Friday 04/09/2015

Objects

1) The net downward surface flux trend (1988-2008) over central Eastern Pacific shows disaggrement between data sets from ERA-Interim atmospheric reanalysis and atmospheric model simulations (Liu et al. 2015, JGR).
2) This area is very important for the understanding of recent "hiatus" (Kosaka and Xie, 2013; Trenberth and Fasullo, 2013; Meehl et al., 2014].
3) Latent heat (LH) change dominates the net surface flux change over the oceans.
4) Investigation using model simulations is complicated, a simple method is used here. LH (see Singh et al 2005) depends on 4 variables: SST, MSLP (mean sea level pressure), wind speed (U) and total column water vapour content (WV).
5) Turbulent energy (LH and SH) is calculated using bulk formula in models and the formula vary from model to model, it is essential to use a unified formula system to compare the results.
6) Identify the sources causing the LH differences.


1. LHs from models and Bulk formula have good agreement in general

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Fig. 1: Anomaly time series of the area mean LH over the global ocean from atmospheric reanalysis (ERAINT, ERA20C and 20thCR) and atmospheric high resolution model simulations (UPSCALE). The reference period is 2001-2005. Twelve month running mean is applied. Lines from UPSCALE are 5 member mean. The LHs calculated from model generated specific humidity at 2m and 1.5 for 20thCR and UPSCALE are also plotted to show the agreement with those using derived specific humidities.

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Fig. 2. The LHF trend over 1988-2008 from model output (left column) and bulk formula (right column) for ERAINT, ERA20C, 20thCR and UPSCALE data sets. The spatial correlations over the global ocean are 0.7, 0.6, 0.6 and 0.8, and 0.7, 0.7, 0.8, 0.8 over oceans between 30oS-30oN. The trend unit is W/m2/yr.
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Fig. 2a. As Fig. 2.

2. Sensitivity test of LH variations with different field combinations


ERA20C to ERAINT

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Fig. 3: The LHF trend sensitivity test using bulk formula. (a) ERA20C, (b) using ERAINT MSLP, (c) using ERAINT SST, (d) using ERAINT U, (e) using ERAINT WV and (f) using ERAINT wind and WV.


20thCR to ERAINT

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Fig. 4: The LHF trend sensitivity test using bulk formula. (a) 20thCR, (b) using ERAINT MSLP, (c) using ERAINT SST, (d) using ERAINT U, (e) using ERAINT WV and (f) using ERAINT wind and WV.


UPSCALE to ERAINT

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Fig. 5: The LHF trend sensitivity test using bulk formula. (a) UPSCALE, (b) using ERAINT MSLP, (c) using ERAINT SST, (d) using ERAINT U, (e) using ERAINT WV and (f) using ERAINT wind and WV.


SSM/I to ERAINT

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Fig. 6: The LHF trend sensitivity test using bulk formula. (a) SSM/I, (b) using ERAINT U, (c) using ERAINT WV and (d) using ERAINT wind and WV.


3. Sources of discrepancies

Trends of SST, MSLP, U and WV

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Fig. 7: Trends of 4 variables from different data sets.



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Fig. 8: As Fig. 7, but for MERRA, JRA55, JRA55C data sets.