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Modelling SST Variability in the Equatorial Atlantic Ocean
Reference
Richter, I., Xie, S.-P., Behera, S.K., Doi, T. and Masumoto, Y. 2014. Equatorial Atlantic variability and its relation to mean state biases in CMIP5. Climate Dynamics 42: 171-188.

Background
The authors write that "the tropical Atlantic is characterized by significant inter-annual variability in sea surface temperatures (SSTs) that exert an important influence on precipitation over the surrounding continents," citing Folland et al. (1986) and Nobre and Shukla (1996).

What was done
Richter et al. evaluated the performance of several general circulation models (CGMs) that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5), based on real-world observations and reanalysis products that characterize the evolution of Atlantic Niņos in terms of surface winds, thermocline depth and SSTs, focusing on the lag between western equatorial surface wind forcing and cold tongue SST response.

What was learned
The five researchers report that (1) "in terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations," that (2) "the seasonal evolution of model biases ... involves [i] weakening of the equatorial easterlies in boreal spring, [ii] subsequent deepening of the eastern equatorial thermocline, and [iii] maximum cold tongue SST bias during the boreal summer upwelling season," that (3) "March-April-May surface wind biases are incipient in the atmospheric model components forced with observed SST," that (4) "they are associated with precipitation biases over the adjacent landmasses and a southward shift of the marine ITCZ [Intertropical Convergence Zone]," that (5) "both [i] magnitude and [ii] seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that [interim] improvements have only been modest," and, finally, that (6) "the weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SSTs."

What it means
If one is looking to foresee the future, CMIP5 models do not look too promising ... at least at the present time.

References
Folland, C.K., Palmer, T.N. and Parker, D.E. 1986. Sahel rainfall and worldwide sea temperatures. Nature 320: 602-607.

Nobre, P. and Shukla, J. 1996. Variations of sea surface temperature, wind stress, and rainfall over the tropical Atlantic and South America. Journal of Climate 9: 2464-2479.

Reviewed 2 April 2014