Diagnosis of the tropical Atlantic near-equatorial SST bias in a directly coupled atmosphere-ocean general circulation model

The current generation of non-flux-corrected coupled atmosphere-ocean general circulation models (CGCMs) have trouble correctly simulating the sign of the annual mean near-equatorial east to west gradient of sea surface temperature (SST) in the Atlantic Ocean. This model pathology is of concern because the distribution of tropical oceanic precipitation is related to the near-equatorial SST distribution. The tropical oceanic precipitation, in turn, influences local and remote precipitation over the tropical land areas through various teleconnection mechanisms. Therefore, understanding and eventually fixing this model error is of interest. In this study, the cause of the Atlantic equatorial SST gradient error in one CGCM is investigated using forced experiments with the CGCM ocean component model. These experiments show that the most likely candidate for this error is the too-weak zonal wind stress along the equator in the coupled model. This wind stress error affects the SST along the equator in two ways. First, it leads to a deepening of the thermocline in the eastern part of the basin and a shallowing in the western part. Second, the weak zonal stress leads to a vertical velocity distribution at the based of the mixed layer that is too weak in the eastern and central Atlantic and too strong in the western Atlantic. Both of these errors lead to insufficient cooling of the eastern near equatorial mixed layer and erroneously enhanced cooling in the western near equatorial mixed layer.