a particular time of the day. These findings will help in improving the representation of orographic effects and the diurnal cycle of rainfall in numerical models.


19. The role of mid-tropospheric moistening and land surface wetting in the progression of the 2016 Indian monsoon. Menon, A., A. G. Turner, A Volonté, C. M. Taylor, S. Webster and G. M. Martin (2022). Quarterly Journal of the Royal Meteorological Society, 148(747): 3033-3055, submitted 3 December 2020, accepted 4 October 2021, first published 11 October 2021. |
    Abstract
    Accurately predicting the Indian monsoon is limited by inadequate understanding of the underlying processes, which feeds into systematic model biases. Here we aim to understand the dynamic and thermodynamic features associated with the progression of the monsoon, using 2016 as a representative year, with the help of convection-permitting simulations of the Met Office Unified Model. Simulations are carried out in a 4 km-resolution limited area model, nested within a coarser global model. Two major processes thought to influence the northwestward progression of the monsoon are: (1) the interaction between the low-level monsoon flow and a mid-tropospheric dry-air intrusion from the northwest and (2) land-atmosphere interactions. We find that the 4-km limited area model simulates the mid-tropospheric moistening that erodes the northwesterly dry intrusion, pushing the northern limit of moist convection northwestwards. The surface soil moisture also plays a major role at the leading edge of the monsoon progression. The heavy rains associated with the local onset wet the soil, reducing the sensitivity of surface fluxes to soil moisture and weakening the land influence on further progression of monsoon rains. The 4 km model is tested with an alternative land surface configuration to explore its sensitivity to land surface processes. We find that the choice of soil and vegetation ancillaries affects the time scales of soil moisture-precipitation feedback and the timing of diurnal convection, thereby affecting the local onset. We further compare these simulations with a parameterised convection run at 17~km resolution to isolate the effects of convective parameterisation and resolution. The model with explicit convection better simulates the dynamic and thermodynamic features associated with the progression of the monsoon.


20. The structure of strong Indian monsoon low-pressure systems in Subseasonal-to-Seasonal prediction models. Deoras, A., K. M. R. Hunt and A. G. Turner (2022). Quarterly Journal of the Royal Meteorological Society, 148(746): 2147-2166, submitted 4 August 2021, accepted 26 April 2022, first published 2 May 2022. |
    Abstract
    The structure of strong Indian monsoon low-pressure systems (SLPSs) up to forecast lead times of 15 days in eleven models of the Subseasonal-to-Seasonal (S2S) prediction project is analysed. SLPS tracks are obtained from a catalogue of LPSs tracked in all ensemble members of the S2S models during a common reforecast period of June-September 1999-2010. SLPSs, which have a minimum intensity equal to at least the upper-quartile intensity of all LPSs, are then composited to generate horizontal and vertical structures of several dynamic and thermodynamic fields. The evolution of fields with forecast lead time and during LPS life-cycle is analysed. Furthermore, the simulation of the lower-tropospheric monsoon circulation, precipitation biases, and the precipitation contribution of LPSs are analysed. All S2S models and the multimodel mean (MMM) simulate the lower-tropospheric monsoon circulation, but prominent dry biases are observed in the Australian Bureau of Meteorology (BoM) and Environment and Climate Change Canada models. The precipitation contribution of LPSs to the summer mean precipitation is smaller in all S2S models than in tracks derived from ERA-Interim reanalysis. The location and amplitude of the lower-tropospheric cold core and the location of maximum precipitation are not well simulated by many models, particularly by the Hydrometeorological Centre of Russia model, in which the cold core is missing altogether. The structure of relative vorticity anomaly in all S2S models and the MMM is shallower and weaker than in ERA-Interim and MERRA-2 reanalyses. Whilst the cold core intensifies through the LPS lifecycle in all models, the warm core features a midlife maximum, except in models such as BoM and China Meteorological Administration. These results demonstrate the potential for S2S models at simulating the structure of SLPSs, benefiting stakeholders that use S2S models for forecasting.


21. Characterising the interaction of tropical and extratropical air masses controlling East Asian summer monsoon progression using a novel frontal detection approach. Volonté, A., A. G. Turner, R. K. Schiemann, P.-L. Vidale and N. P. Klingaman (2022). Weather and Climate Dynamics, 3: 575-599, submitted 24 February 2021, accepted 8 April 2022, published 18 May 2022. | Discussions version, started 26 February 2021. |
    Abstract
    The East Asian summer monsoon (EASM) is a complex phenomenon, influenced by both tropical and midlatitude dynamics and by the presence of the Tibetan Plateau. The EASM front (EASMF) separates tropical and extratropical air masses as the monsoon marches northwards. Although the different factors behind EASM progression are illustrated in a number of studies, their interactions, in particular between tropical and extratropical air masses, still need to be clarified. In this study we apply Eulerian and Lagrangian methods to the ERA5 reanalysis dataset to provide a comprehensive study of the seasonal progression and interannual variability of the EASM, and we highlight the dynamics of the air masses converging at its front. A frontal detection algorithm is used to perform a front-centred analysis of EASM progression. The analysis highlights the primary role of the subtropical westerly jet (STWJ) and of the Western North Pacific subtropical high (WNPSH) in controlling the strength and the poleward progression of the EASMF, in particular during Mei Yu, the primary stage of EASM progression. These forcings act to steer the southerly advection of low-level moist tropical air, modulated by the seasonal cycle of the Asian monsoon. The Mei Yu stage is distinguished by an especially clear interaction between tropical and extratropical air masses converging at the EASMF. The analysis of composites based on the latitude of the EASMF during Mei Yu reveals the influence exerted by the STWJ on the cool extratropical flow impacting on the northern side of the EASMF, whose progression is also dependent on the location of the WNPSH. In turn, this affects the extent of the warm moist advection on its southern side and the distribution and intensity of resultant rainfall over China. This study shows the validity of an analysis of EASM progression focused on its front and on the related low- and mid-level airstreams, at least in the Mei Yu stage. The framework highlighted shows how the regional flow over East Asia drives the low-level airstreams that converge at the EASMF, thus controlling the shape of EASM progression. This framework provides a basis for studies of climate variability and extreme events and for model evaluation.


22. Katabatic and convective processes drive two preferred peaks in the precipitation diurnal cycle of the Central Himalaya. Hunt, K. M. R., A. G. Turner and R. K. H. Schiemann (2022). Quarterly Journal of the Royal Meteorological Society, 148(745): 1731-1751, submitted 25 October 2021, accepted 19 March 2022, first published 27 March 2022. |
    Abstract
    The diurnal cycle of precipitation over the Central Himalaya is governed by a complex interaction between the diurnal cycle of tropical convection and local orographic flow. Understanding this interaction is crucial for model evaluation, where the simulation of such processes is highly sensitive to model resolution and choice of parameterisation schemes. In this study, the mean diurnal cycle is computed using GPM-IMERG data and is shown to be bimodal, with one peak in the late afternoon (1700 IST) and a stronger one in the early morning (0200 IST). This structure is an artefact of compositing, as individual days are associated with single peaks. The late afternoon 'convective' peak is shown to be linked to the diurnal cycle of tropical convection, whereas the nocturnal 'katabatic' peak is shown to be triggered by nocturnal downslope flow converging with the background monsoon circulation. As such, the katabatic peak is strongly favoured by an active monsoon trough, which provides greater south-easterly moisture flux to the foothills, resulting in increased low-level moisture flux convergence upon interaction with the katabatic northerlies. When the trough is less active, precipitation is brought to the region by mesoscale convective systems, ranging in scale from tens to thousands of kilometres, resulting in convective peaks. We hypothesise that these convective afternoon peaks are enhanced by anabatic flow. It is shown that the Boreal Summer Intraseasonal Oscillation (BSISO) does not play a significant role in modulating either the timing or amplitude of the diurnal cycle; however, low-pressure systems do: either by intensifying the trough (and hence the katabatic peak), or, when further north, by providing deep convection (hence supporting the convective peak). Two reanalyses and a separate 17-km model with parameterised convection capture both peaks, but overestimate the magnitude of the convective peak and underestimate the magnitude of the katabatic peak.


23. Effects of anthropogenic aerosol and greenhouse gases emissions on Northern Hemisphere monsoon precipitation: mechanisms and uncertainty. Monerie, P.-A., L. J. Wilcox and A. G. Turner (2022). Journal of Climate, 35(8): 2305-2326, submitted 27 May 2021, revised version submitted 31 December 2021, accepted 4 January 2022, published online 25 March 2022, print 15 April 2022 |
    Abstract
    Northern Hemisphere Land monsoon precipitation (NHLM) exhibits multidecadal variability, decreasing over the second half of the 20th century and increasing after the 1980s. We use a novel combination of CMIP6 simulations and several large ensembles to assess the relative roles of drivers of monsoon precipitation trends, analyzing the effects of anthropogenic aerosol (AA), greenhouse gas (GHG) emissions and natural forcing. We decomposed summer global monsoon precipitation anomalies into dynamic and thermodynamic terms to assess the drivers of precipitation trends. We show that the drying trends are likely to be mainly due to increased AA emissions, which cause shifts of the atmospheric circulation and a decrease in moisture advection. Increases in GHG emissions cause monsoon precipitation to increase due to strengthened moisture advection. The uncertainty in summer monsoon precipitation trends is explored using three initial condition large ensembles. AA emissions have strong controls on monsoon precipitation trends, exceeding the effects of internal climate variability. However, uncertainties in the effects of external forcings on monsoon precipitation are high for specific periods and monsoon domains, and due to differences in how models simulate shifts in atmospheric circulation. The effect of AA emissions is uncertain over the northern African monsoon domain, due to differences among climate models in simulating the effects of AA emissions on net shortwave radiation over the North Atlantic Ocean.


24. Projected changes in the East Asian hydrological cycle for different levels of future global warming. Chevuturi, A., N. P. Klingaman, A. G. Turner, L. Guo and P.-L. Vidale (2022). Atmosphere, 13(3): 405, submitted 15 December 2021, accepted 24 February 2022, published 1 March 2022. |
    Abstract
    Recent decades have shown significant changes to the hydrological cycle over East Asia (EA), and further changes are expected due to future global warming. This study evaluates projected seasonal changes in the EA hydrological cycle using simulations that are 1.5°C, 2.0°C and 3.0°C warmer than pre-industrial, from the Met Office Unified Model (MetUM) Global Ocean Mixed Layer model version 2.0 (GOML2.0), compared against present-day conditions. The moisture sources of the warming-induced precipitation changes are identified over five hydrologically unique regions within EA. Precipitation over EA increases with warming (except over southeastern EA in the spring and autumn) due to the intensified hydrological cycle. The projected seasonal changes in the hydrological cycle are usually nonlinear, with the rate of change between 1.5°C and 2.0°C larger than the rate of change between 2.0°C and 3.0°C of warming. The warming-induced precipitation increases are mainly associated with an increase in remote moisture convergence rather than local moisture recycling, except over the Tibetan Plateau. Decomposition of the changes in moisture sources by direction and flux component indicate that changes from the west are dominated by changes to moisture and changes from the north are more circulation driven. The changes from the south are moisture driven over southern EA and driven by moisture and circulation change over northern EA. Our results highlight the regionally and seasonally diverse projected changes to the EA hydrological cycle due to global warming, which will be useful for region-specific climate mitigation policies and the implementation of seasonally varying adaptation methods.


25. Evaluation of Asian summer precipitation in different configurations of a high-resolution GCM at a range of decision-relevant spatial scales. Muetzelfeldt, M. R., R. K. H. Schiemann, A. G. Turner, N. P. Klingaman, M. J. Roberts and P.-L. Vidale (2021). Hydrology and Earth System Science, submitted 10 December 2020, accepted 11 November 2021, published 16 December 2021. | Discussions version, started 23 February 2020. |
    Abstract
    High-resolution general circulation models (GCMs) can provide new insights into the simulated distribution of global precipitation. We evaluate how summer precipitation is represented over Asia in global simulations with a grid length of 14 km. Three simulations were performed: one with a convection parametrization, one with convection represented explicitly by the model's dynamics, and a hybrid simulation with only shallow and mid-level convection parametrized. We evaluate the mean simulated precipitation and the diurnal cycle of the amount, frequency, and intensity of the precipitation against satellite observations of precipitation from the Climate Prediction Center morphing method (CMORPH). We also compare the high-resolution simulations with coarser simulations that use parametrized convection. The simulated and observed precipitation is averaged over spatial scales defined by the hydrological catchment basins; these provide a natural spatial scale for performing decision-relevant analysis that is tied to the underlying regional physical geography. By selecting basins of different sizes, we evaluate the simulations as a function of the spatial scale. A new BAsin-Scale Model Assessment ToolkIt (BASMATI) is described, which facilitates this analysis. We find that there are strong wet biases (locally up to 72 mm d^-1 at small spatial scales) in the mean precipitation over mountainous regions such as the Himalayas. The explicit convection simulation worsens existing wet and dry biases compared to the parametrized convection simulation. When the analysis is performed at different basin scales, the precipitation bias decreases as the spatial scales increase for all the simulations; the lowest-resolution simulation has the smallest root mean squared error compared to CMORPH. In the simulations, a positive mean precipitation bias over China is primarily found to be due to too frequent precipitation for the parametrized convection simulation and too intense precipitation for the explicit convection simulation. The simulated diurnal cycle of precipitation is strongly affected by the representation of convection: parametrized convection produces a peak in precipitation too close to midday over land, whereas explicit convection produces a peak that is closer to the late afternoon peak seen in observations. At increasing spatial scale, the representation of the diurnal cycle in the explicit and hybrid convection simulations improves when compared to CMORPH; this is not true for any of the parametrized simulations. Some of the strengths and weaknesses of simulated precipitation in a high-resolution GCM are found: the diurnal cycle is improved at all spatial scales with convection parametrization disabled, the interaction of the flow with orography exacerbates existing biases for mean precipitation in the high-resolution simulations, and parametrized simulations produce similar diurnal cycles regardless of their resolution. The need for tuning the high-resolution simulations is made clear. Our approach for evaluating simulated precipitation across a range of scales is widely applicable to other GCMs.


26. 2016 Monsoon Convection and its place in the Large-Scale Circulation using Doppler Radars. Doyle, A. J., T. H. M. Stein and A. G. Turner (2021). Journal of Geophysical Research - Atmospheres, 26(22): 1-25, submitted 25 August 2020, revised version submitted 8 March 2021, accepted 1 November 2021, first published 5 November 2021. | ESSOAr preprint, v3 | DWR-derived convective cell statistics data: CEDA catalogue record / DOI / v2 CEDA catalogue record / DOI |
    Abstract
    Convective cloud development during the Indian monsoon helps moisten the atmospheric environment and drive the monsoon trough northwards each year, bringing a large amount of India's annual rainfall. Therefore, an increased understanding of how monsoon convection develops from observations will help inform model development. In this study, 139 days of India Meteorological Department Doppler weather radar data is analysed for 7 sites across India during the 2016 monsoon season. Convective cell-top heights (CTH) are objectively identified through the season, and compared with near-surface (at 2 km height) reflectivity. These variables are analysed over three time scales of variability during the monsoon: monsoon progression on a month-by-month basis, active-break periods and the diurnal cycle. We find a modal maximum in CTH around 6-8 km for all sites. Cell-averaged reflectivity increases with CTH, at first sharply, then less sharply above the freezing level. Bhopal and Mumbai exhibit lower CTH for monsoon break periods compared to active periods. A clear diurnal cycle in CTH is seen at all sites except Mumbai. For south-eastern India, the phase of the diurnal cycle depends on whether the surface is land or ocean, with the frequency of oceanic cells typically exhibiting an earlier morning peak compared to land, consistent with the diurnal cycle of precipitation. Our findings confirm that Indian monsoon convective regimes are partly regulated by the large-scale synoptic environment within which they are embedded. This demonstrates the excellent potential for weather radars to improve understanding of convection in tropical regions.


27. Skilful seasonal predictions of Global Monsoon summer precipitation with DePreSys3. Monerie, P.-A., J. Robson, N. Dunstone and A. G. Turner (2021). Environmental Research Letters, 16: 104035, published online 28 September 2021. |
    Abstract
    We assess skill of the Met Office's DePreSys3 prediction system at forecasting summer global monsoon precipitation at the seasonal time scale (2-5 month forecast period). DePreSys3 has significant skill at predicting summer monsoon precipitation (r=0.68), but the skill varies by region and is higher in the northern (r=0.68) rather than in the southern hemisphere (r=0.44). To understand the sources of precipitation forecast skill, we decompose the precipitation into several dynamic and thermodynamic components and assess the skill in predicting each. While dynamical changes of the atmospheric circulation primarily contribute to global monsoon variability, skill at predicting shifts in the atmospheric circulation is relatively low. This lower skill partly relates to DePreSys3's limited inability to accurately simulate changes in atmospheric circulation patterns in response to sea surface temperature forcing. Skill at predicting the thermodynamic component of precipitation is generally higher than for the dynamic component, but thermodynamic anomalies only contribute a small proportion of the total precipitation variability. Finally, we show that the use of a large ensemble improves skill for predicting monsoon precipitation, but skill does not increase beyond 20 members.


28. Magnitude, scale, and dynamics of the 2020 Mei Yu rains and floods over China. Volonté, A., R. K. Schiemann, A. G. Turner and N. P. Klingaman (2021). Advances in Atmospheric Science, submitted 26 February 2021, accepted 13 July 2021. | Various news articles about this work by Institute for Atmospheric Physics, Chinese Academy of Sciences: Phys.org, 4 October 2020; Newsbreak, 4 October 2020; Mirage News, 5 October 2020 | Article in AZO Cleantech, 5 October 2020 |
    Abstract
    Large parts of East and South Asia were affected by heavy precipitation and flooding during early summer 2020. This study provides both a statistical and dynamical characterization of rains and floods affecting the Yangtze River Basin (YRB). By aggregating daily and monthly precipitation over river basins across Asia, it is shown that the YRB is one of the areas that was particularly affected. June and July 2020 rainfall was higher than in the previous 20 years, and the YRB experienced anomalously high rainfall across most of its sub-basins. YRB discharge also attained levels not seen since 1998/9. An automated method detecting the daily position of the East Asian Summer Monsoon Front (EASMF) is applied to show that the anomalously high YRB precipitation was associated with a halted northward progression of the EASMF and prolonged Mei Yu conditions over the YRB lasting more than one month. Two 5-day heavy-precipitation episodes (12-16 June and 4-8 July 2020) are selected from this period for dynamical characterisation, including Lagrangian trajectory analysis. Particular attention is devoted to the dynamics of the airstreams converging at the EASMF. Both episodes display heavy precipitation and convergence of monsoonal and subtropical air masses. However, clear differences are identified in the upper-level flow pattern, substantially affecting the balance of airmass advection towards the EASMF. This study contextualises heavy precipitation in Asia in summer 2020 and showcases a number of analysis tools developed by the authors for the study of such events.


29. The four varieties of Indian monsoon low-pressure systems and their modulation by tropical intraseasonal variability. Deoras, A., K. M. R. Hunt and A. G. Turner (2021). Weather, 76(6, Special Issue: Student Conference 2020): 194-200, published online 30 May 2021. |
    Abstract
    South Asian monsoon low-pressure systems (LPSs) are synoptic-scale cyclonic vortices that produce substantial precipitation over the Indian subcontinent. In this study, a catalogue of four regional varieties of LPSs, which occur during June-September 1979-2018, is used for examining the track statistics, thermal and moisture structure, and precipitation contribution of each variety. The modulation of LPS activity by tropical intraseasonal variability, which is commonly monitored by the Boreal Summer Intraseasonal Oscillation, Monsoon Intraseasonal Oscillation and Madden-Julian Oscillation indices, is investigated.


30. Temporal patterns in radar-observed convective cell development during the 2016 monsoon onset. Doyle, A., T. H. M. Stein and A. G. Turner (2021). Weather, 76(6, Special Issue: Student Conference 2020): 180-184, published online 10 May 2021. |
    Abstract
    The onset of the South Asian summer monsoon in different regions in India is highly variable from year-to-year and is tied to the large-scale environment. Convective cloud development is considered to be an important component of the monsoon advance. Therefore, using 12 Doppler radars across India, we examine the relationship between cloud development and monsoon onset at each site. Convective area correlates well with precipitation for all sites. However, only Mumbai, Nagpur and Bhopal show a clear increase in convection after monsoon onset. Elsewhere, complexities associated with the definition of onset and the local environment result in less clarity in cloud development.


31. How interactions between tropical depressions and western disturbances affect heavy precipitation in South Asia. Hunt, K. M. R., A. G. Turner and R. K. H. Schiemann (2021). Monthly Weather Review, 149(6): 1801-1825, published online 5 April 2021. |
    Abstract
    Interactions over South Asia between tropical depressions (TDs) and extratropical storms known as western disturbances (WDs) are known to cause extreme precipitation events, including those responsible for the 2013 floods over northern India. In this study, existing databases of WD and TD tracks are used to identify potential WD-TD interactions from 1979-2015; these are filtered according to proximity and intensity, leaving 59 cases which form the basis of this paper. Synoptic charts, vorticity budgets, and moisture trajectory analyses are employed to identify and elucidate common interaction types among these cases. Two broad families of interaction emerge. Firstly, a dynamical coupling of the WD and TD, whereby either the upper- and lower-level vortices superpose (a vortex merger), or the TD is intensified as it passes into the entrance region of a jet streak associated with the WD (a jet-streak excitation). Secondly, a moisture exchange between the WD and TD, whereby either anomalous moisture is advected from the TD to the WD, resulting in anomalous precipitation near the WD (a TD-to-WD moisture exchange), or anomalous moisture is advected from the WD to the TD (a WD-to-TD moisture exchange). Interactions are most common in the post-monsoon period as the subtropical jet, which brings WDs to the subcontinent, returns south; there is a smaller peak in May and June, driven by monsoon onset vortices. Precipitation is heaviest in dynamically-coupled interactions, particularly jet-streak excitations. Criteria for automated identification of interaction types are proposed, and schematics for each type are presented to highlight key mechanisms.


32. Comparison of the prediction of Indian monsoon low-pressure systems by the Subseasonal-to-Seasonal prediction project. Deoras, A., K. M. R. Hunt and A. G. Turner (2021). Weather and Forecasting, 36(3): 859-877, published online 23 February 2021. | LPS track data in S2S models, ERA-Interim & MERRA-2 |
    Abstract
    This study analyses the prediction of Indian monsoon low-pressure systems (LPSs) on an extended time scale of 15 days by models of the Subseasonal-to-Seasonal (S2S) prediction project. Using a feature-tracking algorithm, LPSs are identified in the eleven S2S models during a common re-forecast period of June September 1999-2010, and then compared with 290 and 281 LPSs tracked in ERA-Interim and MERRA-2 reanalysis datasets. The results show that all S2S models under-simulate the frequency of LPSs. They are able to represent transits, genesis and lysis of LPSs; however, large biases are observed in the Australian Bureau of Meteorology, China Meteorological Administration (CMA) and Hydrometeorological Centre of Russia (HMCR) models. The CMA model exhibits large LPS track position error and the intensity of LPSs is overestimated (underestimated) by most models when verified against ERA-Interim (MERRA-2). The European Centre for Medium-Range Weather Forecasts and UK Met Office models have the best ensemble spread-error relationship for the track position and intensity, whereas the HMCR model has the worst. Most S2S models are underdispersive - more so for the intensity than the position. We find the influence of errors in the LPS simulation on the pattern of total precipitation biases in all S2S models. In most models, precipitation biases increase with forecast lead time over most of the monsoon core zone. These results demonstrate the potential for S2S models at simulating LPSs, thereby giving the possibility of improved disaster preparedness and water resources planning.


