Atmospheric response to seasonal changes in sea surface temperature during the boreal summer in the Tropical Atlantic

We investigate the atmospheric response to seasonal variations in sea surface temperature (SST) in the eastern tropical Atlantic during the boreal summer, using the Weather Research and Forecasting (WRF) regional atmospheric model. Three ensembles of 11 simulations each are produced with different SST forcings: the control ensemble (CTL) uses the observed climatology of the SST in 2000-2009, while the Frozen North (FzN) and Frozen South (FzS) experiments block the seasonal warming or cooling of the SST from June onwards in a region confined to the eastern tropical Atlantic. The result is a cold SST anomaly in the northeastern tropical Atlantic off the coasts of Senegal and Mauritania in FzN, and a warm anomaly in the southeastern region (Gulf of Guinea and the cold tongue zone in the equatorial Atlantic) in FzS. Comparison with CTL reveals significant impacts of these SST anomalies on the position and intensity of the marine intertropical convergence zone (ITCZ) and on West African rainfall during July and August. Over the ocean, the cold anomaly in NETA suppresses convection on the northern side of the ITCZ (north of 10 degrees N), while the warm anomaly in the Gulf of Guinea strengthens convection on its southern flank. The latter is also leading to a sharp increase in precipitation in the coastal regions to the northeast of the Gulf of Guinea. These changes are clearly due to variations in surface pressure gradients and the divergence of low-level moisture in response to SST anomalies, which in turn induce changes in deep atmospheric convection through thermodynamic feedback. On the continent, a substantial reduction in precipitation is observed in the western Sahel (particularly Senegal) following the cold anomaly in NETA, and in the eastern Sahel following the warm anomaly in the Gulf of Guinea: both are explained by a positive anomaly in the divergence of moisture transport in the upper troposphere, associated with an acceleration of the African easterly jet along its southern edge. However, the mechanism by which the SST anomalies create this acceleration in both experiments remains to be elucidated.