Dr. Linda Hirons
MSc student project supervision
2015 | 2014 | 2013 | 2011University of Reading (UoR)
- Karl Johari Chan (2015): The Impact of the Madden-Julian Oscillation on Extreme Rainfall in Malaysia: A case study on the December 2014-January 2015 Flood with Dr. Nick Klingaman.
- Verity Pitts (2015): MJO European weather teleconnections: Why do we not see linkages? with Dr. Nick Klingaman and Dr. Steve Woolnough (UoR).
- Elizabeth Moore (2014): The role of air-sea coupling on variablity in the stratosphere with Dr. Nick Klingaman and Dr. Andrew Charlton-Perez (UoR).
- Vincent Amelie (2014): The relationship between the Madden-Julian Oscillation and wet-season rainfall in the Seychelles with Dr. Nick Klingaman (UoR).
- Eadaoin Clarke (2013): Prediction of the MJO in the ECMWF monthly forecast system with Dr. Steve Woolnough (UoR) and Warwick Norton and Dan Rowlands (Industry partner- City Financial).
- Joseph Portuphy (2011): Methods for comparing GMet rain gauge data with TRMM satellite observations with Dr. Leonard Amekudzi (KNUST)
The Madden-Julian Oscillation (MJO) is a dominant mode of intra-seasonal variability in the tropical equator. It is described as a coupling between the atmospheric circulation and deep convection that propagates eastward around the equator with a period of 30-90 days. The MJO has a major influence on the weather in the tropics, but its impact on rainfall in Malaysia is scarcely documented. This dissertation examines the influence of the MJO on extreme rainfall in Malaysia. A case study was conducted on a major flood that occurred in West Malaysia between December 2014 and January 2015 with the aim of understanding how the MJO modulates rainfall during this specific incident.
Daily rainfall data from 2005-2015 collected from six weather stations around West Malaysia were utilized. Reanalysis data composites of the 850 hPa zonal wind and columnar precipitable water anomalies from 1985-2014 were also used. Simple statistical methods were applied to investigate how different MJO phases modified the probability of rainfall. The results were compared with the reanalysis data to verify the relationship between the MJO, westerly winds and rainfall.
The results suggest that a correlation exist between the MJO and rainfall. The analysis on the MJO and rainfall probabilities established that MJO in phases 2, 3 and 4 are associated with enhanced convection and rainfall, whereas phases 6, 7 and 8 represent supressed convection and rainfall. Specifically during the wet season, the results also show increased rainfall probabilities in phase 1. The rainfall in West Malaysia initiated when the MJO was located over the Indian Ocean (phase 2 and 3) instead of the Maritime Continent (phase 4 and 5). Reanalysis data between the westerly wind anomalies and precipitable water indicated that the moisture in the atmosphere builds-up during the supressed phase. The moisture content then decreases during the active phase due to precipitation. Finally, the case study show that MJO in phases 3, 4 and 5 increased rainfall probabilities between December 2014 and January 2015. The combination of increased rainfall intensities within a short period had contributed to the flooding. Therefore, the analysis from the daily rainfall and reanalysis data predicted the results observed in the case study. Therefore, this dissertation was able validate that the MJO does have a strong influence on rainfall in Malaysia. However, more research is required to understand the extent of the MJO's mechanism and impact on rainfall in Malaysia.
For the European energy industry the Madden-Julian Oscillation (MJO), and its impact on North Atlantic weather, provides a welcome source of predictability at longer lead times. The all-time record MJO amplitude on March 16th, 2015 led to a response across the North Atlantic that did not follow an expected pattern. This study examines ECMWF monthly forecast data and observed data in order to explore the extra-tropical response to this MJO event and its relationship to the tropical forcing. The MJO amplitude and phase, measured by the Wheeler and Hendon (2004) RMM index, are commonly used as two predictors in forecasting responses for the North Atlantic Oscillation (NAO). However, for this event the observed North Atlantic response was not expected given the measured MJO amplitude. A component of the index during this event, the amplitude of the convective component, was found to be a better predictor for the observed NAO. The dynamics of the teleconnection rely strongly on the convective forcing from the tropics. The convective signal associated with this MJO event was weaker than expected given the overall MJO amplitude and this may explain the unexpected extra-tropical response. There was a weak relationship between the MJO amplitude as measured by the Wheeler and Hendon (2004) index and convective amplitude, thus the MJO amplitude was limited in its ability to represent the convective part of MJO dynamics. For weak convection MJO events like this it is suggested that an alternative metric to the MJO amplitude be used to the forecast the extra-tropical response in the NAO.