33. Forecast skill of the Indian monsoon and its onset in the ECMWF seasonal forecasting system 5 (SEAS5). Chevuturi, A., A. G. Turner, S. Johnson, A. Weisheimer, R. Senan, J. K. P. Shonk and T. N. Stockdale (2021). Climate Dynamics, 56(May): 2941-2957, published online 9 February 2021. | UoR press release |
    Abstract
    Accurate forecasting of variations in Indian monsoon precipitation and progression on seasonal time scales remains a challenge for prediction centres. We examine prediction skill for the seasonal-mean Indian summer monsoon and its onset in the European Centre for Medium-Range Weather Forecasts (ECMWF) seasonal forecasting system 5 (SEAS5). We analyse summer hindcasts initialised on 1st of May, with 51 ensemble members, for the 36-year period of 1981-2016. We evaluate the hindcasts against the Global Precipitation Climatology Project (GPCP) precipitation observations and the ECMWF reanalysis 5 (ERA5). The model has significant skill at forecasting dynamical features of the large-scale monsoon and local-scale monsoon onset tercile category one month in advance. SEAS5 shows higher skill for monsoon features calculated using large-scale indices compared to those at smaller scales. Our results also highlight possible model deficiencies in forecasting the all India monsoon rainfall.


34. Intraseasonal soil moisture-atmosphere feedbacks on the Tibetan Plateau circulation. Talib, J., C. M. Taylor, A. Duan and A. G. Turner (2021). Journal of Climate, 34: 1789-1807, submitted 21 May 2020, accepted 22 October 2020, published online 2 November 2020. |
    Abstract
    Substantial intraseasonal precipitation variability is observed across the Tibetan Plateau (TP) during boreal summer associated with the subtropical jet location and the Silk Road pattern. Weather station data and satellite observations highlight a sensitivity of soil moisture and surface fluxes to this variability. During rain-free periods of two or more days, skin temperatures are shown to rise as the surface dries, signalling decreased evaporative fraction. Surface fluxes are further enhanced by relatively clear skies. In this study we use an atmospheric reanalysis to assess how this surface flux response across the TP influences local and remote conditions. Increased surface sensible heat flux induced by decreased soil moisture during a regional dry event leads to a deepening of the planetary boundary-layer and the development of a heat low. Consistent with previous studies, heat low characteristics exhibit pronounced diurnal variability driven by anomalous daytime surface warming. For example, low-level horizontal winds are weakest during the afternoon and intensify overnight when boundary-layer turbulence is minimal. The heat low favours an upper-tropospheric anticyclone which induces an upper-level Rossby wave and leads to negative upper-level temperature anomalies across southern China. The Rossby wave intensifies the upper-level cyclonic circulation across central China, whilst upper-level negative temperature anomalies across south China extends the west Pacific subtropical high westward. These circulation anomalies influence temperature and precipitation anomalies across much of China. The association between land-atmosphere interactions across the TP, large-scale atmospheric circulation characteristics, and precipitation in east Asia highlights the importance of intraseasonal soil moisture dynamics on the TP.


35. Modes of coastal precipitation over southwest India and their relationship to intraseasonal variability. K. M. R. Hunt, A. G. Turner, T. H. M. Stein, J. K. Fletcher and R. K. H. Schiemann (2020) Quarterly Journal of the Royal Meteorological Society, 147(734): 181-201, submitted 1 June 2020, revised version submitted 27 August 2020, accepted 16 September 2020, published 16 October 2020. |
    Abstract
    The west coast of India, dominated by the Western Ghats mountain range, is among the rainiest places in the tropics. The interaction between the land-sea contrast of the coast, the monsoonal westerlies, and the oblique mountains is subject to complex intraseasonal variability, which has not previously been explored in depth. This study investigates that variability from the perspective of the land-sea contrast, using empirical orthogonal function analysis to discern regimes of onshore and offshore rainfall over south-west India and the eastern Indian Ocean. Locally, it is found that the rainfall is most sensitive to mid-tropospheric hu- midity: when this is anomalously high, deep convection is encouraged; when this is anomalously low, it is suppressed. A moisture tracking algorithm is employed to determine the primary sources of the anomalously wet and dry mid-tropospheric air. There are important secondary contributions from low-level vorticity and cross-shore moisture flux. The dominant control on intraseasonal variability in coastal precipitation is found to be the BSISO: over 75% of the strongest offshore events occur during phases 3 and 4; and about 40% of the strongest onshore events occur during phases 5 and 6. The location of monsoon low-pressure systems is also shown to be important in determining the magnitude and location of coastal rainfall.


36. Monsoon Climate Change Assessment. B. Wang, M. Biasutti, M. P. Byrne, C. Castro, C.-P. Chang, K. Cook, R. Fu, A. Grimm, K.-J. Ha, H. Hendon, A. Kitoh, R. Krishnan, J.-Y. Lee, J. Li, J. Liu, A. Moise, S. Pascale, M. K. Roxy, A. Seth, C.-H. Sui, A. G. Turner, S. Yang, K.-S. Yun, L. Zhang and T. Zhou (2021). Bulletin of the American Meteorological Society, 102(1): E1-E19, submitted 31 December 2019, accepted 17 April 2020, early online version 6 May 2020, published 1 January 2021. |
    Abstract
    Monsoon rainfall has profound economic and societal impacts for more than two-thirds of the global population. Here we provide a review on past monsoon changes and their primary drivers, the projected future changes and key physical processes, and discuss challenges of the present and future modeling and outlooks. Continued global warming and urbanization over the past century has already caused a significant rise in the intensity and frequency of extreme rainfall events in all monsoon regions (high confidence). Observed changes in the mean monsoon rainfall vary by region with significant decadal variations. NH land monsoon rainfall as a whole declined from 1950 to 1980 and rebounded after the 1980s, due to the competing influences of internal climate variability and radiative forcing from GHGs and aerosol forcing (high confidence); however, it remains a challenge to quantify their relative contributions. The CMIP6 models simulate better global monsoon intensity and precipitation over CMIP5 models, but common biases and large intermodal spreads persist. Nevertheless, there is high confidence that the frequency and intensity of monsoon extreme rainfall events will increase, alongside an increasing risk of drought over some regions. Also, land monsoon rainfall will increase in South Asia and East Asia (high confidence) and northern Africa (medium confidence), and decrease in North America and unchanged in Southern Hemisphere. Over Asian-Australian monsoon region the rainfall variability is projected to increase on daily to decadal scales. The rainy season will likely be lengthened in the Northern Hemisphere due to late retreat (especially over East Asia), but shortened in the Southern Hemisphere due to delayed onset.


37. Uncertainty in aerosol radiative forcing impacts the simulated global monsoon in the 20th century. Shonk, J., A. G. Turner, A. Chevuturi, L. Wilcox, A. Dittus and E. Hawkins (2020) Atmospheric Chemistry and Physics, 20: 14903-14915, published 3 December 2020. | Discussions version, published 23 June 2020. | Data used in this work |
    Abstract
    Anthropogenic aerosols are dominant drivers of historical monsoon rainfall change. However, large uncertainties in the radiative forcing associated with anthropogenic aerosol emissions, and the dynamical response to this forcing, lead to uncertainty in the simulated monsoon response. We use historical simulations from the "SMURPHS" project, run using HadGEM3-GC3.1, in which the time-varying aerosol emissions are scaled by factors from 0.2 to 1.5 to explore the monsoon sensitivity to historical aerosol forcing uncertainty (present-day versus preindustrial aerosol forcing in the range -0.38 W m^-2 to -1.50 W m^-2). The hemispheric asymmetry in emissions generates a strong relationship between scaling factor and both hemispheric temperature contrast and meridional location of tropical rainfall. Averaged over the period 1950-2014, increasing the scaling factor from 0.2 to 1.5 reduces the hemispheric temperature contrast by 0.9 °C, reduces the tropical summertime land-sea temperature contrast by 0.3 °C and shifts tropical rainfall southwards by 0.28° of latitude. The result is a reduction in global monsoon area by 3% and a reduction in global monsoon intensity by 2%. Despite the complexity of the monsoon system, the monsoon properties presented above vary monotonically and roughly linearly across scalings. A switch in the dominant influence on the 1950-1980 monsoon rainfall trend between greenhouse gases and aerosol is identified as the scalings increase. Regionally, aerosol scaling has a pronounced effect on Northern Hemisphere monsoon rainfall, with the strongest influence on monsoon area and intensity located in the Asian sector, where local emissions are greatest.


38. Effects of horizontal resolution and air-sea coupling on simulated moisture sources for regional East Asian precipitation. Guo, L., R. J. van der Ent, N. P. Klingaman, M.-E. Demory, P.-L. Vidale, A. G. Turner, C. C. Stephan and Amulya Chevuturi (2020). Geoscientific Model Development, 13: 6011-6028, published 1 December 2020. | Discussions version published 8 May 2020. |
    Abstract
    Precipitation over East Asia (EA) simulated in six experiments using the Met Office Unified Model (MetUM) is compared with observations and ERA-Interim reanalysis. These simulations are performed at resolutions of 200 - 40 km and with both atmosphere-only and air-sea coupled configurations. EA precipitation in MetUM is systematically overestimated, especially over southeastern EA and the Tibetan Plateau. Moisture sources for simulated and observed regional EA precipitation are traced using the Water Accounting Model-2layers (WAM-2layers) - a moisture tracking model that traces moisture sources using a combination of evaporation, atmospheric moisture and circulation. Biases in moisture sources are linked to biases in precipitation. For southeastern EA, positive precipitation biases are linked to errors in mid-latitude sources rather than tropical ocean sources. Increasing resolution reduces precipitation biases over the Tibetan Plateau. From the perspective of moisture source, this reduction comes from reduced remote moisture source that is blocked by the better representations of topography at higher resolution. Including coupling does not necessarily improve EA precipitation, however, coupling alters moisture sources. Because the effect of coupling on moisture source varies on location and sign, the collective impact on EA precipitation may not be noticeable. By using WAM-2layers, these changes in moisture sources can be attributed to changes in SST, circulation and associated evaporation. WAM-2layers can be a useful tool to identify model biases that cause biases in regional precipitation.


39. Accelerated increases in global and Asian summer monsoon precipitation from future aerosol reductions. Wilcox, L. J., Z. Liu, B. H. Samset, E. Hawkins, M. T. Lund, K. Nordling, S. Undorf, M. Bollasina, A. M. L. Ekman, S. Krishnan, J. Merikanto and A. G. Turner (2020) Atmospheric Chemistry and Physics, 20: 11955-11977, submitted 22 December 2019, published 26 October 2020. | Discussions version, published 30 January 2020. | Daily Mail news report citing this article Dramatic cuts to air pollution in Europe and Asia could SPEED UP climate change in the short term and lead to heatwaves and heavier rainfall - but 'doing nothing would be worse' | Guardian article discussing this work Drop in pollution may bring hotter weather and heavier monsoons |
    Abstract
    There is a large range of future aerosol emissions scenarios explored in the Shared Socioeconomic Pathways (SSPs), with plausible pathways spanning a range of possibilities from large global reductions in emissions by 2050 to moderate global increases over the same period. Diversity in emissions across the pathways is particularly large over Asia. Rapid reductions in anthropogenic aerosol and precursor emissions between the present day and the 2050s lead to enhanced increases in global and Asian summer monsoon precipitation relative to scenarios with weak air quality policies. However, the effects of aerosol reductions don't persist to the end of the 21st century for precipitation, when instead the response to greenhouse gases dominates differences across the SSPs. The relative magnitude and spatial distribution of aerosol changes is particularly important for South Asian summer monsoon precipitation changes. Precipitation increases here are initially suppressed in SSPs 2-4.5, 3-7.0, and 5-8.5 relative to SSP1-1.9 when the impact of remote emission decreases is counteracted by continued increases in South Asian emissions.


40. Dominant patterns of boreal summer interactions between tropics and mid-latitude: Causal relationships and the role of timescales. Di Capua, G., J. Runge, R. V. Donner, B. van den Hurk, A. G. Turner, R. Vellore, R. Krishnan and D. Coumou (2020). Weather and Climate Dynamics, 1: 519-539, submitted 17 April 2020, published 15 October 2020. | Discussions version, published 23 April 2020. |
    Abstract
    Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemisphere. In winter, the El Niño-Southern Oscillation (ENSO) is the dominant source of predictability in the tropics and extra-tropics, but its role in summer is much less pronounced and the exact teleconnection pathways are not well understood. Here, we assess how tropical convection interacts with mid-latitude summer circulation at different intraseasonal time-scales and how ENSO affects these interactions. First, we apply maximum covariance analysis (MCA) between tropical convective activity and mid-latitude geopotential height fields to identify the dominant modes of interaction. The first MCA mode connects the South Asian monsoon with the mid-latitude circumglobal teleconnection pattern. The second MCA mode connects the western North Pacific summer monsoon in the tropics with a wave-5 pattern centred over the North Pacific High in the mid-latitudes. We show that the MCA patterns are fairly insensitive to the selected intraseasonal time-scale from weekly to 4-weekly data. To study the potential causal interdependencies between these modes and with other atmospheric fields, we apply causal effect networks (CEN) at different time-scales. CENs extend standard correlation analysis by removing the confounding effects of autocorrelation, indirect links and common drivers. In general, there is a two-way causal interaction between the tropics and mid-latitudes but the strength and sometimes sign of the causal link are time-scale dependent. We introduce causal maps that plot the regionally specific causal effect from each MCA mode. Those maps confirm the dominant patterns of interaction and in addition, highlight specific mid-latitude regions that are most strongly connected to tropical convection. In general, the identified causal teleconnection patterns are only mildly affected by ENSO and the tropical-mid-latitude linkages remain similar. Still, La Niña strengthens the South Asian monsoon generating a stronger response in the mid-latitudes, while during El Niño years, the Pacific pattern is reinforced. This study paves the way for process-based validation of boreal summer teleconnections in (sub)seasonal forecast models and climate models and therefore helps to improve sub-seasonal and climate projections.


41. Vegetation forcing modulates global land monsoon and water resources in a CO2-enriched climate. Cui, J., S. Piao, C. Huntingford, X. Wang, X. Lian, A. Chevuturi, A. G. Turner and G. J. Kooperman (2020). Nature Communications, 11: article no. 5184, published 14 October 2020. |
    Abstract
    The global monsoon is characterised by transitions between pronounced dry and wet seasons, affecting food security for two-thirds of the world's population. Rising atmospheric CO2 influences the terrestrial hydrological cycle through climate-radiative and vegetation-physiological forcings. How these two forcings affect the seasonal intensity and characteristics of monsoonal precipitation and runoff is poorly understood. Here we use four Earth System Models to show that in a CO2-enriched climate, radiative forcing changes drive annual precipitation increases for most monsoon regions. Further, vegetation feedbacks substantially affect annual precipitation in North and South America and Australia monsoon regions. In the dry season, runoff increases over most monsoon regions, due to stomatal closure-driven evapotranspiration reductions and associated atmospheric circulation change. Our results imply that flood risks may amplify in the wet season. However, the lengthening of the monsoon rainfall season and reduced evapotranspiration will shorten the water resources scarcity period for most monsoon regions.


42. The impacts of climate change on the winter water cycle of the western Himalaya. Hunt, K. M. R., A. G. Turner and L. C. Shaffrey (2020). Climate Dynamics 55(October): 2287-2307, submitted 11 September 2019, accepted 11 June, published online 21 July 2020. |
    Abstract
    Some 180 million people depend on the Indus River as a key water resource, fed largely by precipitation falling over the western Himalaya. However, the projected response of western Himalayan precipitation to climate change is currently not well constrained: CMIP5 GCMs project a reduced frequency and vorticity of synoptic-scale systems impacting the area, but such systems would exist in a considerably moister atmosphere. In this study, a convection-permitting (4 km horizontal resolution) setup of the Weather Research and Forecasting (WRF) model is used to examine 40 cases of these synoptic-scale systems, known as western disturbances (WDs), as they interact with the western Himalaya. In addition to a present-day control run, three experiments are performed by perturbing the boundary and initial conditions to reflect pre-industrial, RCP4.5 and RCP8.5 background climates respectively. It is found that in spite of the weakening intensity of WDs, net precipitation associated with them in future climate scenarios increases significantly; conversely there is no net change in precipitation between the pre-industrial and control experiments despite a significant conversion of snowfall in the pre-industrial experiment to rainfall in the control experiment, consistent with 25 the changes seen in historical observations. This shift from snowfall to rainfall has profound consequences on water resource management in the Indus Valley, where irrigation is dependent on spring meltwater. Flux decomposition shows that the increase in future precipitation follows directly from the projected moistening of the tropical atmosphere (which increases the moisture flux incident on the western Himalaya by 28%) overpowering the weakened dynamics (which decreases it by 20%). Changes to extreme rainfall events are also examined: it is found that such events may increase significantly in frequency in both future scenarios examined. Two-hour maxima rainfall events that currently occur in 1-in-8 WDs are projected to increase tenfold in frequency in the RCP8.5 scenario; more prolonged (one-week maxima) events are projected to increase fiftyfold.


43. Airmass analysis of the processes driving the progression of the Indian summer monsoon. Volonté, A., A. G. Turner and A. Menon (2020). Quarterly Journal of the Royal Meteorological Society, 146(731): 2949-2980, submitted 28 March 2019, accepted 8 November 2019. |
    Abstract
    The Indian summer monsoon is a vital source of water and a cause of severe impacts for more than a billion people in the Indian subcontinent. The INCOMPASS project investigates the mechanisms driving its onset and progression through an observational field campaign supplemented by high-resolution numerical simulations for the 2016 season using UK Met Office models. A 4.4 km resolution convection-permitting limited-area model simulation (driven at its boundaries by a daily-initialised global model) is used in this study, and verified against observations, along with short-lead-time operational global forecasts. These data show that the monsoon progression towards northwest India in June 2016 is a non-steady process, modulated by the interaction between moist low-level southwesterly flow from the Arabian Sea and a northwesterly incursion of descending dry air from western and central Asia. The location and extent of these two flows are closely linked to mid-latitude dynamics, through the southward propagation of potential vorticity streamers and the associated formation of cyclonic circulations in the region where the two air masses interact. Particular focus is devoted to the use of Lagrangian trajectories to characterise the evolution of the aforementioned airstreams and complement the Eulerian monsoon progression analysis. The trajectories confirm that the interaction of the two airstreams is a primary driver of the general moistening of the troposphere associated with monsoon progression. They also indicate the occurrence of local diabatic processes along the airstreams, such as turbulent mixing and local evaporation from the Arabian Sea, in addition to moisture transport from remote sources. In summary, this combined Eulerian-Lagrangian analysis reveals the non-steady nature of monsoon progression towards northwest India. This process is driven by the interaction of different air masses and influenced by a synergy of factors on different scales, such as mid-latitude dynamics, transient weather systems and local diabatic processes.


44. Performance of the NCMRWF convection-permitting model during contrasting monsoon phases of the 2016 INCOMPASS field campaign. A. Jayakumar, S. J. Abel, A. G. Turner, S. Mohandas, J. Sethunath, D. O'Sullivan and E. N. Rajagopal (2020). Quarterly Journal of the Royal Meteorological Society, 146(731): 2928-2948, submitted 29 March 2019, accepted 22 October 2019. |
    Abstract
    This study uses INCOMPASS aircraft, radiosonde and satellite observations for verifying hydrometeors and associated state variables predicted by the regional model of the NCMRWF (NCUM-R) for contrasting phases of the 2016 monsoon. INCOMPASS flights B957 and B975 took place between Lucknow in northern India and Bhubaneswar near the east coast, and represent a contrast between dry pre-monsoon and active monsoon conditions, respectively. A moist profile above 4 km in Bhubaneswar measured on B957 showed a dry-air intrusion being eroded by mid-level clouds, whereas the Lucknow profile showed a drier, pre-monsoon profile. Aerosol extinction coefficient and cloud-top height measured using lidar showed an influx of continental aerosol, and intermittent multiple clouds below the aircraft in the mid-troposphere and boundary layer. Measurements from B975 match well with cyclonic wind patterns estimated from satellite observations and the convective tendency represented in radiosonde profiles. Extensive clouds were detected below 5 km during the active monsoon. Two-model formulations for cloud representation (prognostic cloud and prognostic condensate, PC2, and diagnostic schemes, Diag) are compared with observations during the campaign. Vertical structures of state variables from both schemes are generally in agreement along the flight tracks. Surface energy budget and cloud diagnoses indicate higher cloud cover in Diag consistent with lower surface temperatures through reduced surface downwelling shortwave flux than in PC2, while the latent heat flux is found to be insensitive to cloud scheme chosen. In-cloud water content is larger in PC2 for lower cloud fraction, and the autoconversion process is faster with respect to Diag. Higher total condensed-water content in the model with respect to aircraft measurements and an enhanced light precipitation bias with respect to satellite data is common to both cloud schemes. Further work to improve the representation of clouds and precipitation for the tropical implementation of the model is clearly warranted.


45. A case study of land-atmosphere coupling during monsoon onset in northern India. E. Barton, C. M. Taylor, D. J. Parker, A. G. Turner, D. Belušiç, S. Böing, J. K. Brooke, R. C. Harlow, P. R. Harris, K. M. R. Hunt, A. Jayakumar and A. K. Mitra (2020). Quarterly Journal of the Royal Meteorological Society, 146(731): 2891-2905, submitted 29 November 2018, accepted 30 March 2019. Authors' preprint |
    Abstract
    This paper presents a land-atmosphere case study for a single day during monsoon onset, incorporating data from a research aircraft, satellite products and model outputs. The unique aircraft observations reveal temperature and humidity contrasts of up to 5 K and 4 gkg^-1 in the planetary boundary layer induced by spatial variations in soil moisture. Both antecedent rain and irrigation were found to be drivers of this atmospheric variability. There is also evidence of soil moisture induced mesoscale circulations above some surfaces. This is the first time such responses have been observed in situ over India. Soil moisture-driven temperature anomalies are larger than those found in previous observational studies in the African Sahel. Moreover, irrigation in the region is extensive, unlike in the Sahel, and has a similar atmospheric effect to antecedent rainfall. This implies that historical changes in irrigation practices are likely to have had an important influence on mesoscale processes within the Indian monsoon. We also examine evidence linking soil moisture and cloud formation. Above wetter soils we observed a suppression of shallow cloud, whilst the initiation of deep convection occurred mostly in areas affected by wet-dry soil moisture boundaries. To investigate the impact of soil moisture heterogeneity on large-scale wind flow, three model depictions of the day are assessed: The European Centre for Medium-range Weather Forecasts ERA-Interim and ERA5 reanalyses and a high-resolution (1.5 km) simulation generated using the Indian National Centre for Medium Range Weather Forecasting regional convection-permitting Unified Model. We find evidence indicating surface flux uncertainties in the models lead to ~3.5 hPa anomalies in the monsoon trough. This does affect the simulation of monsoon circulation and rainfall. Better representation of mesoscale land-atmosphere coupling is likely to improve the depiction of convection within weather and climate models over India.


