Interaction of the Atlantic Equatorial Cold Tongue and the African Monsoon

Abstract

The seasonal cycle of equatorial Atlantic sea surface temperature (SST) is characterized by a rapid cooling from April to July, coinciding with the onset of the West African summer monsoon and followed by a slow warming that lasts 3 times longer. Two ensemble simulations are carried out with an atmospheric general circulation model to investigate the mechanisms for the wind changes that cause this rapid oceanic cooling and its feedback onto the African monsoon. In the control simulation, SST is globally prescribed in its full climatological seasonal cycle, while in the second simulation, equatorial Atlantic SST is held constant in time from 15 April onward. Comparison of these simulations indicates that the equatorial cooling exerts a significant influence on the African monsoon, intensifying the southerly winds in the Gulf of Guinea and pushing the continental rainband inland away from the Guinean coast. The intensification of the cross-equatorial southerlies associated with the onset of the African monsoon, in turn, triggers the oceanic cooling in the east. The equatorial easterlies are also important for the seasonal cooling by inducing local upwelling and raising the thermocline in the east. Three mechanisms are identified for the easterly wind acceleration in the equatorial Atlantic in boreal summer. First, the monsoon rainfall distribution is such that it induces zonal sea level pressure gradients and easterly anomalies in the eastern Atlantic. Second, the strong cross-equatorial southerlies advect the easterly momentum from the south into the equator. Finally, zonal pressure gradients associated with the equatorial ocean cooling accelerate surface easterly winds in the middle and western Atlantic. This interaction of equatorial SST and zonal wind causes their westward copropagation, analogous to that in the equatorial Pacific.