Equatorial Atlantic variability and its relation to mean state biases in CMIP 5

2 ABSTRACT Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. 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. Westerly surface wind biases in boreal spring play an important role by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the Intertropical Convergence Zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. This dependency vanishes when the ITCZ is located near to or south of the equator. Despite serious mean state biases, several models are able to capture some aspects of observed interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the bore-al spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.