46. The dynamic and thermodynamic structure of the monsoon over southern India: New observations from the INCOMPASS IOP. J. Fletcher, D. J. Parker, A. G. Turner, A. Menon, G. M. Martin, C. E. Birch, A. K. Mitra, G. Mrudula, K. M. R. Hunt, C. M. Taylor, R. A. Houze, S. R. Brodzik and G. S. Bhat (2020). Quarterly Journal of the Royal Meteorological Society, 146(731): 2867-2890, accepted 14 November 2018, first published online 21 December 2018. Authors' preprint |
    Abstract
    Some of the highest summer monsoon rainfall in South Asia falls on the windward slopes of the Western Ghats mountains on India's west coast and offshore over the eastern Arabian Sea. Understanding of the processes determining the spatial distribution and temporal variability of this region remains incomplete. In this paper, new Interaction of Convective Organization and Monsoon Precipitation, Atmosphere, Surface and Sea (INCOMPASS) aircraft and ground-based measurements of the summer monsoon over the Western Ghats and upstream of them are presented and placed within the context of remote sensing observations and reanalysis. The transition from widespread rainfall over the eastern Arabian Sea to rainfall over the Western Ghats is documented in high spatial and temporal resolution. Heavy rainfall offshore during the campaign was associated primarily with mid-tropospheric humidity, secondarily with sea surface temperature, and only weakly with orographic blocking. A mid-tropospheric dry intrusion suppressed deep convection offshore in the latter half of the campaign, allowing the build-up of low-level humidity in the onshore flow and enhancing rainfall over the mountains. Rainfall on the lee side of the Western Ghats occurred during the latter half of the campaign in association with enhanced mesoscale easterly upslope flow. Diurnal cycles in rainfall offshore (maximum in the morning) and on the mountains (maximum in the afternoon) were observed. Considerable zonal and temporal variability was seen in the offshore boundary layer, suggesting the presence of convective downdrafts and cold pools. Persistent drying of the subcloud mixed layer several hundred kilometres off the coast was observed, suggesting strong mixing between the boundary layer and the free troposphere. These observations provide quantitative targets to test models and suggest hypotheses on the physical mechanisms determining the distribution and variability in rainfall in the Western Ghats region.


47. Interaction of Convective Organisation with Monsoon Precipitation, Atmosphere, Surface and Sea: the 2016 INCOMPASS field campaign in India. A. G. Turner, G. S. Bhat, G. M. Martin, D. J. Parker, C. M. Taylor, A. K. Mitra, S. N. Tripathi, S. Milton, E. N. Rajagopal, J. G. Evans, R. Morrison, S. Pattnaik, M. Sekhar, B. K. Bhattacharya, R. Madan, Mrudula Govindankutty, J. K. Fletcher, P. D. Willetts, A. Menon, J. H. Marsham and the INCOMPASS team [K. M. R. Hunt, T. Chakraborty, G. George, M. Krishnan, C. Sarangi, D.Belušić, L. Garcia-Carreras, M. Brooks, S.Webster, J. K. Brooke, C. Fox, R. C. Harlow, J. M. Langridge, A. Jayakumar, S. J. Böing, O. Halliday, J. Bowles, J. Kent, D.O'Sullivan, A. Wilson, C. Woods, S. Rogers, R. Smout-Day, D. Tiddeman, D. Desai, R. Nigam, S. Paleri, A. Sattar, M. Smith, D. Anderson, S. Bauguitte, R. Carling, C. Chan, S. Devereau, G. Gratton, D. MacLeod, G. Nott, M. Pickering, H. Price, S. Rastall, C. Reed, J. Trembath, A.Woolley, A. Volonté and B. New] (2020). Quarterly Journal of the Royal Meteorological Society, 146(731): 2828-2852, submitted 14 November 2018, accepted 5 August 2019, first published 12 August 2019. |
    Abstract
    The INCOMPASS field campaign combines airborne and ground measurements of the 2016 Indian monsoon, towards the ultimate goal of better predicting monsoon rainfall. The monsoon supplies the majority of water in South Asia, but forecasting from days to the season ahead is limited by large, rapidly developing errors in model parametrizations. The lack of detailed observations prevents thorough understanding of the monsoon circulation and its interaction with the land surface: a process governed by boundary-layer and convective-cloud dynamics. INCOMPASS used the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft for the first project of this scale in India, to accrue almost 100 hours of observations in June and July 2016. Flights from Lucknow in the northern plains sampled the dramatic contrast in surface and boundary layer structures between dry desert air in the west and the humid environment over the northern Bay of Bengal. These flights were repeated in pre-monsoon and monsoon conditions. Flights from a second base at Bengaluru in southern India measured atmospheric contrasts from the Arabian Sea, over the Western Ghats mountains, to the rain shadow of southeast India and the south Bay of Bengal. Flight planning was aided by forecasts from bespoke 4 km convection-permitting limited-area models at the Met Office and India's NCMRWF. On the ground, INCOMPASS installed eddy-covariance flux towers on a range of surface types, to provide detailed measurements of surface fluxes and their modulation by diurnal and seasonal cycles. These data will be used to better quantify the impacts of the atmosphere on the land surface, and vice versa. INCOMPASS also installed ground instrumentation supersites at Kanpur and Bhubaneswar. Here we motivate and describe the INCOMPASS field campaign. We use examples from two flights to illustrate contrasts in atmospheric structure, in particular the retreating mid-level dry intrusion during the monsoon onset.


48. Assessing the performance of cloud microphysical parameterization over the Indian region: Simulation of monsoon depressions and validation with INCOMPASS observations V. Hazra, S. Pattnaik, A. Sisodiya, H. Baisya, A. G. Turner and G. S.Bhat (2020). Atmospheric Research, 239(July, 104925): 1-13, published online 21 February 2020. | Discussion of this paper in articles by Sahana Ghosh in Mongabay, A journey through the clouds to improve Indian monsoon forecast and Quartz India An Indian scientist flew along with the clouds to understand the monsoon on the INCOMPASS field campaign and the importance of cloud microphysics |
    Abstract
    This study validates the performance of four different cloud microphysics parameterization (CMP) with the INCOMPASS aircraft observations during monsoon 2016 and assesses its impact on simulations of two monsoondepressions (MDs) using the Weather Research and Forecasting (WRF) model. The simulations are carried outwith a lead time up to 96 h. It is found that the Aerosol Aware Thompson (AAT) scheme showed better result interms of wind and the WRF Double Moment Six Class microphysical scheme (WDM6) showed better correlations for temperature and dew point temperature compared to aircraft measurements. It is noted that the choice of CMP significantly impacts the key characteristics of the MDs such as rainfall, wind, temperature, hydrometeorsand associated convective processes (e.g. moist static energy, moisture convergence). In general, CMPs have overestimated the rainfall compared to satellite estimates Tropical Rainfall Measuring Mission (TRMM), with WDM6 producing the least errors. Therefore, inter-comparisons of simulations of CMPs are carried out using WDM6 as the benchmark. Inter-comparison results suggest that there is a substantial reduction in rainfall for theMorrison due to drier lower and middle troposphere leading to subdued convective activity compared to others. Further, WDM6 has produced the least errors in the distribution of frozen hydrometer compared to ERA5. By examining the water budget, it is found that moisture convergence is the major driver for the rainfall, and themagnitude of moisture convergence is strongly affected by the choice of CMPs. Additionally, the local and advection terms of the moisture budget equation provide minimal contributions towards rainfall generation.


49. The Recent Decline and Recovery of Indian Summer Monsoon Rainfall: Relative Roles of External Forcing and Internal Variability. X. Huang, T. Zhou, A. G. Turner, A. Dai, X. Chen, R. Clark, J. Jiang, W. Man, J. Murphy, J. Rostron, B. Wu, L. Zhang, W. Zhang and L. Zou (2020). Journal of Climate, 33(June): 5035-5060. Submitted 5 November 2019, revised version submitted 3 March 2020, accepted 12 March 2020, version of record online 13 May 2020. | Article in The Hindu Business Line by TV Jayan citing this paper, Nature, not man, has a greater impact on monsoon, says study |
    Abstract
    The Indian summer monsoon (ISM) rainfall affects a large population in South Asia. Observations show a decline in ISM rainfall from 1950-1999 and a recovery from 1999-2013. While the decline has been attributed to global warming, aerosol effects, deforestation, and a negative-to-positive phase transition of the Interdecadal Pacific Oscillation (IPO), the cause for the recovery remains largely unclear. Through analyses of a 57-member perturbed-parameter ensemble of model simulations, this study shows that the externally-forced rainfall trend is relatively weak and is overwhelmed by large internal variability during both 1950-1999 and 1999-2013. The IPO is identified as the internal mode that helps modulate the recent decline and recovery of the ISM rainfall. The IPO induces ISM rainfall changes through moisture convergence anomalies associated with an anomalous Walker circulation and meridional tropospheric temperature gradients and the resultant anomalous convection and zonal moisture advection. The negative-to-positive IPO phase transition from 1950-1999 reduces what would have been an externally-forced weak upward rainfall trend of 0.01 mm day^-1 decade^-1 to -0.15 mm day^-1 decade^-1 during that period, while the rainfall trend from 1999-2013 increases from the forced value of 0.42 to 0.68 mm day^-1 decade^-1 associated with a positive-to-negative IPO phase transition. Such a significant modulation of the historical ISM rainfall trends by the IPO is confirmed by another 100-member ensemble of simulations using perturbed initial conditions. Our findings highlight that the interplay between the effects of external forcing and the IPO needs be considered for climate adaptation and mitigation strategies in South Asia.


50. A tale of two futures: contrasting scenarios of future precipitation for West Africa from an ensemble of Regional Climate Models. A. Dosio, A. G. Turner, A. Tamoffo, M. Sylla, C. Lennard, R. G. Jones, L. Terray, G. Nikulin and B. Hewitson (2020) Environmental Research Letters, 15(6, June): 064007, published 19 May 2020. |
    Abstract
    The results of a large ensemble of regional climate models lead to two contrasting but plausible scenarios for the precipitation characteristics over West Africa; one where mean precipitation is projected to decrease significantly over the Gulf of Guinea in spring and the Sahel in summer, and the other one where summer precipitation over both regions is projected to increase. Dry and wet models show similar patterns of the dynamic and thermodynamic terms of the moisture budget, although their magnitudes are larger in the dry models. Largest discrepancies are found in the strength of the land-atmosphere coupling, with dry models showing a marked decrease in soil moisture and evapotranspiration. Some changes in precipitation characteristics are consistent for both sets of models. In particular, precipitation frequency is projected to decrease in spring over the Gulf of Guinea and in summer over the Sahel, but precipitation is projected to become more intense.


51. Black carbon physical and optical properties across northern India during pre-monsoon and monsoon seasons. J. Brooks, D. Liu, J. D. Allen, P. I. Willians, J. Haywood, E. J. Highwood, S. K. Kompalli, S. Suresh Babu, S. K. Satheesh, A. G. Turner and Hugh Coe (2019). Atmospheric Chemistry and Physics, 19: 13079-13096, published 23 October 2019. | ACPD discussion version |
    Abstract
    Black carbon (BC) is known to have major impacts on both climate and human health and is therefore of global importance, particularly in regions close to large populations that have strong sources. The size-resolved mixing state of BC=contining particles was characterised using a single-particle soot photometer (SP2). The study focuses on the Idno-Gangetic Plain (IGP) during the pre-monsoon and monsoon seasons. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft during the pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons of 2016. Over the IGP, BC mass concentrations were greater (1.95 μm^-3) compared to north-west India (1.5 μm^-3) and north-east India (0.70 μm^-3) during the pre-monsoon season. Across northern India, two distinct BC modes were recorded; a mode of small BC particles (core diameter <0.16 μm and coating thickness <50 nm) and a mode of moderately coated BC (core diameter <0.22 μm and coating thickness of 50-200 nm). The IGP and north-east India locations exhibited moderately coated black carbon particle with enhanced coating thicknesses, core sizes, mass absorption cross sections, and scattering enhancement values compared to much lower values present in the north-west. The coating thickness and mass absorption cross section increased with altitude (13%) compared to those in the boundary layer. As the monsoon arrived across the region, mass concentration of BC decreased over the central IGP and north-east locations (38% and 28% respectively), whereas for the north-west location BC properties remained relatively consistent. Post-monsoon onset, the coating thickness, core size, mass absorption cross section, and scattering enhancement values were all greatest over the central IGP much like the pre-monsoon season but were considerably reduced over both north-east and north-west India. Increases in mass absorption cross section through the atmospheric column were still present during the monsoon for the north-west and central IGP locations, but less so over the northe-east due to lack of long-range transport aerosol aloft. Across the Indo-Gangetic Plain and north-east India during the pre-monsoon and monsoon seasons, solid-fuel (wood burning) emissions form the greatest proportion of BC with moderately coated particles. However, as the monsoon develops in the north-east there was a switch to small uncoated BC particles indicative of traffic emissions, but the solid-fuel emissions remained in the IGP into the monsoon. For both seasons in the north-west, traffic emissions form the greatest proportion of BC particles. Our findings will prove important for greater understanding of the BC physical and optical properties, with important consequences for the atmospheric radiative forcing of BC-containing particles. The findings will also help constrain the regional aerosol models for a variety of applications such as space-based remote sensing, chemistry transport modelling, air quality, and BC source and emissions inventories.


52. The impact of air-sea coupling and ocean biases on the seasonal cycle of southern West African precipitation. C. M. Wainwright, L. C. Hirons, N. P. Klingaman, R. P. Allan, E. Black and A. G. Turner (2019). Climate Dynamics, 53: 7027-7044, published online 16 September 2019. |
    Abstract
    The biannual seasonal rainfall regime over the southern part of West Africa is characterised by two wet seasons, separated by the 'Little Dry Season' in July-August. Lower rainfall totals during this intervening dry season may be detrimental for crop yields over a region with a dense population that depends on agricultural output. Coupled Model Intercomparison Project Phase 5 (CMIP5) models do not correctly capture this seasonal regime, and instead generate a single wet season, peaking at the observed timing of the Little Dry Season. Hence, the realism of future climate projections over this region is questionable. Here, the representation of the Little Dry Season in coupled model simulations is investigated, to elucidate factors leading to this misrepresentation. The Global Ocean Mixed Layer configuration of the Met Office Unified Model is particularly useful for exploring this misrepresentation, as it enables separating the effects of coupled model ocean biases in different ocean basins while maintaining air-sea coupling. Atlantic Ocean SST biases cause the incorrect seasonal regime over southern West Africa. Upper level descent in August reduces ascent along the coastline, which is associated with the observed reduction in rainfall during the Little Dry Season. When coupled model Atlantic Ocean biases are introduced, ascent over the coastline is deeper and rainfall totals are higher during July-August. Hence, this study indicates detrimental impacts introduced by Atlantic Ocean biases, and highlights an area of model development required for production of meaningful climate change projections over the West Africa region.


53. Simulation of location-specific severe thunderstorm events using high-resolution land data assimilation. A. Sisodiya, S. Pattnaik, H. Baisya, G. S. Bhat and A. G. Turner (2019). Dynamics of Atmospheres and Oceans, 87: September 2019, article number 10198, published online 28 June 2019. |
    Abstract
    In this study, the impact of different initial land conditions on the simulation of thunderstorms and monsoon depressions is investigated using the Weather Research and Forecasting (WRF) model. A control run (CNTL) and a simulation with an improved land state (soil moisture and temperature) using the High Resolution Land Data Assimilation System (HRLDAS) are compared for three different rainfall cases in order to examine the robustness of the assimilation system. The study comprises two thunderstorm cases (one in the pre-monsoon and one during the monsoon) and one monsoon depression case that occurred during the Interaction of Convective Organisation, Atmosphere, Surface and Sea (INCOMPASS) field campaign of the 2016 Indian monsoon. HRLDAS is shown to yield improvements in the representation of location-specific rainfall, particularly over land. Further, it is found that the surface fluxes as well as the convective indices are better captured for the pre-monsoon thunderstorm case in HRLDAS. By analysing components of the vorticity tendency equation, it is found that the vertical advection term is the major contributor towards the positive vorticity tendency in HRLDAS compared to CNTL, hence improving localised convection and consequently facilitating rainfall. Significant improvements in the simulation of the pre-monsoon thunderstorm are noted, as seen using Automatic Weather Station (AWS) validation, whereas improvements in the monsoon depression is minimal. Further, it is found that vertical advection (moisture flux convergence) is the major driver modulating the convective circulation in localised thunderstorm (monsoon depression) cases and these dynamics are better represented by HRLDAS compared to CNTL. These findings underline the importance of accurate and high resolution land-state conditions in model initial conditions for forecasting severe weather systems, particularly the simulation of localised thunderstorms over India.


54. Falling trend of western disturbances in future climate simulations. K. M. R. Hunt, A. G. Turner and L. C. Shaffrey (2019). Journal of Climate, 32: 5037-5051, published online 24 May 2019. Authors' preprint | CEDA catalogue records RCP4.5 RCP8.5 / track data RCP4.5 RCP8.5 |
    Abstract
    Western disturbances (WDs) are synoptic-scale cyclonic weather systems advected over Pakistan and north India by the subtropical westerly jet stream. There, they are responsible for most of the winter precipitation, crucial for agriculture of the rabi crop, as well as more extreme precipitation events, which can lead to local flooding and avalanches. Despite their importance, there has not yet been an attempt to objectively determine the fate of WDs in future climate GCMs. Here, a tracking algorithm is used to build up a catalogue of WDs in both CMIP5 historical and representative concentration pathway (RCP) experiments of the future. It is shown that in business-as-usual (RCP8.5) future climate simulations, WD frequency falls by around 10% by the end of the twenty-first century, with the largest relative changes coming in pre- and post-monsoon months. Meanwhile, mean WD intensity will decrease, with vorticity expected to become less cyclonic by about 8% over the same period. Changes in WD frequency can be attributed to the projected widening and poleward shift of the winter subtropical jet, whereas the changes in intensity are explained by decreasing meridional wind shear and mid-tropospheric baroclinic vorticity tendency. Finally, the impact of these changes on regional precipitation is explored. The decline in WD frequency and intensity will cause a decrease in mean winter rainfall over Pakistan and north India amounting to about 15% of the seasonal mean. The effect on extreme precipitation events, however, remains unclear.


55. The role of the subtropical jet in deficient winter precipitation across the mid-Holocene Indus basin. K. M. R. Hunt and A. G. Turner (2019). Geophysical Research Letters, 46(10): 5452-5459, published online 20 May 2019. Authors' preprint | CEDA catalogue record / track data |
    Abstract
    The mid-Holocene (7-5 ka) was a period with an increased seasonal insolation cycle, resulting from decreased insolation during northern hemisphere winter. Here, a set of six CMIP5 models is used to show that the decreased insolation reduced the upper-tropospheric meridional temperature gradient, producing a weaker subtropical jet with less horizontal shear. These effects work to reduce the baroclinic and barotropic instability available for perturbations to grow, and in consequence, storm-tracking results show that there are fewer winter storms over India and Pakistan (known as western disturbances). These western disturbances are weaker, resulting in a reduction in winter precipitation of around 15% in the north Indus Basin. Combined with previous work showing greater northwestward extent of the Indian monsoon during the mid-Holocene, our GCM-derived results are consistent with the Indus Basin changing from a summer-growing season in the mid-Holocene to a winter-growing season in the present day.


56. Indian summer monsoon onset forecast skill in the UK Met Office initialized coupled seasonal forecasting system (GloSea5-GC2). A. Chevuturi, A. G. Turner, S. J. Woolnough, G. M. Martin and C. MacLachlan (2019). Climate Dynamics, 52(11): 6599-6617, published online 24 November 2018. Authors' preprint |
    Abstract
    Accurate and precise forecasting of the Indian monsoon is important for the socio-economic security of India, with improvements in agriculture and associated sectors from prediction of the monsoon onset. In this study we establish the skill of the UK Met Office coupled initialized global seasonal forecasting system, GloSea5-GC2, in forecasting Indian monsoon onset. We build on previous work that has demonstrated the good skill of GloSea5 at forecasting interannual variations of the seasonal mean Indian monsoon using measures of large-scale circulation and local precipitation. We analyze the summer hindcasts from a set of three springtime start-dates in late April/early May for the 20-year hindcast period (1992-2011). The hindcast set features at least fifteen ensemble members for each year and is analyzed using five different objective monsoon indices. These indices are designed to examine large and local-scale measures of the monsoon circulation, hydrological changes, tropospheric temperature gradient, or rainfall for single value (area-averaged) or grid-point measures of the Indian monsoon onset. There is significant correlation between onset dates in the model and those found in reanalysis. Indices based on large-scale dynamic and thermodynamic indices are better at estimating monsoon onset in the model rather than local-scale dynamical and hydrological indices. This can be attributed to the model's better representation of large-scale dynamics compared to local-scale features. GloSea5 may not be able to predict the exact date of monsoon onset over India, but this study shows that the model has a good ability at predicting category-wise monsoon onset, using early, normal or late tercile categories. Using a grid-point local rainfall onset index, we note that the forecast skill is highest over parts of central India, the Gangetic plains, and parts of coastal India - all zones of extensive agriculture in India. El Niño Southern Oscillation (ENSO) forcing in the model improves the forecast skill of monsoon onset when using a large-scale circulation index, with late monsoon onset coinciding with El Niño conditions and early monsoon onset more common in La Niñ years. The results of this study suggest that GloSea5's ensemble-mean forecast may be used for reliable Indian monsoon onset prediction a month in advance despite systematic model errors.