The Madden-Julian Oscillation (MJO) is the main form of sub-seasonal variability across the tropics. It is described as a coupling of convection to the large-scale circulation that moves eastward around the tropics within a period of 30-90 days. It affects weather in many parts of the world, but active convection is more prominent over the Indian and western Pacific Oceans where the sea surface temperature is warmer. The MJO has major influence on the active-break phases of the Indian and Australian monsoons. But little is known about its impact on rainfall in the Seychelles. To determine whether there is any relationship and if there is, whether it is significant enough to be used to improve forecasting skill, Seychelles daily rainfall is compared against different phases and amplitudes of the MJO using simple statistical approaches.
The study reveals that there is no direct correlation between the daily MJO amplitude and daily rainfall. There is however a relationship between the MJO phase and the rainfall probabilities. MJO phases 6 and 7 are both associated with 'suppressed convection' over the Indian Ocean and 'enhanced convection' in the Pacific Ocean. Surprisingly, MJO phase 6 has the greater influences over the Seychelles and increases the probability of daily rainfall, whereas phase 7 has the stronger influence to increase the probability of consecutive days of rainfall above a rainfall threshold that can cause flooding. The influence of these two phases suggests that regions of suppressed convection can still produce rainfall above the climatological values. This was supported by wind computation by showing westerly wind anomalies in these same phases, an indication of an unusual weather system such as tropical cyclones in the vicinity. MJO phase 4 has the greatest potential to reduce rainfall. The influence of MJO phases 1, 2, 3, and 5 is not strong and consistent, whereas phase 8 shows no major impact as expected.
The study concludes by proposing other type of datasets that could be used in future research and also some of the specific investigations that could be done to help explain the results obtained in this project.
The 30-90 day oscillation associated with eastward propagating convection anomalies around the tropics, known as the Madden-Julian Oscillation (MJO), is one of the most important features of tropical subseasonal variability. The MJO is a complex system which is still not fully understood. Teleconnections associated with the MJO may provide a potential source of predictability in the extratropics.
Using different methods of verification techniques, including bivariate correlations and contingency tests amongst others, this project analyses 3 years of data to evaluate the skill of the European Centre for Medium-Range Weather Forecasts (ECMWF) monthly forecast system at forecasting the MJO. The ECMWF monthly forecast is a 51-member ensemble of forecasts with a 32-day lead time. The index used to identify MJO events in this project is constructed in a similar way to the Wheeler and Hendon index.
This study revealed that the ensemble mean forecast has better skill than any individual member and showed a skill out to 23 days. The model shows a better ability at forecasting strong active MJO events. The phase dependence of the forecast skill is highly variable, phases 2 and 3 (Indian Ocean) often show a poorer MJO propagation. Tests also found that the model has a relatively good skill at capturing successive MJO events, although it suffers in its ability to accurately predict primary MJO events. These results also suggest that at later lead time, amplitude is the predominant aspect which causes the reduction in forecast skill. This project produced varied results when different methods of verification were used, which was unexpected. The ensemble spread of the forecast was analysed and it was found that initially the forecast has small spread suggesting the ensemble forecast is confident at identifying MJO events. When the forecast is dependent on amplitude the ensemble becomes split with approximately half the forecasts identifying a strong active MJO event and half identifying a weak MJO event at later lead times.
The skill of the ECMWF monthly forecast system is highly dependent on the amplitude and phase of the MJO. The majority of tests find that amplitude is the dominant factor in determining the skill of the forecast. A larger data-set would provide a better evaluation of the model skill. Further studies on the initiation of MJO events would also provide additional information to this study. Since MJO events display such varying characteristics in terms of seasonality, amplitude and phase, it is challenging to assess the true skill of the forecast system.