57. Vertical and horizontal distribution of sub-micron aerosol chemical composition and physical characteristics across Northern India, during the pre-monsoon and monsoon seasons. J. Brooks, J. Allan, P. Williams, D. Liu, C. Fox, J. Haywood, J. Langridge, E. Highwood, S. K. Kompalli, D. O'Sullivan, S. Babu, S. Satheesh, A. G. Turner and H. Coe (2019). Atmospheric Chemistry and Physics, 19: 5615-5634, Special Issue on Interactions between aerosols and the South West Asian monsoon, published online 30 April 2019. | ACPD discussion version |
    Abstract
    The vertical distribution in the physical and chemical properties of submicron aerosol has been characterised across northern India for the first time using airborne in-situ measurements. This study focusses primarily on the Indo-Gangetic Plain, a low-lying area in the north of India which commonly experiences high aerosol mass concentrations prior to the monsoon season. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights in the region during the 2016 pre-monsoon (11th and 12th June) and monsoon (30th June to 11th July) seasons. Inside the Indo-Gangetic Plain boundary layer, organic matter dominated the submicron aerosol mass (43%) followed by sulphate (29%), ammonium (14%), nitrate (7%) and black carbon (7%). However, outside the Indo-Gangetic Plain, sulphate was the dominant species contributing 44% to the total submicron aerosol mass in the boundary layer, followed by organic matter (30%), ammonium (14%), nitrate (6%) and black carbon (6%). Chlorine mass concentrations were negligible throughout the campaign. Black carbon mass concentrations were higher inside the Indo-Gangetic Plain (2µg/m³ std) compared to outside (1µg/m³ std). Nitrate appeared to be controlled by thermodynamic processes, with increased mass concentration in conditions of lower temperature and higher relative humidity. Increased mass and number concentrations were observed inside the Indo-Gangetic Plain and the aerosol was more absorbing in this region, whereas outside the Indo-Gangetic Plain the aerosol was larger in size and more scattering in nature, suggesting greater dust presence especially in northwest India. The aerosol composition remained largely similar as the monsoon season progressed, but the total aerosol mass concentrations decreased by ~50% as the rainfall arrived; the pre-monsoon average total mass concentration was 30µg/m³ std compared to a monsoon average total mass concentration of 10-20µg/m³ std. However, this mass concentration decrease was less noteworthy (~20-30%) over the Indo-Gangetic Plain, likely due to the strength of emission sources in this region. Decreases occurred in coarse mode aerosol, with the fine mode fraction increasing with monsoon arrival. In the aerosol vertical profile, inside the Indo-Gangetic Plain during the pre-monsoon, organic aerosol and absorbing aerosol species dominated in the lower atmosphere (<1.5km) with sulphate, dust and other scattering aerosol species enhanced in an elevated aerosol layer above 1.5km with maximum aerosol height ~6km. As the monsoon progressed into this region, the elevated aerosol layer diminished, the aerosol maximum height reduced to ~2km and the total mass concentrations decreased by ~50%. The dust and sulphate-dominated aerosol layer aloft was removed upon monsoon arrival, highlighted by an increase in fine mode fraction throughout the profile.


58. Moisture sources for East Asian precipitation: mean seasonal cycle and interannual variability. L. Guo, R. J. van der Ent, N. P. Klingaman. M.-E. Demory, P. L. Vidale, A. G. Turner, C. C. Stephan and A. Chevuturi (2019). Journal of Hydrometeorology, 20(4): 657-672. Authors' preprint |
    Abstract
    Moisture sources that supply East Asian (EA) precipitation and their interannual variability, are investigated in this study. Moisture sources are tracked using the Water Accounting Model-2layers (WAM-2layers), based on the Eulerian framework. The WAM-2layers is applied to five subregions over EA, driven by the ERA-Interim reanalysis from 1979 to 2015. Due to differences in hydrological and topographic features, the mean moisture sources vary among EA subregions. The tropical oceanic source dominates southeastern EA, while the extratropical continental source dominates other EA subregions. The moisture sources experience large seasonal variations, due to the seasonal cycle of the EA monsoon, the freeze-thaw cycle of the Eurasian continent and local moisture recycling over the Tibetan Plateau. The interannual variability of moisture sources is linked to interannual modes of the coupled ocean-atmosphere system. The negative phase of the North Atlantic Oscillation increases moisture transport to northwestern EA in winter by driving a southern shift in the mid-latitude westerly jet over the Mediterranean Sea, the Black Sea and the Caspian Sea. Atmospheric moisture lifetime is also reduced due to the enhanced westerlies. In summers following El Niños, an anti-cyclonic anomaly over the western North Pacific increases moisture supplied from the South China Sea to the southeastern EA and shortens the travelling distance. A stronger Somali Jet in summer increases moisture to the Tibetan Plateau and therefore increases precipitation over the eastern Tibetan Plateau. The WAM-2layers and findings in this study can be used to evaluate hydrological features in climate simulations.


59. Representation of western disturbances in CMIP5 models. K. M. R. Hunt, A. G. Turner and L. C. Shaffrey (2019). Journal of Climate, 32(7): 1997-2011. Authors' preprint | CEDA catalogue record / CMIP5 historical track data |
    Abstract
    Western disturbances (WDs) are synoptic extratropical disturbances embedded in the subtropical westerly jet stream. They are an integral part of the South Asian winter climate, both for the agriculture-supporting precipitation they bring to the region and for the associated isolated extreme events that can induce devastating flash flooding. Here, WD behaviour and impacts are characterised in 23 CMIP5 historical simulations and compared with reanalysis and observations. It is found that WD frequency has a strong relationship with model resolution: higher resolution models produce significantly more WDs, and a disproportionately high fraction of extreme events. Exploring metrics of jet strength and shape, we find that the most probable cause of this relationship is that the jet is wider in models with coarser resolution, and therefore the northern edge in which WDs are spun up sits too far north of India. The frequency of WDs in both winter and summer is found to be overestimated by most models, and thus the winter frequency of WDs estimated from the multi-model mean (30 winter^-1) is above the reanalysis mean (26 winter^-1). In this case, the error cannot be adequately explained by local jet position and strength. Instead, we show that it is linked with a positive bias in upstream mid-tropospheric baroclinicity. Despite a positive winter precipitation bias in CMIP5 models over most of India and Pakistan and a dry bias in the western Himalaya, the fraction of winter precipitation for which WDs are responsible is accurately represented. Using partial correlation, it is shown that the overestimation in WD frequency is the largest contributor to this bias, with a secondary, spatially heterogeneous contribution coming from the overestimation of WD intensity.


60. Climatology of Tibetan Plateau vortices in reanalysis data and a high-resolution global climate model. J. Curio, R. K. Schiemann, K. I. Hodges and A. G. Turner (2019). Journal of Climate, 32(6): 1933-1950. Authors' preprint | Supplementary material |
    Abstract
    The Tibetan Plateau (TP) and surrounding high mountain ranges constitute an important forcing of the atmospheric circulation over Asia due to their height and extent, and thereby impacts weather and climate in downstream regions of East Asia. Mesoscale Tibetan Plateau vortices (TPVs) are one of the major precipitation-bearing systems on the TP. A fraction of TPVs move off the TP to the east and can trigger extreme precipitation in parts of China, e.g. the Szechuan province and the Yangtze River valley. In this study, the climatology of TPV occurrence is examined in two reanalyses and, for the first time, in a high-resolution global climate model using an objective feature tracking algorithm, identifying relative vorticity maxima at the 500 hPa level. Most TPVs are generated in the north-western part of the TP; the centre of this main genesis region is small and stable throughout the year. The strength and position of the subtropical westerly jet controls the distance TPVs can travel eastwards and shapes the annual cycle of TPV occurrence frequency. TPV-associated precipitation can account for up to 40% of the total precipitation in parts of China in selected months, often due to individual TPVs. The results show that the global climate model is able to simulate TPVs at N512 (~ 25 km) horizontal resolution and in general agrees with the reanalyses. The fact that the global climate model can represent the TPV climatology opens a wide range of options for future model-based research on TPVs.


61. A mechanism for the interdecadal variability of the Silk Road Pattern. C. C. Stephan, N. P. Klingaman and A. G. Turner (2019). Journal of Climate, 32: 717-736. Authors' preprint |
    Abstract
    The Silk Road Pattern (SRP) teleconnection manifests in summer over Eurasia, where it is associated with substantial temperature and precipitation anomalies. The SRP varies on interannual and decadal scales; reanalyses show an increase in its decadal variability around the mid-1970s. Understanding what drives this decadal variability is particularly important, because contemporary seasonal prediction models struggle to predict the phase of the SRP. Based on analysis of observations and multiple targeted numerical experiments, this study proposes a mechanism for decadal SRP variability. Causal Effect Network analysis confirms a positive feedback loop between the eastern portion of the SRP pattern and vertical motion over India on synoptic timescales. Anomalies over a larger region of subtropical South Asia can reinforce a background state that projects onto the positive or negative SRP through this mechanism. This effect is isolated and confirmed in targeted numerical simulations. The transition from weak to strong decadal variability in the mid-1970s is consistent with more spatially coherent interannual precipitation variability over subtropical South Asia. Furthermore, results suggest that oceanic variability does not directly force the SRP. Nevertheless, sea surface temperatures in the North Atlantic and the North Pacific may indirectly affect the SRP by modulating South Asian rainfall on decadal timescales.


62. Impact of ENSO longitudinal position on teleconnections to the NAO. W. Zhang, Z. Wang, M. F. Stuecker, A. G. Turner, F.-F. Jin and X. Geng (2019). Climate Dynamics, 52(1-2): 257-274. Authors' preprint |
    Abstract
    While significant improvements have been made in understanding how the El Niño-Southern Oscillation (ENSO) impacts both North American and Asian climate, its relationship with the North Atlantic Oscillation (NAO) remains less clear. Observations indicate that ENSO exhibits a highly complex relationship with the NAO-associated atmospheric circulation. One critical contribution to this ambiguous ENSO/NAO relationship originates from ENSO's diversity in its spatial structure. In general, both eastern (EP) and central Pacific (CP) El Niño events tend to be accompanied by a negative NAO-like atmospheric response. However, for two different types of La Niña the NAO response is almost opposite. Thus, the NAO responses for the CP ENSO are mostly linear, while nonlinear NAO responses dominate for the EP ENSO. These contrasting extra-tropical atmospheric responses are mainly attributed to nonlinear air-sea interactions in the tropical eastern Pacific. The local atmospheric response to the CP ENSO sea surface temperature (SST) anomalies is highly linear since the air-sea action center is located within the Pacific warm pool, characterized by relatively high climatological SSTs. In contrast, the EP ENSO SST anomalies are located in an area of relatively low climatological SSTs in the eastern equatorial Pacific. Here only sufficiently high positive SST anomalies during EP El Niño events are able to overcome the SST threshold for deep convection, while hardly any anomalous convection is associated with EP La Niña SSTs that are below this threshold. This ENSO/NAO relationship has important implications for NAO seasonal prediction and places a higher requirement on models in reproducing the full diversity of ENSO.


63. A nonstationary ENSO-NAO relationship due to AMO modulation. W. Zhang, X. Wei, X. Geng, A. G. Turner and F.-F. Jin (2019). Journal of Climate, 32: 33-43. Authors' preprint |
    Abstract
    Many previous studies have demonstrated a high uncertainty in the relationship between the El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO). In the present work, decadal modulation by the Atlantic Multidecadal Oscillation (AMO) is investigated as a possible cause of the nonstationary ENSO-NAO relationship based on observed and reanalysis data. It is found that the negative ENSO-NAO correlation in late winter is significant only when ENSO and the AMO are in-phase (AMO+/El Niño and AMO-/La Niña). However, no significant ENSO-driven atmospheric anomalies can be observed over the North Atlantic when ENSO and the AMO are out-of-phase (AMO-/El Niño and AMO+/La Niña). Further analysis indicates that the sea surface temperature anomaly (SSTA) in the tropical North Atlantic (TNA) plays an essential role in this modulating effect. Due to broadly analogous TNA SSTA responses to both ENSO and the AMO during late winter, a warm SSTA in the TNA is evident when El Niño occurs during a positive AMO phase, resulting in a significantly weakened NAO, and vice versa when La Niña occur during a negative AMO phase. In contrast, neither the TNA SSTA nor the NAO show a prominent change under out-of-phase combinations of ENSO and AMO. The AMO modulation and associated effect of the TNA SSTA are shown to be well reproduced by historical simulations of the HadCM3 coupled model and further verified by forced experiments using an atmospheric circulation model. These offer hope that similar models will be able to make predictions for the NAO when appropriately initialized.


64. The contributions of local and remote atmospheric moisture fluxes to East Asian precipitation and its variability. L. Guo, N. P. Klingaman, M.-E. Demory, P.-L. Vidale, A. G. Turner and C. C. Stephan (2018). Climate Dynamics, 51(11-12): 4139-4156, first published online 13 January 2018. Authors' preprint |
    Abstract
    We investigate the contribution of the local and remote atmospheric moisture fluxes to East Asia (EA) precipitation and its interannual variability during 1979-2012. We use and expand the Brubaker et al. (J Clim 6:1077-1089, 1993) method, which connects the area-mean precipitation to area-mean evaporation and the horizontal moisture flux into the region. Due to its large landmass and hydrological heterogeneity, EA is divided into five sub-regions: Southeast (SE), Tibetan Plateau (TP), Central East (CE), Northwest (NW) and Northeast (NE). For each region, we first separate the contributions to precipitation of local evaporation from those of the horizontal moisture flux by calculating the precipitation recycling ratio: the fraction of precipitation over a region that originates as evaporation from the same region. Then, we separate the horizontal moisture flux across the region's boundaries by direction. We estimate the contributions of the horizontal moisture fluxes from each direction, as well as the local evaporation, to the mean precipitation and its interannual variability. We find that the major contributors to the mean precipitation are not necessarily those that contribute most to the precipitation interannual variability. Over SE, the moisture flux via the southern boundary dominates the mean precipitation and its interannual variability. Over TP, in winter and spring, the moisture flux via the western boundary dominates the mean precipitation; however, variations in local evaporation dominate the precipitation interannual variability. The western moisture flux is the dominant contributor to the mean precipitation over CE, NW and NE. However, the southern or northern moisture flux or the local evaporation dominates the precipitation interannual variability over these regions, depending on the season. Potential mechanisms associated with interannual variability in the moisture flux are identified for each region. The methods and results presented in this study can be readily applied to model simulations, to identify simulation biases in precipitation that relate to the simulated moisture supplies and transport.


65. Subtropical Westerly Jet Influence on Occurrence of Western Disturbances and Tibetan Plateau Vortices. K. M. R. Hunt, J. Curio, A. G. Turner and R. Schiemann (2018). Geophysical Research Letters, 45(16): 8629-8636, published online 13 August 2018. Authors' preprint |
    Abstract
    Western disturbances (WDs) are midtropospheric to upper-tropospheric mesoscale vortices, which typically propagate along the subtropical westerly jet stream and bring heavy rainfall to Pakistan and northern India during boreal winter. They are dynamically similar to Tibetan Plateau vortices (TPVs), which affect southwest China during spring and summer and emanate from the Tibetan Plateau. Here we propose that their similarity implies the existence of a more general group of upper-tropospheric vortices featuring interactions with the orography of the Hindu Kush-Himalaya-Tibetan Plateau region. Using existing track databases for WDs and TPVs derived from ERA-Interim reanalysis, we show that their respective occurrence frequencies are highly anticorrelated with each other through the seasonal cycle, yet both are strongly correlated with jet latitude. Our findings imply that the incidence of hazards due to WDs and TPVs is correlated on intra-annual and interannual time scales, particularly through upper-level baroclinicity.


66. The impact of Indian Ocean mean-state biases on the representation of the East African short rains. L. C. Hirons and A. G. Turner (2018). Journal of Climate, 31: 6611-6631. Authors' preprint |
    Abstract
    The role of the Indian Ocean Dipole (IOD) in controlling interannual variability in the East African short rains, from October to December, is examined in state-of-the-art models and in detail in one particular climate model. In observations, a wet short-rainy season is associated with the positive phase of the IOD and anomalous easterly low-level flow across the equatorial Indian Ocean. A model's ability to capture the teleconnection to the positive IOD is closely related to its representation of the mean-state. During the short-rains season, the observed low-level wind in the equatorial Indian Ocean is westerly. However, half of the models analysed exhibit mean-state easterlies across the entire basin. Specifically, those models that exhibit mean-state low-level equatorial easterlies in the Indian Ocean, rather than the observed westerlies, are unable to capture the latitudinal structure of moisture advection into East Africa during a positive IOD. Furthermore, the associated anomalous easterly surface wind stress causes upwelling in the eastern Indian Ocean. This upwelling draws up cool sub-surface waters, enhancing the zonal sea-surface temperature gradient between west and east and strengthening the positive IOD pattern, further amplifying the easterly wind stress. This positive Bjerknes coupled feedback is stronger in easterly mean-state models, resulting in a wetter East African short rain precipitation bias in those models.


67. Intraseasonal summer rainfall variability over China in the MetUM GA6 and GC2 configurations. C. C. Stephan, N. Klingaman, P.-L. Vidale, A. G. Turner, M.-E. Demory and L. Guo (2018). Geoscientific Model Development, 11: 3215-3288. GMDD discussion version |
    Abstract
    The simulation of intraseasonal precipitation variability over China in extended summer (May-October) is evaluated based on six climate simulations of the Met Office Unified Model. Two simulations use the Global Atmosphere 6.0 (GA6), and four the Global Coupled 2.0 (GC2) configuration. Model biases are large, such that mean precipitation and intraseasonal variability reach twice their observed values, particularly in southern China. To test the impact of air-sea coupling and horizontal resolution, GA6 and GC2 at horizontal resolutions corresponding to ~25, 60 and 135km at 50°N are analyzed. Increasing the horizontal resolution and adding air-sea coupling have little effect on these biases. Pre-monsoon rainfall in the Yangtze River basin is too strong in all simulations. Simulated rainfall amounts in June are too high along the southern coast and persist in the coastal region through July, with only a weak northward progression. The observed northward propagation of the Meiyu/Baiu/Changma rainband from spring to late summer is poor in all GA6 and GC2 simulations. To assess how well the MetUM simulates spatial patterns of temporally coherent precipitation, empirical orthogonal teleconnection (EOT) analysis is applied to pentad-mean precipitation. Patterns are connected to large-scale processes by regressing atmospheric fields onto the EOT pentad timeseries. Most observed patterns of intraseasonal rainfall variability are found in all simulations, including the associated observed mechanisms. This suggests that GA6 and GC2 may provide useful predictions of summer intraseasonal variability, despite their substantial biases in mean precipitation and overall intraseasonal variance.


68. Different Asian monsoon rainfall responses to idealised orography sensitivity experiments in the HadGEM3-GA6 and FGOALS-FAMIL global climate models. K.-C. Wong, S. Liu, A. G. Turner and R. K. Schiemann (2018). Advances in Atmospheric Sciences, 35(8): 1049-1062. Authors' preprint | AAS URL | YouTube slides about this work |
    Abstract
    Recent work has shown the dominance of the Himalayas in supporting the Indian summer monsoon (ISM), perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier mid-latitude air. Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau. However, given the large biases in simulating the monsoon in CMIP5 models, such results may be model dependent. This study investigates the impact of orographic forcing from the Tibetan Plateau, Himalayas and Iranian Plateau on the ISM and East Asian summer monsoons (EASM) in the UK Met Office HadGEM3-GA6 and China's Institute of Atmospheric Physics FGOALS-FAMIL GCMs. The models chosen feature opposite-signed biases in their simulation of the ISM rainfall and circulation climatology. The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies. However, considerable differences exist in the rainfall responses over India and China, and in the detailed aspects such as onset and retreat dates. In particular, the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed. Our results show that a multi-model approach, as suggested in the forthcoming Global Monsoon Model Intercomparison Project (GMMIP) associated with CMIP6, is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error.


69. Comparison of a manual and an automated tracking method for Tibetan Plateau Vortices. J. Curio, Y. Chen, R. Schiemann, A. G. Turner, K. C. Wong, K. Hodges and Y. Li (2018). Advances in Atmospheric Sciences, 35(8): 965-980. Authors' preprint | AAS URL | YouTube slides about this work |
    Abstract
    Tibetan Plateau vortices (TPVs) are mesoscale cyclones originating over the Tibetan Plateau (TP) during the extended summer season (April-September). Most TPVs stay on the TP, but a small number can move off the TP to the east. TPVs are known to be one of the main precipitation-bearing systems on the TP and moving-off TPVs have been associated with heavy precipitation and flooding downstream of the TP (e.g., in Sichuan province or over the Yangtze River Valley). Identifying and tracking TPVs is difficult because of their comparatively small horizontal extent (400-800 km) and the limited availability of soundings over the TP, which in turn constitutes a challenge for short-term predictions of TPV-related impacts and for the climatological study of TPVs. In this study, (i) manual tracking (MT) results using radiosonde data from a network over and downstream of the TP are compared with (ii) results obtained by an automated tracking (AT) algorithm applied to ERA-Interim data. Ten MT-TPV cases are selected based on method (i) and matched to and compared with the corresponding AT-TPVs identified with method (ii). Conversely, 10 AT-TPVs are selected and compared with the corresponding MT-TPVs. In general, the comparison shows good results in cases where the underlying data are in good agreement, but considerable differences are also seen in some cases and explained in terms of differences in the tracking methods, data availability/coverage and disagreement between sounding and ERA-Interim data. Recommendations are given for future efforts in TPV detection and tracking, including in an operational weather forecasting context.


70. Modelling the moistening of the free troposphere during the northwestward progression of Indian monsoon onset. A. Menon, A. G. Turner, G. M. Martin and C. MacLachlan (2018). Quarterly Journal of the Royal Meteorological Society, 144(713): 1152-1168. Authors' preprint |
    Abstract
    Understanding and prediction of the Indian monsoon onset is of paramount importance for agricultural planning that affects food production and the gross domestic product of the country. A recent observational study suggests that the progression of the Indian monsoon onset in a northwestward direction, which is perpendicular to the mean flow, is reinforced by the moistening of the free troposphere by pre-monsoon showers and wetting of the land surface. As the onset progresses, the mid-tropospheric dry layer is thought to be constantly moistened from below by detrainment from shallow cumulus and congestus clouds from the southeast. The dry layer becomes much shallower towards southeast India, making the profile closer to moist adiabatic, providing favourable conditions for deep cumulus convection. Increased moistening of the free troposphere thereby pushes the northern limit of moist convection to move northwestwards. Here we examine the representation of this process in hindcast simulations from the fully coupled atmosphere-ocean seasonal forecast system of the UK Met Office, GloSea5. The model effectively captures the mid-level dry air intrusion from the northwest which suppresses convection over the northwestern parts of India. We also show that detrainment from shallow convection, measured by moisture tendencies around the freezing level, acts to saturate the free troposphere ahead of the monsoon onset, eroding the dry-layer from the southeast. This work suggests that initialized coupled models are capable of simulating dynamic and thermodynamic processes inherent in monsoon progression during the onset.


71. A comprehensive analysis of coherent rainfall patterns in China and potential drivers. Part II: Intraseasonal variability. C. C. Stephan, N. P. Klingaman, P. L. Vidale, A. G. Turner, M.-E. Demory and L. Guo (2018). Climate Dynamics, 51(1-2): 17-33. Authors' preprint |
    Abstract
    The causes of subseasonal precipitation variability in China are investigated using observations and reanalysis data for extended winter (November-April) and summer (May-October) seasons from 1982 to 2007. For each season, the three dominant regions of coherent intraseasonal variability are identified with Empirical Orthogonal Teleconnection (EOT) analysis. While previous studies have focused on particular causes for precipitation variability or on specific regions, here a comprehensive analysis is carried out with an objective method. Furthermore, the associated rainfall anomaly timeseries are tied to specific locations in China, which facilitates their interpretation. To understand the underlying processes associated with spatially coherent patterns of rainfall variability, fields from observations and reanalysis are regressed onto EOT timeseries. The three dominant patterns in winter together explain 43% of the total space-time variance and have their origins in midlatitude disturbances that appear two pentads in advance. Winter precipitation variability along the Yangtze River is associated with wave trains originating over the Atlantic and northern Europe, while precipitation variability in southeast China is connected to the Mediterranean storm track. In summer, all patterns have a strong relationship with the Boreal Summer Intraseasonal Oscillation and are modulated by the seasonal cycle of the East Asian summer monsoon. The wet and dry phases of the regional patterns can substantially modulate the frequency of daily rainfall across China. The discovered links between weather patterns, precursors, and effects on local and remote precipitation may provide a valuable basis for hydrological risk assessments and the evaluation of numerical weather prediction models.


72. A comprehensive analysis of coherent rainfall patterns in China and potential drivers. Part I: Interannual variability. C. C. Stephan, N. P. Klingaman, P. L. Vidale, A. G. Turner, M.-E. Demory and L. Guo (2018). Climate Dynamics, 50(11-12): 4405-4424. Authors' preprint |
    Abstract
    Interannual rainfall variability in China affects agriculture, infrastructure and water resource management. To improve its understanding and prediction, many studies have associated precipitation variability with particular causes for specific seasons and regions. Here, a consistent and objective method, Empirical Orthogonal Teleconnection (EOT) analysis, is applied to 1951-2007 high-resolution precipitation observations over China in all seasons. Instead of maximizing the explained space-time variance, the method identifies regions in China that best explain the temporal variability in domain-averaged rainfall. The EOT method is validated by the reproduction of known relationships to the El Niño Southern Oscillation (ENSO): high positive correlations with ENSO are found in eastern China in winter, along the Yangtze River in summer, and in southeast China during spring. New findings include that wintertime rainfall variability along the southeast coast is associated with anomalous convection over the tropical eastern Atlantic and communicated to China through a zonal wavenumber-three Rossby wave. Furthermore, spring rainfall variability in the Yangtze valley is related to upper-tropospheric midlatitude perturbations that are part of a Rossby wave pattern with its origin in the North Atlantic. A circumglobal wave pattern in the northern hemisphere is also associated with autumn precipitation variability in eastern areas. The analysis is objective, comprehensive, and produces timeseries that are tied to specific locations in China. This facilitates the interpretation of associated dynamical processes, is useful for understanding the regional hydrological cycle, and allows the results to serve as a benchmark for assessing general circulation models.


73. The role of potential vorticity anomalies in the Somali Jet on Indian Summer Monsoon Intraseasonal Variability. P. Rai, M. Joshi, A. P. Dimri and A. G. Turner (2018). Climate Dynamics, 50(11-12): 4149-4169. Authors' preprint |
    Abstract
    The climate of the Indian subcontinent is dominated by rainfall arising from the Indian summer monsoon (ISM) during June to September. Intraseasonal variability during the monsoon is characterized by periods of heavy rainfall interspersed by drier periods, known as active and break events respectively. Understanding and predicting such events is of vital importance for forecasting human impacts such as water resources. The Somali Jet is a key regional feature of the monsoon circulation. In the present study, we find that the spatial structure of Somali Jet potential vorticity (PV) anomalies varies considerably during active and break periods. Analysis of these anomalies shows a mechanism whereby sea surface temperature (SST) anomalies propagate north/northwestwards through the Arabian Sea, caused by a positive feedback loop joining anomalies in SST, convection, modification of PV by diabatic heating and mixing in the atmospheric boundary layer, wind-stress curl, and ocean upwelling processes. The feedback mechanism is consistent with observed variability in the coupled ocean-atmosphere system on timescales of approximately 20 days. This research suggests that better understanding and prediction of monsoon intraseasonal variability in the South Asian monsoon may be gained by analysis of the day-to-day dynamical evolution of PV in the Somali Jet.


74. Interannual rainfall variability over China in the MetUM GA6 and GC2 configurations. C. C. Stephan, N. P. Klingaman, P.-L. Vidale, A. G. Turner, M.-E. Demory and L. Guo (2018). Geoscientific Model Development, 11: 1823-1847. Publisher version | GMDD discussion version |
    Abstract
    Six climate simulations of the Met Office Unified Model Global Atmosphere 6.0 and Global Coupled 2.0 configurations are evaluated against observations and reanalysis data for their ability to simulate the mean state and year-to-year variability of precipitation over China. To analyze the sensitivity to air-sea coupling and horizontal resolution, atmosphere-only and coupled integrations at atmospheric horizontal resolutions of N96, N216 and N512 (corresponding to ~200, 90, and 40 km in the zonal direction at the equator, respectively) are analyzed. The mean and interannual variance of seasonal precipitation are too high in all simulations over China, but improve with finer resolution and coupling. Empirical Orthogonal Teleconnection (EOT) analysis is applied to simulated and observed precipitation to identify spatial patterns of temporally coherent interannual variability in seasonal precipitation. To connect these patterns to large-scale atmospheric and coupled air-sea processes, atmospheric and oceanic fields are regressed onto the corresponding seasonal-mean timeseries. All simulations reproduce the observed leading pattern of interannual rainfall variability in winter, spring and autumn; the leading pattern in summer is present in all but one simulation. However, only in two simulations are the four leading patterns associated with the observed physical mechanisms. Coupled simulations capture more observed patterns of variability and associate more of them with the correct physical mechanism, compared to atmosphere-only simulations at the same resolution. However, finer resolution does not improve the fidelity of these patterns or their associated mechanisms. This shows that evaluating climate models by only geographical distribution of mean precipitation and its interannual variance is insufficient. The EOT analysis adds knowledge about coherent variability and associated mechanisms.


75. Extreme daily rainfall in Pakistan and north India: scale-interactions, mechanisms, and precursors. K. M. R. Hunt, A. G. Turner and L. C. Shaffrey (2018). Monthly Weather Review, 146: 1005-1022, April 2018. Authors' preprint |
    Abstract
    While much of India is used to heavy precipitation and frequent low-pressure systems during the summer monsoon, towards the northwest and into Pakistan, such events are uncommon. Here, as much as a third of the annual rainfall is delivered sporadically during the winter monsoon by western disturbances. Such events of sparse but heavy precipitation in this region of typically mountainous valleys in the north and desert in the south can be catastrophic, as in the case of the Pakistan floods of July 2010. In this study, we identify extreme precipitation events (EPEs) in a box approximately covering this region (65-78°E, 25-38°N) using the APHRODITE gauge-based precipitation product. The role of the large-scale circulation in causing EPEs is investigated: it is found that, during winter, they often coexist with an upper-tropospheric Rossby wave train that has prominent anomalous southerlies over the region of interest. These winter EPEs are also found to be strongly colocated with incident western disturbances whereas those occurring during the summer are found to have a less direct relationship. Conversely, summer EPEs are found to have a strong relationship with tropical lows. A detailed Lagrangian method is used to explore possible sources of moisture for such events, and suggests that in winter, the moisture is mostly drawn from the Arabian Sea, whereas during the summer, it comes from along the African coast and the Indian monsoon trough region.


76. Projected changes in the Asian-Australian monsoon region in 1.5°C and 2.0°C global-warming scenarios. A. Chevuturi, N. P. Klingaman, A. G. Turner and S. Hannah (2018). Earth's Future, 6(3): 339-358, March 2018. Authors' preprint |
    Abstract
    In light of the Paris Agreement, it is essential to identify regional impacts of half a degree additional global warming to inform climate adaptation and mitigation strategies. We investigate the effects of 1.5°C and 2.0°C global warming above pre-industrial conditions, relative to present day (2006-2015), over the Asian-Australian monsoon region (AAMR) using five models from the Half a degree Additional warming, Prognosis and Projected Impacts (HAPPI) project. There is considerable inter-model variability in projected changes to mean climate and extreme events in 2.0°C and 1.5°C scenarios. There is high confidence in projected increases to mean and extreme surface temperatures over AAMR, as well as more-frequent persistent daily temperature extremes over East Asia, Australia and northern India with an additional 0.5°C warming, which are likely to occur. Mean and extreme monsoon precipitation amplify over AAMR, except over Australia at 1.5°C where there is uncertainty in the sign of the change. Persistent daily extreme precipitation events are likely to become more frequent over parts of East Asia and India with an additional 0.5°C warming. There is lower confidence in projections of precipitation change than in projections of surface temperature change. These results highlight the benefits of limiting the global-mean temperature change to 1.5°C above pre-industrial, as the severity of the above effects increases with an extra 0.5°C warming.


77. Vertical Structure and Radiative Forcing of Monsoon Clouds over Kanpur during the INCOMPASS Campaign. G. George, C. Sarangi, S. N. Tripathi, T. Chakraborty and A. G. Turner (2018). Journal of Geophysical Research - Atmospheres, 123 (4): 2152-2174. Authors' preprint | CEDA catalogue record / radiosonde data |
    Abstract
    An overview of cloud vertical structure (CVS) and cloud radiative forcing (CRF) during Indian summer monsoon is obtained over Kanpur, through observations made during the Interaction of Convective Organisation and Monsoon Precipitation, Atmosphere, Surface and Sea (INCOMPASS) field campaign of 2016. Associations of CVS parameters with CRF at surface and top of atmosphere (TOA) are also investigated. 137 radiosondes were launched at Indian Institute of Technology Kanpur (IITK), between 5th and 28th July, 2016. CVS is determined using an algorithm that identifies cloud layers from vertical profiles of relative humidity (RH), with altitude-dependent RH thresholds. CVS is analysed by separating the campaign period on the basis of presence and absence of depressions/low-pressure systems. Compared to non-depression periods, low-pressure events showed significant difference in all CVS and CRF parameters except cloud-top height. CVS was multi-layered in ~75% launches, with deep, mixed-phase clouds being present in ~70% launches. CRF was calculated from clear-sky measurements and TOA observations from Clouds and the Earth's Radiant Energy System (CERES) satellite retrievals, and surface measurements. A net cooling effect was found overall, with instantaneous shortwave forcing (SWCRF) (mean values of -95.92 and -101.89 W/m² at surface and TOA, respectively) dominating longwave forcing (LWCRF) (mean values of 15.33 and 66.55 W/m² at surface and TOA, respectively). Results suggest that SWCRF depends on total depth of cloud layers, and is independent of cloud altitude, whereas LWCRF depends on both depth and vertical location of cloud layers, with base and top heights regulating LWCRF at surface and TOA, respectively.


78. Maritime Continent seasonal climate biases in AMIP experiments of the CMIP5 multimodel ensemble. Y. Y. Toh, A. G. Turner, S. J. Johnson and C. E. Holloway (2018). Climate Dynamics, 50(3-4): 777-800. Authors' preprint |
    Abstract
    The fidelity of 28 Coupled Model Intercomparison Project phase 5 (CMIP5) models in simulating mean climate over the Maritime Continent in the Atmospheric Model Intercomparison Project (AMIP) experiment is evaluated in this study. The performance of AMIP models varies greatly in reproducing seasonal mean climate and the seasonal cycle. The multi-model mean has better skill at reproducing the observed mean climate than the individual models. The spatial pattern of 850 hPa wind is better simulated than the precipitation in all four seasons. We found that model horizontal resolution is not a good indicator of model performance. Instead, a model's local Maritime Continent biases are somewhat related to its biases in the local Hadley circulation and global monsoon. The comparison with coupled models in CMIP5 shows that AMIP models generally performed better than coupled models in the simulation of the global monsoon and local Hadley circulation but less well at simulating the Maritime Continent annual cycle of precipitation. To characterize model systematic biases in the AMIP runs, we performed cluster analysis on Maritime Continent annual cycle precipitation. Our analysis resulted in two distinct clusters. Cluster I models are able to capture both the winter monsoon and summer monsoon shift, but they overestimate the precipitation; especially during the JJA and SON seasons. Cluster II models simulate weaker seasonal migration than observed, and the maximum rainfall position stays closer to the equator throughout the year. The tropics-wide properties of these clusters suggest a connection between the skill of simulating global properties of the monsoon circulation and the skill of simulating the regional scale of Maritime Continent precipitation.


79. The evolution, seasonality, and impacts of western disturbances. K. M. R. Hunt, A. G. Turner and L. C. Shaffrey (2018). Quarterly Journal of the Royal Meteorological Society, 144(710): 278-290. Authors' preprint | CEDA catalogue record / ERA-Interim track data |
    Abstract
    Western disturbances (WDs) are upper-level synoptic-scale systems embedded in the subtropical westerly jet stream (STWJ), often associated with extreme rainfall events in north India and Pakistan during boreal winter. Here, a tracking algorithm is applied to the upper-tropospheric vorticity field in 37 years of ERA-Interim reanalysis data, giving a catalogue of over 3000 events. These events are analysed in a composite framework: the vertical structure is explored across a large number of dynamic and thermodynamic fields, revealing a significant northwestward tilt with height, strong ascent ahead of the centre which sits above the maximum surface precipitation and a warm-overcold, dry-over-moist structure among other signatures of strong baroclinicity. Evolution of the structures of cloud cover and vertical wind speed are investigated as the composite WD passes across northern India. Cloud cover in particular is found to be particularly sensitive to the presence of the Himalayan foothills, with a significant maximum at 300 hPa approximately one day after the WD reaches peak intensity. k-means clustering is used to classify WDs both according to dynamical structure and precipitation footprint, and the relationship between the two sets is explored. Finally, the statistical relationship between the STWJ position and WDs on interannual time scales is explored, showing that WD frequency in north India is highly sensitive to the jet location over Eurasia. Years with a greater number of WDs feature a STWJ shifted to the south, a pattern that is substantially more coherent and reaches as far west as North America during boreal winter. This suggests that it may be possible to predict the statistics of western disturbance events on seasonal time scales if suitable indicators of jet position can also be predicted.


80. The effect of soil moisture perturbations on Indian Monsoon Depressions in a numerical weather prediction model. K. M. R. Hunt and A. G. Turner (2017). Journal of Climate, 30(21): 8811-8823. Authors' preprint |
    Abstract
    Indian monsoon depressions (MDs) are synoptic-scale cyclonic systems that propagate across peninsular India three or four times per monsoon season. They are responsible for the majority of rainfall in agrarian north India, thus constraining precipitation estimates is of high importance. Here, we use a case study from August 2014 to explore the relationship between varying soil moisture and the resulting track and structure of an incident MD using the Met Office Unified Model. We use this case study with the view to increasing understanding of the general impact of soil moisture perturbations on monsoon depressions. It is found that increasing soil moisture in the monsoon trough region results in deeper inland penetration and a more developed structure - e.g. a warmer core in the mid-troposphere and a stronger bimodal potential vorticity core in the middle/lower troposphere - with more precipitation, and a structure that in general more closely resembles that found in depressions over the ocean, indicating that soil moisture may enhance the convective mechanism that drives depressions over land. This experiment also shows that these changes are most significant when the depression is deep, and negligible when it is weakening. Increasing soil moisture in the sub-Himalayan arable zone, a region with large irrigation coverage, also caused deeper inland penetration and some feature enhancement in the upper troposphere but no significant changes were found in the track heading or lower-tropospheric structure.


81. Boreal summer sub-seasonal variability of the South Asian monsoon in the Met Office GloSea5 initialized coupled model. A. Jayakumar, A. G. Turner, S. J. Johnson, E. N. Rajagopal, Saji Mohandas and A. K. Mitra (2017). Climate Dynamics, 49(5-6, September): 2035-2059. Authors' preprint |
    Abstract
    Boreal summer sub-seasonal variability in the Asian monsoon, otherwise known as the monsoon intra-seasonal oscillation (MISO), is one of the dominant modes of intraseasonal variability in the tropics, with large impacts on total monsoon rainfall and India's agricultural production. However, our understanding of the mechanisms involved in MISO is incomplete and its simulation in various numerical models is often flawed. In this study, we focus on the objective evaluation of the fidelity of MISO simulation in the Met Office Global Seasonal forecast system version 5 (GloSea5), an initialized coupled model. We analyze a series of nine-member hindcasts from GloSea5 over 1996-2009 during the peak monsoon period (July-August) over the South-Asian monsoon domain focusing on aspects of the time-mean background state and air-sea interaction processes pertinent to MISO. Dominant modes during this period are evident in power spectrum analysis, but propagation and evolution characteristics of the MISO are not realistic. We find that simulated air-sea interactions in the central Indian Ocean are not supportive of MISO initiation in that region, likely a result of the low surface wind variance there. As a consequence, the expected near-quadrature phase relationship between SST and convection is not represented properly over the central equatorial Indian Ocean, and northward propagation from the equator is poorly simulated. This may reinforce the equatorial rainfall mean state bias in GloSea5.


82. The 2015 Indian summer monsoon onset - Phenomena, forecasting and research flight planning. Willetts, P., A. G. Turner, G. M. Martin, G. Mrudula, K. M. R. Hunt, D. J. Parker, C. M. Taylor, C. E. Birch, A. K. Mitra, J. T. Heming and M. E. Brooks (2017) Weather 72(6, June): 168-175. Authors' preprint |
    Abstract
    From May to July 2016, as part of the INCOMPASS project, the Facility for Airborne AtmosphericMeasurements (FAAM, jointly funded by the Met Office and NERC) BAe-146 research aircraft travelled to India to record key aspects of the Indian summer monsoon onset and evolution. As part of the planning for the campaign, partners in the UK and India took part in a dry-run forecasting exercise during 2015, to assess the reliability of the forecast products and develop a set of flight plans, in advance of the real campaign, and to get a real-time feel for the monsoon onset. 5-day forecasts from the Met Office and the Indian National Centre for Medium Range Weather Forecasting (NCMRWF) showed good skill in terms of predicting the advance of rainfall in regions key for the campaign in north and south India, and captured transitions from active (wet) monsoon conditions to break (dry) periods and back again. Key phenomena seen during the dry-run exercise include (1) a western disturbance, which had a major effect on the extreme pre-monsoon heatwave conditions over India; (2) dry intrusions, which are thought to be important in the progression of the monsoon onset against the synoptic flow; and (3) cyclones Ashobaa and Komen.


83. Contribution of Tropical Cyclones to atmospheric moisture transport and rainfall over East Asia. L. Guo, N. P. Klingaman, P. L. Vidale, A. G. Turner M.-E. Demory and A. Cobb (2017). Journal of Climate, 30(10, May): 3853-3865. Authors' preprint |
    Abstract
    The coastal region of East Asia (EA) is one of the regions with the most frequent impacts from tropical cyclones (TCs). In this study, rainfall and moisture transports related to TCs are measured over the EA, and the contribution of TCs to the regional water budget is compared with other contrubutors, especially the mean circulation of the EA summer monson (EASM). Based on ERA-Interim re-analysis (1979-2012), the trajectories of TCs are identified using an objective feature tracking method. Over 60% of TCs occur from July to October (JASO). During JASO, TC rainfall contributes 10-30% of the total monthly rainfall over the coastal region of EA; this contribution is highest over the south/southeast coast of China in September. TCs make a larger contribution to daily extreme rainfall (above the 95th percentile): 50-60% over the EA coast and as high as 70% over Taiwan island. Compared with the mean EASM, TCs transport less moisture over the EA. However, as the peak of the mean seasonal cycle of TCs lags two months behind that of the EASM, the moisture transported by TCs is an important source for the water budget over the EA region when the EASM withdraws. The moisture transport is largely performed by westward-moving TCs. These results improve our understanding of the water cycle of EA and provide a useful test bed for evaluating and improving seasonal forecasts and coupled climate models.


84. The effect of horizontal resolution on Indian monsoon depressions in the Met Office NWP model. K. M. R. Hunt and A. G. Turner (2017). Quarterly Journal of the Royal Meteorological Society, 143(705, April, Part B): 1756-1771. Authors' preprint |
    Abstract
    Monsoon depressions are synoptic-scale features that are responsible for a significant fraction of the rain over northern India during the summer monsoon season, and, as such, it is important to quantify their structure and behaviour in numerical weather prediction models. It is known that increasing model resolution is strongly correlated with improved forecasts in the short term and global circulation in the longer term, as well as better representation of tropical cyclones; here, we explore the sensitivity of depressions to changes in resolution using the Met Office Unified Model. Seven numerical weather prediction (NWP) case studies of depressions from 2013-15 were run at eight resolutions corresponding to equatorial grid spacing of between 16 and 208km, and compared with data from the same events from TRMM and ERA-Interim reanalysis. We found that at the low resolution end of the spectrum, increases in resolution led to improvements in composite structure, but with diminishing returns; that is to say, the improvements in forecast track and structure become smaller. The model also persistently overestimated the depression intensity, in particular the wind speed and the warm core aloft - with the source appearing to originate in the mid-troposphere. The sensitivity of the diurnal cycle to resolution was also explored: the stratiform component was found to be very well represented by the model, whereas the convective component was described quite poorly. Improvement in most componentns of structure with increasing model resolution were marginal beyond N320 (63km) and N512 (39km) for dynamic and thermodynamic fields respectively.


85. An assessment of Indian monsoon seasonal forecasts and mechanisms underlying monsoon interannual variability in the Met Office GloSea5-GC2 system. S. J. Johnson, A. G. Turner, S. J. Woolnough, G. M. Martin and C. MacLachlan (2017). Climate Dynamics, 48(5, March): 1447-1465. Authors' preprint |
    Abstract
    We assess Indian summer monsoon seasonal forecasts in GloSea5-GC2, the Met Office fully coupled subseasonal-to-seasonal ensemble forecasting system. Using several metrics, GloSea5-GC2 shows similar skill to other state-of-the-art forecast systems. The prediction skill of the large-scale South Asian monsoon circulation is higher than that of Indian monsoon rainfall. Using multiple linear regression analysis we evaluate relationships between Indian monsoon rainfall and five possible dirvers of monsoon interannual variability. Over the time period studied (1992-2011), the El Nino-Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD) are the most important of these drivers in both observations and GloSea5-GC2. Our analysis indicates that ENSO and its teleconnection with the Indian rainfall are well represented in GloSea5-GC2. However the relationship between the IOD and Indian rainfall anomalies is too weak in GloSea5-GC2, which may be limiting the prediction skill of the local monsoon circulation and Indian rainfall. We show that this weak relationship likely results from a couple mean-state bias that limits the impact of anomalous winds forcing on SST variability, resulting in erroneous IOD SST anomalies. Known difficulties in representing convective precipitation oer India may also play a role. Since Indian rainfall responds weakly to the IOD, it responds more consistently to ENSO than in observations. Our assessment identifies specific coupled biases that are likely limiting GloSea5-GC2 prediction skill, providing targets for model improvement.


86. The Spatiotemporal Structure of Precipitation in Indian Monsoon Depressions. K. M. R. Hunt, A. G. Turner and D. E. Parker (2016). Quarterly Journal of the Royal Meteorological Society, 142(701): 3195-3210.
    Abstract
    Indian monsoon depressions are synoptic scale events typically spun up in the Bay of Bengal. They usually last 4-6 days, during which they propagate northwestward across the Indian subcontinent before dissipating over northwest India or Pakistan. They can have a significant effect on monsoon precipitation, particularly in primarily agrarian northern India, and therefore quantifying their structure and variability and evaluating these in NWP models and GCMs is of critical importance. In this study, satellite data from the CloudSat and recently concluded TRMM missions are used in conjunction with an independently evaluated tracking algorithm to form a three-dimensional composite image of cloud structure and precipitation within monsoon depressions. The composite comprises 34 depressions from the 1998-2014 TRMM mission and 12 from the 2007-present CloudSat mission, and is statistically robust enough to allow significant probing of the spatiotemporal characteristics of moisture and hydrometeor fields. Among the key results of this work are the discovery and characterisation of a bimodal, diurnal cycle in surface precipitation; the first picture of the structure of cloud type and density in depressions, showing that deep convection dominates south of the centre and prominent cirrus throughout; the first composite picture of vertical hydrometeor structure in depressions, showing significant precipitation for hundreds of kilometres outside the centre and well past the mid-troposphere; and novel discussion of drop size distributions (showing significant uniformity across the depression) and resulting latent heat profiles, showing average heating rates near the centre can reach 2K/hr.


87. GMMIP (v1.0) contribution to CMIP6: Global Monsoons Model Inter-comparison Project. T. Zhou, A. G. Turner, J. Kinter, B. Wang, Y. Qian, X. Chen, B. Wu, B. Wang, B. Liu, L. Zou and B. He (2016). Geoscientific Model Development, 9: 3589-3604. Publisher link | GMDD discussion version |
    Abstract
    The Global Monsoons Model Inter-comparison Project (GMMIP) has been endorsed by the panel of Coupled Model Inter-comparison Project (CMIP) as one of the participating MIPs in the sixth phase of CMIP (CMIP6). The focus of GMMIP is on monsoon climatology, variability, prediction and projection, which is relevant to four of the Grand Challenges proposed by the World Climate Research Programme. At present, 21 international modelling groups are committed to joining GMMIP. This overview paper introduces the motivation behind GMMIP and the scientific questions it intends to answer. Three tiers of experiments, of decreasing priority, are designed to examine: (a) model skill in simulating the climatology and interannual-to-multidecadal variability of global monsoons during SST-forced experiments of the historical climate period; (b) the roles of the Interdecadal Pacific Oscillation and Atlantic Multidecadal Oscillation in driving variations of the global and regional monsoons; and (c) the effects of large orographic terrain on the establishment of the monsoons. The outputs of the CMIP6 DECK, historical simulation and other MIPs will also be used in the diagnostic analysis of GMMIP to give a comprehensive understanding of the roles played by different external forcings, potential improvements in the simulation of monsoon rainfall at high resolution and predictability at decadal time scales. The implementation of GMMIP will improve our understanding of the fundamental physics of changes in the global and regional monsoons over the past 140 years and ultimately benefit monsoon prediction and projection in the current century.


88. Local and remote impacts of aerosol species on Indian summer monsoon rainfall in a GCM. L. Guo, A. G. Turner and E. J. Highwood (2016). Journal of Climate, 29(19): 6937-6955. Open Access PDF | Supplementary material | CEDA catalogue record (model experiment data download) |
    Abstract
    The HadGEM2 AGCM is used to determine the most important anthropogenic aerosols in the Indian monsoon using experiments in which observed trends in individual aerosol species are imposed. Sulphur dioxide (SD) emissions are shown to impact rainfall more strongly than black carbon (BC) aerosols, causing reduced rainfall especially over northern India. Significant perturbations due to BC are not noted until its emissions are scaled up in a sensitivity test, resulting in rainfall increases over northern India due to the Elevated Heat Pump mechanism, enhancing convection during the premonsoon and bringing forward the monsoon onset. Secondly, the impact of anthropogenic aerosols is compared to that of increasing greenhouse-gas concentrations and observed sea-surface temperature (SST) warming. The tropospheric temperature gradient driving the monsoon shows weakening whenforced by either SD or imposed SST trends. However the observed SST trend is dominated by warming in the deep tropics; when the component of SST trend related to aerosol emissions is removed, further warming is found in the extratropical northern hemisphere that tends to offset monsoon weakening. This suggests caution is needed when using SST forcing as a proxy for greenhouse warming. Finally, aerosol emissions are decomposed into those from the Indian region and those elsewhere, in pairs of experiments with SD and BC. Both local and remote aerosol emissions are found to lead to rainfall changes over India; for SD, remote aerosols contribute around 75% of the rainfall decrease over India, while for BC the remote forcing is even more dominant.


89. On the Structure and Dynamics of Indian Monsoon Depressions. Kieran M. R. Hunt, Andrew G. Turner, Peter M. Inness, David Parker, and Richard C. Levine (2016). Monthly Weather Review, 144: 3391-3416. Open access PDF version |
    Abstract
    ERA-Interim reanalysis data from the past 35 years have been used with a newly-developed feature tracking algorithm to identify Indian monsoon depressions originating in or near the Bay of Bengal. These were then rotated, centralised and combined to give a fully three-dimensional 106-depression composite structure - a considerably larger sample than any previous detailed study on monsoon depressions and their structure. Many known features of depression structure are confirmed, particularly the existence of a maximum to the southwest of the centre in rainfall and other fields, and a westward axial tilt in others. Additionally, the depressions are found to have significant asymmetry due to the presence of the Himalayas; a bimodal mid-tropospheric potential vorticity core; a separation into thermally cold- (~-1.5K) and neutral- (~0K) cores near the surface with distinct properties; and that the centre has very large CAPE and very small CIN. Variability as a function of background state has also been explored, with land/coast/sea, diurnal, ENSO, active/break and Indian Ocean Dipole contrasts considered. Depressions are found to be markedly stronger during the active phase of the monsoon, as well as during La Niña. Depressions on land are shown to be more intense and more tightly constrained to the central axis. A detailed schematic diagram of a vertical cross-section through a composite depression is also presented, showing its inherent asymmetric structure.


90. The interaction of moist convection and mid-level dry air in the advance of the onset of the Indian monsoon. D. J. Parker, P. Willetts, C. Birch, A. G. Turner, J. H. Marsham, C. M. Taylor, S. Kolusu and G. M. Martin (2016). Quarterly Journal of the Royal Meteorological Society, 142(699): 2256-2272. Authors' preprint |
    Abstract
    The advance of the onset of the Indian monsoon is here explained in terms of a balance between the low-level monsoon flow and an over-running intrusion of mid-tropospheric dry air. The monsoon advances, over a period of about 6 weeks, from the south of the country to the northwest. Given that the low-level monsoon winds are westerly or southwesterly, and the midlevel winds northwesterly, the monsoon onset propagates upwind relative to midlevel flow, and perpendicular to the low-level flow, and is not directly caused by moisture flux toward the northwest. Lacking a conceptual model for the advance means that it has been hard to understand and correct known biases in weather and climate prediction models. The mid-level northwesterlies form a wedge of dry air that is deep in the far northwest of India and over-runs the monsoon flow. The dry layer is moistened from below by shallow cumulus and congestus clouds, so that the profile becomes much closer to moist adiabatic, and the dry layer is much shallower in the vertical, toward the southeast of India. The profiles associated with this dry air show how the most favourable environment for deep convection occurs in the south, and onset occurs here first. As the onset advances across India, the advection of moisture from the Arabian Sea becomes stronger, and the mid-level dry air is increasingly moistened from below. This increased moistening makes the wedge of dry air shallower throughout its horizontal extent, and forces the northern limit of moist convection to move toward the northwest. Wetting of the land surface by rainfall will further reinforce the north-westward progression, by sustaining the supply of boundary layer moisture and shallow cumulus. The local advance of the monsoon onset is coincident with weakening of the mid-level northwesterlies, and therefore weakened mid-level dry advection.


91. A New Understanding of El Niño's Impact over East Asia: Dominance of the ENSO Combination Mode. W. Zhang, H. Li, M. Stuecker, F.-F. Jin and A. G. Turner (2016). Journal of Climate, 29(12), 4347-4359, published online 8 June 2016. Authors' preprint |
    Abstract
    Previous studies have shown that the Indo-Pacific atmospheric response to ENSO comprises two dominant modes of variability: a meridionally quasi-symmetric response (independent from the annual cycle) and an anti-symmetric response (arising from the nonlinear atmospheric interaction between ENSO variability and the Western Pacific annual cycle), referred to as the combination mode (C-Mode). This study demonstrates that the direct El Niño signal over the tropics is confined to the equatorial region and has no significant impact on the atmospheric response over East Asia. The El Niño-associated equatorial anomalies can be expanded towards off-equatorial regions by the C-Mode through ENSO's interaction with the annual cycle. The C-Mode is the prime driver for the development of an anomalous low-level anticyclone over the western North Pacific (WNP) during the El Niño decay phase, which usually transports more moisture to East Asia and thereby causes more precipitation over southern China. We use an Atmospheric General Circulation Model that well reproduces the WNP anticyclonic anomalies when both El Niño sea surface temperature (SST) anomalies as well as the SST annual cycle are prescribed as boundary conditions. However, no significant WNP anticyclonic circulation anomaly appears during the El Niño decay phase when excluding the SST annual cycle. Our analyses of observational data and model experiments suggest that the Western Pacific annual cycle plays a key role in the East Asian climate anomalies associated with El Niño through their nonlinear atmospheric interaction. Hence, a realistic simulation of the Western Pacific annual cycle is crucial in order to correctly capture the ENSO-associated climate anomalies over East Asia.


92. The resolution sensitivity of the South Asian monsoon and Indo-Pacific in a global 0.35° AGCM. S. J. Johnson*, R. C. Levine, A. G. Turner, G. M. Martin, S. J. Woolnough, R. Schiemann, M. S. Mizielinski, M. J. Roberts, P. L. Vidale, M.-E. Demory and J. Strachan (2016). Climate Dynamics, 46(3): 807-831. *Note that S. J. Johnson was formerly known as S. J. Bush. Open access PDF version |
    Abstract
    The South Asian monsoon is one of the most significant manifestations of the seasonal cycle. It directly impacts nearly one third of the world's population and also has substantial global influence. Using 27-year integrations of a high-resolution atmospheric general circulation model (Met Office Unified Model), we study changes in South Asian monsoon precipitation and circulation when horizontal resolution is increased from approximately 200-40km at the equator (N96-N512, 1.9°-0.35°). The high resolution, integration length and ensemble size of the dataset make this the most extensive dataset used to evaluate the resolution sensitivity of the South Asian monsoon to date. We find a consistent pattern of JJAS precipitation and circulation changes as resolution increases, which include a slight increase in precipitation over peninsular India, changes in Indian and Indochinese orographic rain bands, increasing wind speeds in the Somali Jet, increasing precipitation over the Maritime Continent islands and decreasing precipitation over the northern Maritime Continent seas. To diagnose which resolution-related processes cause these changes, we compare them to published sensitivity experiments that change regional orography and coastlines. Our analysis indicates that improved resolution of the East African Highlands results in the improved representation of the Somali Jet and further suggests that improved resolution of orography over Indochina and the Maritime Continent results in more precipitation over the Maritime Continent islands at the expense of reduced precipitation further north. We also evaluate the resolution sensitivity of monsoon depressions and lows, which contribute more precipitation over northeast India at higher resolution. We conclude that while increasing resolution at these scales does not solve the many monsoon biases that exist in GCMs, it has a number of small, beneficial impacts.


93. Impact of different El Niño types on the El Niño/IOD relationship W. Zhang, Y. Wang, F.-F. Jin, M. F. Stucker and A. G. Turner (2015). Geophysical Research Letters, 42, 8570-8576, first published 19 October 2015. Authors' preprint |
    Abstract
    Previous studies reported that positive phases of the Indian Ocean Dipole (IOD) tend to accompany El Niño during boreal autumn. Here we show that the El Niño/IOD relationship can be better understood when considering the two different El Niño flavors. Eastern-Pacific (EP) El Niño events exhibit a strong correlation with the IOD dependent on their magnitude. In contrast, the relationship between Central-Pacific (CP) El Niño events and the IOD depends mainly on the zonal location of the sea surface temperature anomalies rather than their magnitude. CP El Niño events lying further west than normal are not accompanied by significant anomalous easterlies over the eastern Indian Ocean along the Java/Sumatra coast, which is unfavorable for the local Bjerknes feedback and correspondingly for an IOD development. The El Niño/IOD relationship has experienced substantial changes due to the recent decadal El Niño regime shift, which has important implications for seasonal prediction.


94. The annual cycle modulation of meridional asymmetry in ENSO's atmospheric response and its depenence on ENSO zonal structure. W. Zhang, H. Li, F.-F. Jin, M. F. Stucker, A. G. Turner, and N. P. Klingaman (2015). Journal of Climate, 28(14), 5795-5812. Authors' preprint |
    Abstract
    Previous studies documented that a distinct southward shift of central-Pacific low-level wind anomalies occurring during the ENSO decaying phase, is caused by an interaction between the Western Pacific annual cycle and El Niño-Southern Oscillation (ENSO) variability. The present study finds that the meridional movement of the central-Pacific wind anomalies appears only during traditional Eastern-Pacific (or EP) El Niño events rather than in Central-Pacific (CP) El Niño events in which sea surface temperature (SST) anomalies are confined to the central Pacific. The zonal structure of ENSO-related SST anomalies therefore has an important effect on meridional asymmetry in the associated atmospheric response and its modulation by the annual cycle. In contrast to EP El Niño events, the SST anomalies of CP El Niño events extend further west towards to the warm pool region with its climatological warm SSTs. In the warm pool region, relatively small SST anomalies are thus able to excite convection anomalies on both sides of the equator, even with a meridionally asymmetric SST background state. Therefore, almost meridionally symmetric precipitation and wind anomalies are observed over the central Pacific during the decaying phase of CP El Niño events. The SST anomaly pattern of La Niña events is similar to CP El Niño events with a reversed sign. Accordingly, no distinct southward displacement of the atmospheric response occurs over the central Pacific during the La Niña decaying phase. These results have important implications for ENSO climate impacts over East Asia, since the anomalous low-level anticyclone over the western North Pacific is an integral part of the annual cycle-modulated ENSO response.


95. Seasonal intercomparison of observational rainfall datasets over India during the southwest monsoon season. S. Prakash, A.K. Mitra, I.M. Momin, E.N. Rajagopal, S. Basu, M. Collins, A. G. Turner, K. AchutoRao and K. Ashok (2015). International Journal of Climatology, 35(9), 2326-2338, first published online 12 August 2014. Authors' preprint |
    Abstract
    The Indian monsoon is an important component of Earth's climate system, accurate forecasting of its mean rainfall being essential for regional food and water security. Accurate measurement of rainfall is essential for various water-related applications, the evaluation of numerical models and detection and attribution of trends, but a variety of different gridded rainfall datasets are available for these purposes. In this study, six gridded rainfall datasets are compared against the India Meteorological Department (IMD) gridded rainfall dataset, chosen as the most representative of the observed system due to its high gauge density. The datasets comprise those based solely on rain gauge observations and those merging rain gauge data with satellite-derived products. Various skill metrics and subjective comparisons are carried out for the Indian region during the southwest monsoon season (June-September). Relative biases and skill metrics are documented at all-India and subregional scales. In the gauge-based (land-only) category, Asian Precipitation - Highly-Resolved Observational Data Integration Towards Evaluation of water resources (APHRODITE) and Global Precipitation Climatology Center (GPCC) datasets perform better relative to the others in terms of a variety of skill metrics. In the merged category, the Global Precipitation Climatology Project (GPCP) dataset is shown to perform better than the Climate Prediction Center Merged Analysis of Precipitation (CMAP) for the Indian monsoon in terms of various metrics, when compared with the IMD gridded data. Most of the datasets have difficulties in representing rainfall over orographic regions including the Western Ghats mountains, in Northeast India and the Himalayan foothills. The wide range of skill metrics seen among the datasets and even the change of sign of bias found in some years are causes of concern. This uncertainty between datasets is largest in Northeast India. These results will help those studying the Indian monsoon region to select an appropriate dataset depending on their application and focus of research.


96. Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models. L. Guo, A. G. Turner and E. J. Highwood (2015). Atmospheric Chemistry and Physics, 15, 6367-6378, published 11 June 2015 | Online version (open access) | ACPD Discussion version (open access) | UoR Meteorology blog entry | CEDA catalogue record / data for this article |
    Abstract
    Comparison of single-forcing varieties of 20th century historical experiments in a subset of models from the Fifth Coupled Model Intercomparison Project (CMIP5) reveals that South Asian summer monsoon rainfall increases towards the present day in Greenhouse Gas (GHG)-only experiments with respect to pre-industrial levels, while it decreases in anthropogenic aerosol-only experiments. Comparison of these single-forcing experiments with the all-forcings historical experiment suggests aerosol emissions have dominated South Asian monsoon rainfall trends in recent decades, especially during the 1950s to 1970s. The variations in South Asian monsoon rainfall in these experiments follows approximately the time-evolution of inter-hemispheric temperature gradient over the same period, suggesting a contribution from the large-scale background state relating to the asymmetric distribution of aerosol emissions about the equator. By examining the twenty-five available all-forcings historical experiments, we show that models including aerosol indirect effects dominate the negative rainfall trend. Indeed, models including only the direct radiative effect of aerosol show an increase in monsoon rainfall, consistent with the dominance of increasing greenhouse gas emissions and planetary warming on monsoon rainfall in those models. For South Asia, reduced rainfall in the models with indirect effects is related to decreased evaporation at the land surface rather than from anomalies in horizontal moisture flux, suggesting the impact of indirect effects on local aerosol emissions. This is confirmed by examination of aerosol loading and cloud droplet number trends over the South Asia region. Thus while remote aerosols and their asymmetric distribution about the equator play a role in setting the inter-hemispheric temperature distribution on which the South Asian monsoon, as one of the global monsoons, operates, the addition of indirect aerosol effects acting on very local aerosol emissions also plays a role in declining monsoon rainfall. The disparity between the response of monsoon rainfall to increasing aerosol emissions in models containing direct aerosol effects only and those also containing indirect effects needs to be urgently investigated since the suggested future decline in Asian anthropogenic aerosol emissions inherent to the representative concentration pathways (RCPs) used for future climate projection may turn out to be optimistic. In addition, both groups of models show declining rainfall over China, also relating to local aerosol mechanisms. We hypothesize that aerosol emissions over China are large enough, in the CMIP5 models, to cause declining monsoon rainfall even in the absence of indirect aerosol effects. The same is not true for India.


97. The impact of monsoon intraseasonal variability on renewable power generation in India. C. M. Dunning, A. G. Turner and D. J. Brayshaw (2015). Environmental Research Letters, 10(6), 064002, published 2 June 2015. Open access PDF version | Blog article about this work (David Brayshaw) | Poster for ICEM conference | Walker Institute paper of the month precis | environmentalresearchweb news article (Kate Ravilious) | NCAS Science Highlight (PDF) |
    Abstract
    India is increasingly investing in renewable technology to meet rising energy demands, with hydropower and other renewables comprising one-third of current installed capacity. Installed wind-power is projected to increase 5-fold by 2035 (to nearly 100GW) under the International Energy Agency's New Policies scenario. However, renewable electricity generation is dependent upon the prevailing meteorology, which is strongly influenced by monsoon variability. Prosperity and widespread electrification are increasing the demand for air conditioning, especially during the warm summer. This study uses multi-decadal observations and meteorological reanalysis data to assess the impact of intraseasonal monsoon variability on the balance of electricity supply from wind-power and temperature-related demand in India. Active monsoon phases are characterised by vigorous convection and heavy rainfall over central India. This results in lower temperatures giving lower cooling energy demand, while strong westerly winds yield high wind-power output. In contrast, monsoon breaks are characterised by suppressed precipitation, with higher temperatures and hence greater demand for cooling, and lower wind-power output across much of India. The opposing relationship between wind-power supply and cooling demand during active phases (low demand, high supply) and breaks (high demand, low supply) suggests that monsoon variability will tend to exacerbate fluctuations in the socalled demand-net-wind (i.e., electrical demand that must be supplied from non-wind sources). This study may have important implications for the design of power systems and for investment decisions in conventional schedulable generation facilities (such as coal and gas) that are used to maintain the supply/demand balance. In particular, if it is assumed (as is common) that the generated wind-power operates as a price-taker (i.e., wind farm operators always wish to sell their power, irrespective of price) then investors in conventional facilities will face additional weather-volatility through the monsoonal impact on the length and frequency of production periods (i.e. their load-duration curves).


98. The effect of increased convective entrainment on Asian monsoon biases in the MetUM General Circulation Model. Bush, S. J., A. G. Turner, S. J. Woolnough, Gill M. Martin and N. P. Klingaman (2015) Quarterly Journal of the Royal Meteorological Society, 141: 311-326, January 2015. Open access PDF version |
    Abstract
    We demonstrate that summer precipitation biases in the South Asian monsoon domain are sensitive to increasing the convective parametrisation's entrainment and detrainment rates in the Met Office Unified Model. We explore this sensitivity to improve our understanding of the biases and inform efforts to improve convective parametrisation. We perform novel targeted experiments in which we increase the entrainment and detrainment rates in regions of especially large precipitation bias. We use these experiments to determine whether the sensitivity at a given location is a consequence of the local change to convection or is a remote response to the change elsewhere. We find that a local change leads to different mean-state responses in comparable regions. When the entrainment and detrainment rates are increased globally, feedbacks between regions usually strengthen the local responses. We choose two regions of tropical ascent that show different mean-state responses, the western equatorial Indian Ocean and western north Pacific, and analyse them as case studies to determine the mechanisms leading to the different responses. Our results indicate that several aspects of a region's mean-state, including moisture content, sea surface temperature and circulation, play a role in local feedbacks that determine the response to increased entrainment and detrainment.


99. Increasing Autumn Drought over Southern China associated with ENSO Regime Shift. W. Zhang, F.-F. Jin and A. G. Turner (2014) Geophysical Research Letters, 41(11): 4020-4026, 16 June 2014. Authors' preprint | Highlight in Nature Climate Change | NCAS Science Highlight (PDF) |
    Abstract
    In the two most recent decades, more frequent drought struck southern China during autumn, causing an unprecedented water crisis. We found that the increasing autumn drought is largely attributed to an ENSO regime shift. Compared to traditional eastern-Pacific (EP) El Niño, central-Pacific (CP) El Niño events have occurred more frequently, with maximum sea surface temperature anomalies located near the dateline. Southern China usually experiences precipitation surplus during the autumn of EP El Niño years, while the CP El Niño tends to produce precipitation deficits. Since the CP El Niño has occurred more frequently while EP El Niño has become less common after the early 1990s, there has been a significant increase in the frequency of autumn drought. This has implications for increasing precipitation shortages over southern China in a warming world, in which CP El Niño events have been suggested to become more common.


100. The Asian Summer Monsoon: An Intercomparison of CMIP5 vs. CMIP3 Simulations of the Late 20th Century. Sperber, K. R., H. Annamalai, I.-S. Kang, A. Kitoh, A. Moise, A. Turner, B. Wang and T. Zhou (2013) Climate Dynamics, 41: 2711-2744. Authors' preprint |
     Abstract
     The boreal summer Asian monsoon has been evaluated in 25 Coupled Model Intercomparison Project-5 (CMIP5) and 22 CMIP3 GCM simulations of the late 20th Century. Diagnostics and skill metrics have been calculated to assess the time-mean, climatological annual cycle, interannual variability, and intraseasonal variability. Progress has been made in modeling these aspects of the monsoon, though there is no single model that best represents all of these aspects of the monsoon. The CMIP5 multi-model mean (MMM) is more skillful than the CMIP3 MMM for all diagnostics in terms of the skill of simulating pattern correlations with respect to observations. Additionally, for rainfall/convection the MMM outperforms the individual models for the time mean, the interannual variability of the East Asian monsoon, and intraseasonal variability. The pattern correlation of the time (pentad) of monsoon peak and withdrawal is better simulated than that of monsoon onset. The onset of the monsoon over India is typically too late in the models. The extension of the monsoon over eastern China, Korea, and Japan is underestimated, while it is overestimated over the subtropical western/central Pacific Ocean. The anti-correlation between anomalies of all-India rainfall and Niño-3.4 sea surface temperature is overly strong in CMIP3 and typically too weak in CMIP5. For both the ENSO-monsoon teleconnection and the East Asian zonal wind-rainfall teleconnection, the MMM interannual rainfall anomalies are weak compared to observations. Though simulation of intraseasonal variability remains problematic, several models show improved skill at representing the northward propagation of convection and the development of the tilted band of convection that extends from India to the equatorial west Pacific. The MMM also well represents the space-time evolution of intraseasonal outgoing longwave radiation anomalies. Caution is necessary when using GPCP and CMAP rainfall to validate (1) the time-mean rainfall, as there are systematic differences over ocean and land between these two data sets, and (2) the timing of monsoon withdrawal over India, where the smooth southward progression seen in India Meteorological Department data is better realized in CMAP data compared to GPCP data.


101. Observational Challenges in Evaluating Climate Models. Collins, M., K. AchutaRao, K. Ashok, S. Bhandari, A. Mitra, S. Prakash, R. Srivastava and A. G. Turner (2013) Nature Climate Change, 3: 941-942. Authors' preprint |
     Abstract
     Activities such as the Coupled Model Intercomparison Project (CMIP) have revolutionized climate modelling in terms of our ability to compare models and to process information about climate projections and their uncertainties. The evaluation of models against observations is now considered a key component of multimodel studies. Although there are a number of outstanding scientific issues surrounding model evaluation, notably the open question of how to link model performance to future projections, here we highlight a specific but growing problem - that of uncertainties in the observational data that are used to evaluate the models. We illustrate this using an example obtained from studies of the South Asian monsoon, but we believe the problem is a generic one that arises in many different areas of climate model evaluation and requires some attention by the community.


102. 20th century intraseasonal Asian monsoon dynamics viewed from Isomap. Hannachi, A. and Turner, A. G. (2013) Nonlinear Processes in Geophyics, 20: 725-741. Open access PDF version |
     Abstract
     The Asian summer monsoon is a high dimensional and highly nonlinear phenomenon involving considerable moisture transport towards land from the ocean, and is critical for the whole region. We have used daily ECMWF reanalysis (ERA-40) sea-level pressure (SLP) anomalies to the seasonal cycle, over the region 50-145°E, 20°S-35°N to study the nonlinearity of the Asian monsoon using Isomap. We have focused on the two-dimensional embedding of the SLP anomalies for ease of interpretation. Unlike the unimodality obtained from tests performed in empirical orthogonal function space, the probability density function, within the two-dimensional Isomap space, turns out to be bimodal. But a clustering procedure applied to the SLP data reveals support for three clusters, which are identified using a three-component bivariate Gaussian mixture model. The modes are found to appear similar to active and break phases of the monsoon over South Asia in addition to a third phase, which shows active conditions over the Western North Pacific. Using the low-level wind field anomalies the active phase over South Asia is found to be characterised by a strengthening and an eastward extension of the Somali jet whereas during the break phase the Somali jet is weakened near southern India, while the monsoon trough in northern India also weakens. Interpretation is aided using the APHRODITE gridded land precipitation product for monsoon Asia. The effect of large-scale seasonal mean monsoon and lower boundary forcing, in the form of ENSO, is also investigated and discussed. The outcome here is that ENSO is shown to perturb the intraseasonal regimes, in agreement with conceptual ideas.


103. The role of northern Arabian Sea surface temperature biases in CMIP5 model simulations and future projections of Indian summer monsoon rainfall. Levine, R. C., A. G. Turner, D. Marathayil and G. M. Martin (2013) Climate Dynamics, 41(1), pgs 155-172. Authors' preprint |
     Abstract
     Many climate models have problems simulating Indian summer monsoon rainfall and its variability, resulting in considerable uncertainty in future projections. Problems may relate to many factors, such as local effects of the formulation of physical parametrisation schemes, while common model biases that develop elsewhere within the climate system may also be important. Here we examine the extent and impact of cold sea surface temperature (SST) biases developing in the northern Arabian Sea in the CMIP5 multi-model ensemble, where such SST biases are shown to be common. Such biases have previously been shown to reduce monsoon rainfall in the Met Office Unified Model (MetUM) by weakening moisture fluxes incident upon India. The Arabian Sea SST biases in CMIP5 models consistently develop in winter, via strengthening of the winter monsoon circulation, and persist into spring and summer. A clear relationship exists between Arabian Sea cold SST bias and weak monsoon rainfall in CMIP5 models, similar to effects in the MetUM. Part of this effect may also relate to other factors, such as forcing of the early monsoon by spring-time excessive equatorial precipitation. Atmosphere-only future time-slice experiments show that Arabian Sea cold SST biases have potential to weaken future monsoon rainfall increases by limiting moisture flux acceleration through non-linearity of the Clausius-Clapeyron relationship. Analysis of CMIP5 model future scenario simulations suggests that, while such effects are likely small compared to other sources of uncertainty, models with large Arabian Sea cold SST biases suppress the range of potential outcomes for changes to future early monsoon rainfall.


104. Isomap nonlinear dimensionality reduction and bimodality of monsoon convection. Hannachi, A. and A. G. Turner (2013) Geophys. Res. Letts., 40(8), 1653-1658. Authors' preprint |
     Abstract
     It is known that the EOF method is unable to detect possible nonlinear structure in climate data. Here Isomap, as a tool for nonlinear dimensionality reduction, is applied to 1958-2001 ERA-40 SLP anomalies to study nonlinearity of the Asian summer monsoon intraseasonal variability. Using the leading two Isomap time series, the probability density function is shown to be bimodal. A two-dimensional bivariate Gaussian mixture model is then applied to identify the monsoon phases; the obtained regimes representing enhanced and suppressed phases respectively. The relationship with the large-scale seasonal mean monsoon indicates that the frequency of monsoon regime occurrence is significantly perturbed in agreement with conceptual ideas, with preference for enhanced convection on intraseasonal timescales during large-scale strong monsoons. Trend analysis suggests a shift in concentration of monsoon convection, with less emphasis on South Asia and more on East China Sea.


105. Systematic winter SST biases in the northern Arabian Sea in HiGEM and the CMIP3 models. Marathayil, D., A. G. Turner, L. C. Shaffrey and R. C. Levine (2013) Environ. Res. Letts., 8(1), article no. 014028. Open acess PDF version | environmentalresearchweb Insight article | NCAS Science Highlight (PDF) |
     Abstract
     Analysis of 20th century simulations of the High resolution Global Environment Model (HiGEM) and the Third Coupled Model Intercomparison Project (CMIP3) models shows that most have a cold sea-surface temperature (SST) bias in the northern Arabian Sea during boreal winter. The association between Arabian Sea SST and the South Asian monsoon has been widely studied in observations and models, with winter cold biases known to be detrimental to rainfall simulation during the subsequent monsoon in coupled general circulation models (GCMs). However, the causes of these SST biases are not well understood. Indeed this is one of the first papers to address causes of the cold biases. The models show anomalously strong north-easterly winter monsoon winds and cold air temperatures in north-west India, Pakistan and beyond. This leads to the anomalous advection of cold, dry air over the Arabian Sea. The cold land region is also associated with an anomalously strong meridional surface temperature gradient during winter, contributing to the enhanced low-level convergence and excessive precipitation over the western equatorial Indian Ocean seen in many models.


106. The use of land/sea warming contrast under climate change to improve impact metrics. Joshi, M. M., A. G. Turner and C. Hope (2013) Climatic Change, 117(4): 951-960. Published online 19 February 2013. Authors' preprint |
     Abstract
     A favoured method of assimilating information from state-of-the-art climate models into integrated assessment models of climate impacts is to use the transient climate response (TCR) of the climate models as an input, sometimes accompanied by a pattern matching approach to provide spatial information. More recent approaches to the problem use TCR with another independent piece of climate model output: the land-sea surface warming ratio (φ). In this paper we show why the use of φ in addition to TCR has such utility. Multiple linear regressions of surface temperature change onto TCR and φ in 22 climate models from the CMIP3 multi-model database show that the inclusion of φ explains a much greater fraction of the inter-model variance than using TCR alone. The improvement is particularly pronounced in North America and Eurasia in the boreal summer season, and in the Amazon all year round. The use of φ as the second metric is beneficial for three reasons: firstly it is uncorrelated with TCR in state-of-the-art climate models and can therefore be considered as an independent metric; secondly, because of its projected time-invariance, the magnitude of φ is better constrained than TCR in the immediate future; thirdly, the use of two variables is much simpler than approaches such as pattern scaling from climate models. Finally we show how using the latest estimates of φ from climate models with a mean value of 1.6 - as opposed to previously reported values of 1.4 - can significantly increase the mean time-integrated discounted damage projections in a state-of-the-art integrated assessment model by about 15%. When compared to damages calculated without the inclusion of the land-sea warming ratio, this figure rises to 65 %, equivalent to almost 200 trillion dollars over 200 years.


107. The effect of regional changes in anthropogenic aerosols on rainfall of the East Asian summer monsoon. Guo, L., E. J. Highwood, L. C. Shaffrey and A. G. Turner (2013) Atmos. Chem. Phys., 13: 1521-1534. Published online 6 February 2013. Authors' preprint | Open access PDF version |
     Abstract
     The response of East Asian Summer Monsoon (EASM) precipitation to long term changes in regional anthropogenic aerosols (sulphate and black carbon) is explored in an atmospheric general circulation model, the atmospheric component of the UK High-Resolution Global Environment Model v1.2 (HiGAM). Separately, sulphur dioxide (SO₂) and black carbon (BC) emissions in 1950 and 2000 over East Asia are used to drive model simulations, while emissions are kept constant at year 2000 levels outside this region. The response of the EASM is examined by comparing simulations driven by aerosol emissions representative of 1950 and 2000. The aerosol radiative effects are also determined using an off-line radiative transfer model. During June, July and August, the EASM was not significantly changed as either SO₂ or BC emissions increased from 1950 to 2000 levels. However, in September, precipitation is significantly decreased by 26.4% for sulphate aerosol and 14.6% for black carbon when emissions are at the 2000 level. Over 80% of the decrease is attributed to changes in convective precipitation. The cooler land surface temperature over China in September (0.8°C for sulphate and 0.5°C for black carbon) due to increased aerosols reduces the surface thermal contrast that supports the EASM circulation. However, mechanisms causing the surface temperature decrease in September are different between sulphate and BC experiments. In the sulphate experiment, the sulphate direct and the 1st indirect radiative effects contribute to the surface cooling. In the BC experiment, the BC direct effect is the main driver of the surface cooling, however, a decrease in low cloud cover due to the increased heating by BC absorption partially counteracts the direct effect. This results in a weaker land surface temperature response to BC changes than to sulphate changes. The resulting precipitation response is also weaker, and the responses of the monsoon circulation are different for sulphate and black carbon experiments. This study demonstrates a mechanism that links regional aerosol emission changes to the precipitation changes of the EASM, and it could be applied to help understand the future changes in EASM precipitation in CMIP5 simulations.


108. Will the South Asian monsoon overturning circulation stabilize any further? Krishnan, R., T. P. Sabin, D. C. Ayantika, A. Kitoh, M. Sugi, H. Murakami, A. G. Turner, J. M. Slingo and K. Rajendran (2013) Clim. Dyn., 40(1): 187-211. Published online 29 February 2012. Authors' preprint |
     Abstract
     Understanding the response of the South Asian monsoon (SAM) system to global climate change is an interesting scientific problem that has enormous implications from the societal viewpoint. While the CMIP3 projections of future changes in monsoon precipitation used in the IPCC AR4 show major uncertainties, there is a growing recognition that the rapid increase of moisture in a warming climate can potentially enhance the stability of the large-scale tropical circulations. In this work, the authors have examined the stability of the SAM circulation based on diagnostic analysis of climate datasets over the past half century; and addressed the issue of likely future changes in the SAM in response to global warming using simulations from an ultrahigh resolution (20 km) global climate model. Additional sensitivity experiments using a simplified atmospheric model have been presented to supplement the overall findings. The results here suggest that the intensity of the boreal summer monsoon overturning circulation and the associated southwesterly monsoon flow have significantly weakened during the past 50-years. The weakening trend of the monsoon circulation is further corroborated by a significant decrease in the frequency of moderate-to-heavy monsoon rainfall days and upward vertical velocities particularly over the narrow mountain ranges of the Western Ghats. Based on simulations from the 20-km ultra high-resolution model, it is argued that a stabilization (weakening) of the summer monsoon Hadley-type circulation in response to global warming can potentially lead to a weakened large-scale monsoon flow thereby resulting in weaker vertical velocities and reduced orographic precipitation over the narrow Western Ghat mountains by the end of the twenty-first century. Supplementary experiments using a simplified atmospheric model indicate a high sensitivity of the large-scale monsoon circulation to atmospheric stability in comparison with the effects of condensational heating.


109. Climate Change and the South Asian summer monsoon. Turner, A.G. and H. Annamalai (2012) Nature Climate Change, 2: 587-595. Published online 24 June 2012. Authors' preprint | Various media activity about this article | University of Hawaii/SOEST press release |
     Abstract
     The vagaries of South Asian summer monsoon rainfall on short and long timescales impact the lives of more than one billion people. Understanding how the monsoon will change in the face of global warming is a challenge for climate science, not least because our state-of-the-art general circulation models still have difficulty simulating the regional distribution of monsoon rainfall. However, we are beginning to understand more about processes driving the monsoon, its seasonal cycle and modes of variability. This gives us the hope that we can build better models and ultimately reduce the uncertainty in our projections of future monsoon rainfall.


110. The effect of Arabian Sea optical properties on SST biases and the South Asian summer monsoon in a coupled GCM. Turner, A.G., M. Joshi, E.S. Robertson and S.J. Woolnough (2012) Clim. Dyn., 39(3-4): 811-826. Published online 11 December 2011. Authors' preprint |
     Abstract
     This study examines the effect of seasonally varying chlorophyll on the climate of the Arabian Sea and South Asian monsoon. The effect of such seasonality on the radiative properties of the upper ocean is often a missing process in coupled general circulation models and its large amplitude in the region makes it a pertinent choice for study to determine any impact on systematic biases in the mean and seasonality of the Arabian Sea. In this study we examine the effects of incorporating a seasonal cycle in chlorophyll due to phytoplankton blooms in the UK Met Office coupled atmosphere-ocean GCM HadCM3. This is achieved by performing experiments in which the optical properties of water in the Arabian Sea - a key signal of the semi-annual cycle of phytoplankton blooms in the region - are calculated from a chlorophyll climatology derived from Sea-viewing Wide Field-of-View Sensor (SeaWiFS) data. The SeaWiFS chlorophyll is prescribed in annual mean and seasonally-varying experiments. In response to the chlorophyll bloom in late spring, biases in mixed layer depth are reduced by up to 50% and the surface is warmed, leading to increases in monsoon rainfall during the onset period. However when the monsoons are fully established in boreal winter and summer and there are strong surface winds and a deep mixed layer, biases in the mixed layer depth are reduced but the surface undergoes cooling. The seasonality of the response of SST to chlorophyll is found to depend on the relative depth of the mixed layer to that of the anomalous penetration depth of solar fluxes. Thus the inclusion of the effects of chlorophyll on radiative properties of the upper ocean acts to reduce biases in mixed layer depth and increase seasonality in SST.


111. Dependence of Indian monsoon rainfall on moisture fluxes across the Arabian Sea and the impact of coupled model sea surface temperature biases. Levine, R.C. and A.G. Turner (2012) Clim. Dyn. 38(11-12): 2167-2190. Published online 24 May 2011. Authors' preprint |
     Abstract
     The Arabian Sea is an important moisture source for Indian monsoon rainfall. The skill of climate models in simulating the monsoon and its variability varies widely, while Arabian Sea cold sea surface temperature (SST) biases are common in coupled models and may therefore influence the monsoon and its sensitivity to climate change. We examine the relationship between monsoon rainfall, moisture fluxes and Arabian Sea SST in observations and climate model simulations. Observational analysis shows strong monsoons depend on moisture fluxes across the Arabian Sea, however detecting consistent signals with contemporaneous summer SST anomalies is complicated in the observed system by air/sea coupling and large-scale induced variability such as the El Niño-Southern Oscillation feeding back onto the monsoon through development of the Somali Jet. Comparison of HadGEM3 coupled and atmosphere-only configurations suggests coupled model cold SST biases significantly reduce monsoon rainfall. Idealised atmosphere-only experiments show that the weakened monsoon can be mainly attributed to systematic Arabian Sea cold SST biases during summer and their impact on the monsoon-moisture relationship. The impact of large cold SST biases on atmospheric moisture content over the Arabian Sea, and also the subsequent reduced latent heat release over India, dominates over any enhancement in the land-sea temperature gradient and results in changes to the mean state. We hypothesize that a cold base state will result in underestimation of the impact of larger projected Arabian Sea SST changes in future climate, suggesting that Arabian Sea biases should be a clear target for model development.


112. Using idealized snow forcing to test teleconnections with the Indian summer monsoon in the Hadley Centre GCM. Turner, A.G. and J.M. Slingo (2011) Clim. Dyn., 36(9): 1717-1735. Published online 11 April 2010. Authors' preprint | UoR press release | UoR Knowledge Exchange Newsletter featuring this work |
     Abstract
     Anomalous heavy snow during winter or spring has long been regarded as a possible precursor of deficient Indian monsoon rainfall during the subsequent summer. However previous work in this field is inconclusive, in terms of the mechanism that communicates snow anomalies to the monsoon summer, and even the region from which snow has the most impact. In this study we explore these issues in coupled and atmosphere-only versions of the Hadley Centre model. A 1050-year control integration of the HadCM3 coupled model, which well represents the seasonal cycle of snow cover over the Eurasian continent, is analysed and shows evidence for weakened monsoons being preceded by strong snow forcing (in the absence of ENSO) over either the Himalaya/Tibetan Plateau or north/west Eurasia regions. However, empirical orthogonal function (EOF) analysis of springtime interannual variability in snow depth shows the leading mode to have opposite signs between these two regions, suggesting that competing mechanisms may be possible. To determine the dominant region, ensemble integrations are carried out using HadAM3, the atmospheric component of HadCM3, and a variety of anomalous snow forcing initial conditions obtained from the control integration of the coupled model. Forcings are applied during spring in separate experiments over the Himalaya/Tibetan Plateau and north/west Eurasia regions, in conjunction with climatological SSTs in order to avoid the direct effects of ENSO. With the aid of idealized forcing conditions in sensitivity tests, we demonstrate that forcing from the Himalaya region is dominant in this model via a Blanford-type mechanism involving reduced surface sensible heat and longwave fluxes, reduced heating of the troposphere over the Tibetan Plateau and consequently a reduced meridional tropospheric temperature gradient which weakens the monsoon during early summer. Snow albedo is shown to be key to the mechanism, explaining around 50% of the perturbation in sensible heating over the Tibetan Plateau, and accounting for the majority of cooling through the troposphere.


113. Is there regime behaviour in Monsoon Convection in the Late 20th Century? Turner, A.G. and A. Hannachi (2010) Geophys. Res. Lett., 37, L16706. Published online 25 August 2010. Authors' preprint |
     Abstract
     Mixture model techniques are applied to a daily index of monsoon convection from ERA-40 reanalysis to show regime behavior. The result is the existence of two significant regimes showing preferred locations of convection within the Asia/Western-North Pacific domain, with some resemblance to active-break events over India. Simple trend analysis over 1958-2001 shows that the first regime has become less frequent while the second becomes much more dominant. Both undergo a change in structure contributing to the total OLR trend over the ERA-40 period. Stratifying the data according to a large-scale dynamical index of monsoon interannual variability, we show the regime occurrence to be strongly perturbed by the seasonal condition, in agreement with conceptual ideas. This technique could be used to further examine predictability issues relating the seasonal mean and intraseasonal monsoon variability or to explore changes in monsoon behavior in centennial-scale model integrations.


114. A regime view of the North Atlantic Oscillation and its response to anthropogenic forcing. Woollings, T.J., A. Hannachi, B.J. Hoskins and A.G. Turner (2010) J. Clim., 23(6): 1291-1307. Published online 15 March 2010. Authors' preprint |
     Abstract
     The distribution of the daily wintertime North Atlantic Oscillation (NAO) index in the 40-yr ECMWF Re-Analysis (ERA-40) is significantly negatively skewed. Dynamical and statistical analyses both suggest that this skewness reflects the presence of two distinct regimes - referred to as "Greenland blocking" and "subpolar jet". Changes in both the relative occurrence and in the structure of the regimes are shown to contribute to the long-term NAO trend over the ERA-40 period. This is contrasted with the simulation of the NAO in 100-yr control and doubled CO2 integrations of the third climate configuration of the Met Office Unified Model (HadCM3). The model has clear deficiencies in its simulation of the NAO in the control run, so its predictions of future behavior must be treated with caution. However, the subpolar jet regime does become more dominant under anthropogenic forcing and, while this change is small it is clearly statistically significant and does represent a real change in the nature of NAO variability in the model.


115. Uncertainties in future projections of extreme precipitation in the Indian monsoon region. Turner, A.G. and J.M. Slingo (2009) Atmos. Sci. Lett. 10(3): 152-168. Published online 3 June 2009. Authors' preprint |
     Abstract
     Uncertainties in changes to the spatial distribution and magnitude of the heaviest extremes of daily monsoon rainfall over India are assessed in the doubled CO2 climate change scenarios in the IPCC Fourth Assessment Report. Results show diverse changes to the spatial pattern of the 95th and 99th subseasonal percentiles, which are strongly tied to the mean precipitation change during boreal summer. In some models, the projected increase in heaviest rainfall over India at CO2 doubling is entirely predictable based upon the surface warming and the Clausius-Clapeyron relation, a result which may depend upon the choice of convection scheme.


116. Subseasonal extremes of precipitation and active-break cycles of the Indian summer monsoon in a climate change scenario. Turner, A.G. and J.M. Slingo (2009) Q. J. R. Meteorol. Soc. 135(640): 549-567. Published online 23 March 2009. Authors' preprint | UoR press release | NERC Science achievement 2007-08 article on this work | Article on this work in the University of Reading's summer 2009 Research Review |
     Abstract
     Changes to the behaviour of subseasonal precipitation extremes and active-break cycles of the Indian summer monsoon are assessed in this study using pre-industrial and 2 x CO2 integrations of the HadCM3 coupled GCM, which is able to reasonably simulate the monsoon seasonal cycle. At 2 x CO2, mean summer rainfall increases slightly, especially over central and northern India. The mean intensity of daily precipitation during the monsoon is found to increase consistent with fewer wet days, and there are increases to heavy rain events beyond changes in the mean alone. The chance of reaching particular thresholds of heavy rainfall is found to approximately double over northern India, increasing the likelihood of damaging floods on a seasonal basis. The local distribution of such projections is uncertain, however, given the large spread in mean monsoon rainfall change and associated extremes amongst even the most recent coupled climate models. The measured increase of the heaviest precipitation events over India is found to be broadly in-line with the degree of atmospheric warming and associated increases in specific humidity, lending a degree of predictability to changes in rainfall extremes. Active-break cycles of the Indian summer monsoon, important particularly due to their effect on agricultural output, are shown to be reasonably represented in HadCM3, in particular with some degree of northward propagation. We note an intensification of both active and break events, particularly when measured against the annual cycle, although there is no suggestion of any change to the duration or likelihood of monsoon breaks.


117. Preferred structures in large scale circulation and the effect of doubling greenhouse gas concentration in HadCM3. Hannachi, A. and A.G. Turner (2008) Q. J. R. Meteorol. Soc. 134(631): 469-480. Published online 4 April 2008.
     Abstract
     Preferred structures in the surface pressure variability are investigated in and compared between two 100-year simulations of the Hadley Centre climate model HadCM3. In the first (control) simulation, the model is forced with pre-industrial carbon dioxide concentration (1 x CO2) and in the second simulation the model is forced with doubled CO2 concentration (2 x CO2). Daily winter (December-January-February) surface pressures over the Northern Hemisphere are analysed. The identification of preferred patterns is addressed using multivariate mixture models. For the control simulation, two significant flow regimes are obtained at 5% and 2.5% significance levels within the state space spanned by the leading two principal components. They show a high pressure centre over the North Pacific/Aleutian Islands associated with a low pressure centre over the North Atlantic, and its reverse. For the 2 x CO2 simulation, no such behaviour is obtained. At higher-dimensional state space, flow patterns are obtained from both simulations. They are found to be significant at the 1% level for the control simulation and at the 2.5% level for the 2 x CO2 simulation. Hence under CO2 doubling, regime behaviour in the large-scale wave dynamics weakens. Doubling greenhouse gas concentration affects both the frequency of occurrence of regimes and also the pattern structures. The less frequent regime becomes amplified and the more frequent regime weakens. The largest change is observed over the Pacific where a significant deepening of the Aleutian low is obtained under CO2 doubling.


118. The effect of doubled CO₂ and model basic state biases on the monsoon-ENSO system. II: Changing ENSO regimes. Turner, A.G., P.M. Inness and J.M. Slingo (2007) Q. J. R. Meteorol. Soc. 133(622): 1159-1173. Published online 10 July 2007. Authors' preprint |
     Abstract
     Integrations of a fully-coupled climate model with and without flux adjustments in the equatorial oceans are performed under 2 x CO2 conditions to explore in more detail the impact of increased greenhouse gas forcing on the monsoon-ENSO system. When flux adjustments are used to correct some systematic model biases, ENSO behaviour in the modelled future climate features distinct irregular and periodic (biennial) regimes. Comparison with the observed record yields some consistency with ENSO modes primarily based on air-sea interaction and those dependent on basinwide ocean wave dynamics. Simple theory is also used to draw analogies between the regimes and irregular (stochastically forced) and self-excited oscillations respectively. Periodic behaviour is also found in the Asian-Australian monsoon system, part of an overall biennial tendency of the model under these conditions related to strong monsoon forcing and increased coupling between the Indian and Pacific Oceans. The tropospheric biennial oscillation (TBO) thus serves as a useful descriptor for the coupled monsoon-ENSO system in this case. The presence of obvious regime changes in the monsoon-ENSO system on interdecadal timescales, when using flux adjustments, suggests there may be greater uncertainty in projections of future climate, although further modelling studies are required to confirm the realism and cause of such changes.


119. The effect of doubled CO₂ and model basic state biases on the monsoon-ENSO system. I: Mean response and interannual variability. Turner, A.G., P.M. Inness and J.M. Slingo (2007) Q. J. R. Meteorol. Soc. 133(622): 1143-1157. Published online 3 July 2007. Authors' preprint |
     Abstract
     The impact of doubled CO2 concentration on the Asian summer monsoon is studied using a coupled ocean-atmosphere model. Both the mean seasonal precipitation and interannual monsoon variability are found to increase in the future climate scenario presented. Systematic biases in current climate simulations of the coupled system prevent accurate representation of the monsoon-ENSO teleconnection, of prime importance for seasonal prediction and for determining monsoon interannual variability. By applying seasonally varying heat flux adjustments to the tropical Pacific and Indian Ocean surface in the future climate simulation, some assessment can be made of the impact of systematic model biases on future climate predictions. In simulations where the flux adjustments are implemented, the response to climate change is magnified, with the suggestion that systematic biases may be masking the true impact of increased greenhouse gas forcing. The teleconnection between ENSO and the Asian summer monsoon remains robust in the future climate, although the Indo-Pacific takes on more of a biennial character for long periods of the flux-adjusted simulation. Assessing the teleconnection across interdecadal timescales shows wide variations in its amplitude, despite the absence of external forcing. This suggests that recent changes in the observed record cannot be distinguished from internal variations and as such are not necessarily related to climate change.


120. The role of the basic state in the ENSO-Monsoon relationship and implications for predictability Turner, A.G., P.M. Inness and J.M. Slingo (2005) Q. J. R. Meteorol. Soc. 131, 607, pgs 781-804. Authors' preprint |
     Abstract
     The impact of systematic model errors on a coupled simulation of the Asian summer monsoon and its interannual variability is studied. Although the mean monsoon climate is reasonably well captured, systematic errors in the equatorial Pacific mean that the monsoon-ENSO teleconnection is rather poorly represented in the general-circulation model. A system of ocean-surface heat flux adjustments is implemented in the tropical Pacific and Indian Oceans in order to reduce the systematic biases. In this version of the general-circulation model, the monsoon-ENSO teleconnection is better simulated, particularly the lag-lead relationships in which weak monsoons precede the peak of El Niño. In part this is related to changes in the characteristics of El Niño, which has a more realistic evolution in its developing phase. A stronger ENSO amplitude in the new model version also feeds back to further strengthen the teleconnection. These results have important implications for the use of coupled models for seasonal prediction of systems such as the monsoon, and suggest that some form of flux correction may have significant benefits where model systematic error compromises important teleconnections and modes of interannual variability.

Non-refereed publications

• Climate Science: A Summary for Actuaries - What the IPCC Climate Change Report 2021 Means for the Actuarial Profession. Connors, S., M. Dionne, G. Hanák, R. Musulin, N. Aellen, M. Amjad, S. Bowen, D. R. Carrascal, E. Coppola, E. D. Moro, A. Dosio, S. H. Faria, T. Y. Gan, M. Gomis, J. M. Gutiérrez, P. Hope, R. Kopp, S. Krakovska, K. Leitzell, D. Maraun, V. Masson-Delmotte, R. Matthews, T. Maycock, S. Paddam, G.-K. Plattner, A. Pui, M. Rahimi, R. Ranasinghe, J. Rogelj, A. C. Ruane, S. Szopa, A. Turner, R. Vautard, Y. Velichkova, A. Weigel and X. Zhang (2022). International Actuarial Association, published March 2022. | PDF |
  Abstract
  This Summary, based on the IPCC Working Group I Sixth Assessment Report released in August 2021, is tailored to the actuarial community to provide helpful insights into what the IPCC report means for the Actuarial profession. The IPCC Working Group I report addresses the most up-to-date physical understanding of the climate system and climate change. It brings together the latest advances in climate science, combining multiple lines of evidence from paleoclimate, observations, process understanding, and global and regional climate simulations to get the clearest picture of past, present, and possible future climate. Actuaries, as risk professionals, need to understand the physical impacts of climate systems and climate changes. Such impacts will affect how risks are underwritten, priced, managed, and reported, whether for general, life or health insurance, pensions, other financial institutions, or social security. It is important for actuaries to understand the magnitude of the potential changes, the uncertainty of their frequency and intensity, and the inherent volatility of such risks. Each of the physical changes analyzed in the latest IPCC Working Group I report could have an impact on human well-being and the long-term sustainability of the environment. Within these changes, actuaries are particularly interested in the effect of climate change on floods, droughts, fires, storms, rise of sea level, air pollution and the long-term effects of climate change. The Summary focuses on the physical changes affecting the most common perils analysed by actuaries and is supplemented with two Annexes on data and regional specificities and a glossary to support its users.


• Indo-UK Joint Monsoon Campaign: Projects under Monsoon Mission. Turner, A.G. (2020). CLIVAR Exchanges, 79 (Special Issue on India's Monsoon Mission): 38-41.


• Preface to the INCOMPASS Special Collection Turner, A. G. and G. S. Bhat (2020) Quarterly Journal of the Royal Meteorological Society, 146(731): 2826-2827, first published 13 August 2020. | The INCOMPASS Special Collection


• The Indian Monsoon in a changing climate Turner, A. G. (2020) Weather 75(1): 18-18. | This article is part of the Climate Change Insights series. | Authors' preprint


• Solving the mysteries of the monsoon. Turner, A. G. Planet Earth (Autumn 2016) Natural Environment Research Council. | PDF of Autumn 2016 Planet Earth.


• RMetS conference workshop: monsoons and their teleconnections. Williams, C. J. R. and A. G. Turner (2013) Atmos. Sci. Letts. 14(1), pgs 52-53. Authors' preprint |
  Abstract
  This short report details the presentations and subsequent discussion that occured during the Monsoons and their teleconnections session, one of the workshops at the 2011 National Conference of the Royal Meteorological Society.


• Atmospheric Low Frequency Variability: The Examples of the North Atlantic and the Indian Monsoon Hannachi, A., T. Woollings and A. G. Turner (2012). In Climate Variability - Some Aspects, Challenges and Prospects (Ed. A. Hannachi) ISBN: 978-953-307-699-7, InTech. PDF |
  Abstract
  (no abstract for this work)


• Forthcoming International workshop on interdecadal variability of the global monsoons Turner, A., B. Wang and C. Ereño. CLIVAR Exchanges 58(Vol 17, No. 1) February 2012, pp53-54.


• Monsoon collapse following super-volcanic eruption? Turner, A. G., B. M. Harris and E. J. Highwood. Extended abstract for the OCHAMP-2012 conference, IITM, Pune, India, 21-25 February, 2012.
  Abstract
  Based on conceptual models, the Asian summer monsoon has been suggested as a tipping element of the Earth System that could shift into a low rainfall regime following suitably large changes in land-use or anthropogenic aerosol loading that lead to changes in planetary albedo. A similar impact on the monsoon system could be effected by super-volcanic eruptions, which populate the stratosphere with sulphate aerosols that scatter solar radiation back to space. Using a series of integrations of the coupled ocean-atmosphere GCM HadCM3, this study considers whether tropical super-volcanic eruptions akin to the Pinatubo eruption of 1991 can lead to monsoon collapse. Using an empirical time and latitude-dependent function for the stratospheric aerosol optical depth (AOD) that closely matches observations following the Pinatubo eruption, we compare the results of 30-year integrations in which the AOD is scaled between one and 100 times that of Pinatubo, to determine at what stage monsoon collapse occurs. We find that in the summer following an eruption of around 15 x Pinatubo, the South Asian monsoon begins to collapse, the south-westerly monsoon flow deflecting from the Indian peninsula towards the equator. At 100 x Pinatubo, the monsoon flow resembles that normally found during winter. Comparison with a 1050-year control integration of the model reveals negative precipitation anomalies over India to be well outside those occurring due to natural variability. We also discuss the time taken for the Asian monsoon to recover following eruption and the likelihood of such an event.


• The effect of Arabian Sea optical properties on SST biases and the South Asian summer monsoon in a coupled GCM. Turner, A. G., M. Joshi, E. S. Robertson and S. J. Woolnough. Extended abstract for the OCHAMP-2012 conference, IITM, Pune, India, 21-25 February, 2012.
  Abstract
  This study examines the effect of seasonally varying chlorophyll on the climate of the Arabian Sea and South Asian monsoon. The effect of such seasonality on the radiative properties of the upper ocean is often a missing process in coupled general circulation models and its large amplitude in the region makes it a pertinent choice for study to determine any impact on systematic biases in the mean and seasonality of the Arabian Sea. In this study we examine the effects of incorporating a seasonal cycle in chlorophyll due to phytoplankton blooms in the UK Met Office coupled atmosphere-ocean GCM HadCM3. This is achieved by performing experiments in which the optical properties of water in the Arabian Sea - key signal of the semi-annual cycle of phytoplankton blooms in the region - are calculated from a chlorophyll climatology derived from Sea-viewing Wide Field-of-View Sensor (SeaWiFS) data. The SeaWiFS chlorophyll is prescribed in annual mean and seasonally-varying experiments. In response to the chlorophyll bloom in late spring, biases in mixed layer depth are reduced by up to 50% and the surface is warmed, leading to increases in monsoon rainfall during the onset period. However when the monsoons are fully established in boreal winter and summer and there are strong surface winds and a deep mixed layer, biases in the mixed layer depth are reduced but the surface undergoes cooling. The seasonality of the response of SST to chlorophyll is found to depend on the relative depth of the mixed layer to that of the anomalous penetration depth of solar fluxes. Thus the inclusion of the effects of chlorophyll on radiative properties of the upper ocean acts to reduce biases in mixed layer depth and increase seasonality in SST.


• The Global Monsoon Systems factsheet, October 2011. Produced as a joint effort among the CLIVAR Asian-Australian Monsoon Panel and the GEWEX Monsoon Asian Hydro-Atmosphere Scientific Research and Prediction Initiative.
  Abstract
  Monsoon rainfall is the life-blood of more than half the world's population, for whom agriculture is the main source of subsistence. Extensive research is being conducted to increase our understanding of monsoon predictability, improve the accuracy of predictions, and refine projections of the impact of man-made climate change on monsoonal systems worldwide. This has the potential to provide significant socio-economic returns by maximizing the benefits of monsoon rainfall and reducing the impact of extreme events such as those witnessed during the northern hemisphere summer monsoon of 2010 in Pakistan, China, and India, and the southern hemisphere summer monsoon of 2011 in Australia.


• Report on WMO IWM-IV, Beijing, October 2008. Turner, A.G. UCLA TROPICAL METEOROLOGY AND CLIMATE NEWSLETTER 85 SECTION A (May 11, 2009)


• Modelling Monsoons: Understanding and predicting current and future behaviour. Slingo, J., A. Giannini, M. Kimoto, C. Roberto Mechoso, G. Meehl, K. Sperber and A. Turner. Proceedings of the Fourth WMO International Workshop on Monsoons (IWM-IV), (Beijing, October 2008) World Meteorological Organization


• Indian summer. Turner, A.G. Planet Earth, (Spring 2007) Natural Environment Research Council. Planet Earth Online version.


• Indian Monsoon: Contribution to Stern Review. Challinor, A.J., J.M. Slingo, A.G. Turner and T. Wheeler (2006). Also available from www.sternreview.org.uk


• UGAMP newsletter article (November 2003). | Web version.

Book chapters

IPCC AR6 Lead Author

• Linking Global to Regional Climate Change. Doblas-Reyes, F. J., A. A. Sörensson, M. Almazroui, A. Dosio, W. J. Gutowski, R. Haarsma, R. Hamdi, B. Hewitson, W.-T. Kwon, B. L. Lamptey, D. Maraun, T. S. Stephenson, I. Takayabu, L. Terray, A. Turner, Z. Zuo (2021). In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press. | Supplementary Material


• IPCC, 2021: Annex V: Monsoons [Cherchi, A., A. Turner (eds.)]. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.

IPCC AR6 Contributing Author

• Technical Summary
• Chapter 2: Changing State of the Climate System
• Chapter 4: Future Global Climate: Scenario-Based Projections and Near-Term Information
• Interdecadal variability of the Asian monsoons. A. G. Turner and B. Wang. Chapter 23 in The Global Monsoon System: Research and Forecast, 3rd Edition, World Scientific Publishing Company, Singapore, submitted May 2014, in press September 2016. Authors' preprint |
  Abstract
  The Asian monsoon represents one of the major modes of the seasonal cycle on the planet and undergoes variations on intraseasonal through interannual time scales that affect the water supply for more than half the global population. But the monsoon also undergoes appreciable variability on decadal time scales, noted both in the Asian monsoon and also as part of more coherent variations in the global monsoon. In the observed data, large variations on multi-decadal time scales are present but are confounded by external drivers including anthropogenic aerosols and greenhouse gas emissions. This review examines some of the mechanisms linking internal multi decadal ocean modes with Asian monsoon decadal variability, including from the Pacific and Atlantic. Since much focus is still being given to seasonal monsoon prediction, we also examine perturbations caused by long time-scale oceanic modes to the monsoon-ENSO teleconnection, affecting seasonal monsoon predictability. Finally, we examine new prospects for decadal monsoon prediction in research considering the CMIP5 decadal hindcasts and forecasts, which are particularly encouraging for Atlantic modes. If future work can establish links between synoptic or intraseasonal monsoon variability and longer-term variations in the mean state, then there is potential for a cascade of predictability, improving our forecasts of the monsoon.


• Modelling Monsoons. K. R. Sperber, E. Cusiner, A. Kitoh, C. R. Mechoso, A. Moise, W. Moufouma-Okia, K. Schiro and A. G. Turner. Chapter 7 in The Global Monsoon System: Research and Forecast, 3rd Edition, World Scientific Publishing Company, Singapore, submitted April 2014, in press July 2016. Authors' preprint |
  Abstract
  The simulation of monsoons remains a challenging problem given the complexity of the multiscale interactions and the modulating influences that operate on a broad range of time scales. Recent studies have demonstrated an improvement in the simulation of monsoons in CMIP5 relative to CMIP3, though many outstanding issues remain. For example, although mean state biases have been reduced in amplitude, the spatial error pattern remains virtually unchanged between the two vintages of experimentation. Systematic errors in the onset time of monsoons indicate that the simulated atmosphere-land-ocean interactions are not responding properly to the annual cycle of solar forcing. As such, the CMIP5 models have early monsoon onset over the Sahel and the North American domain, and late monsoon onset over India, the South American domain, and the Gulf of Guinea. This indicates that a regional process study approach is warranted for improving our understanding of atmosphere-land-ocean interactions inherent to monsoon development and onset. The simulation of intraseasonal variability remains a grand challenge problem, especially given its importance for initiating monsoon onset and being associated with precipitation extremes. Experimental prediction of intraseasonal variability suggests forecast skill to 3 weeks, with the potential for increased skill with the implementation of different projection basis function for boreal summer vs. boreal winter. Despite improvements in the simulation of the El Niño/Southern Oscillation, the interannual monsoon teleconnections are sensitive to biases in the regional rainfall that can compromise the response to the remote forcing. At best, interdecadal variations of Sahel rainfall are qualitatively captured in CMIP5, with the amplitude of the 1970s-1980s drought strongly underestimated. Conversely, mechanisms for observed interdecadal trends in rainfall over East Asia and northern Australia are yet to be understood, and these trends remain to be simulated. Higher horizontal resolution has been beneficial in the representation of orographic rainfall, as well as larger scale aspects of the circulation (e.g., Baiu front) though improvements to model physics are most essential for improving the simulation and prediction of monsoons.


• Modelling Monsoons: Understanding and Predicting Current and Future Behaviour. Turner, A., K. Sperber, J. Slingo, G. Meehl, C. R. Mechoso, M. Kimoto and A. Giannini. In The Global Monsoon System: Research and Forecast, 2nd Edition, World Scientific Series on Asia-Pacific Weather and Climate. Vol. 5, pgs 421-454 (Eds. C.-P. Chang, Y. Ding, G. N.-C. Lau, R. H. Johnson, B. Wang and T. Yasunari), World Scientific Publishing Company, Singapore. ISBN: 978-981-4343-40-4, published 20 March 2011.
  Abstract
  The global monsoon system is so varied and complex that understanding and predicting its diverse behaviour remains a challenge that will occupy modellers for many years to come. Despite the difficult task ahead, an improved monsoon modelling capability has been realized through the inclusion of more detailed physics of the climate system and higher resolution in our numerical models. Perhaps the most crucial improvement to date has been the development of coupled ocean-atmosphere models. From subseasonal to interdecadal timescales, only through the inclusion of air-sea interaction can the proper phasing and teleconnections of convection be attained with respect to sea surface temperature variations. Even then, the response to slow variations in remote forcings (e.g., El Niño-Southern Oscillation) does not result in a robust solution, as there are a host of competing modes of variability that must be represented, including those that appear to be chaotic. Understanding the links between monsoons and land surface processes is not as mature as that explored regarding air-sea interactions. A land surface forcing signal appears to dominate the onset of wet season rainfall over the North American monsoon region, though the relative role of ocean versus land forcing remains a topic of investigation in all the monsoon systems. Also, improved forecasts have been made during periods in which additional sounding observations are available for data assimilation. Thus, there is untapped predictability that can only be attained through the development of a more comprehensive observing system for all monsoon regions. Additionally, improved parameterizations - for example, of convection, cloud, radiation, and boundary layer schemes as well as land surface processes - are essential to realize the full potential of monsoon predictability. Dynamical considerations require ever increased horizontal resolution (probably to 0.5 degree or higher) in order to resolve many monsoon features including, but not limited to, the Mei-Yu/Baiu sudden onset and withdrawal, low-level jet orientation and variability, and orographic forced rainfall. Under anthropogenic climate change many competing factors complicate making robust projections of monsoon changes. Without aerosol effects, increased land-sea temperature contrast suggests strengthened monsoon circulation due to climate change. However, increased aerosol emissions will reflect more solar radiation back to space, which may temper or even reduce the strength of monsoon circulations compared to the present day. A more comprehensive assessment is needed of the impact of black carbon aerosols, which may modulate that of other anthropogenic greenhouse gases. Precipitation may behave independently from the circulation under warming conditions in which an increased atmospheric moisture loading, based purely on thermodynamic considerations, could result in increased monsoon rainfall under climate change. The challenge to improve model parameterizations and include more complex processes and feedbacks pushes computing resources to their limit, thus requiring continuous upgrades of computational infrastructure to ensure progress in understanding and predicting the current and future behaviour of monsoons.

Thesis

• Behaviour of the monsoon-ENSO system in current and future climates of a general circulation model. Turner, A.G. University of Reading Ph.D. Thesis, supervised by Prof. Julia Slingo and Dr. Pete Inness (December 2005). Examined by Prof. Mojib Latif and Prof. Anthony Illingworth. British Library record for this thesis.