James A. Carton and Bohua Huang

Sea surface temperature in the eastern equatorial Atlantic Ocean undergoes anomalous warming events of $1-2 deg C$ every few years. The warm anomalies reach their maximum strength in northern hemisphere summer, when equatorial upwelling normally brings cold thermocline water to the surface. By compositing surface observations from a 28-year record, we are able to identify consistent features in anomalies of SST and winds. The composites show that the SST anomalies in northern summer are confined to the eastern equatorial region, with reduced zonal winds to the west and reduced northward trade winds to the east. Accompanying these changes in winds are enhanced convection near the equator caused by a southward shift and intensification of the Intertropical Convergence Zone. Later in the year SST south of the equator becomes elevated. As a result, by spring of the year following the equatorial anomaly, convection in the western side of the basin is much higher than normal.

To understand the ocean dynamics that give rise to these warm anomalies we examine a simulation of the ocean circulation during the 1980s. We find that the cause of the warm event in 1984 stretches back to the intense trade winds during the summer and fall of 1983. The unusual winds led to Ekman-deepening of the thermocline in the west on both sides of the equator. Late in 1983 the trade winds in the west relaxed, which led to a surge of warm water eastward along the equatorial waveguide. The arrival of anomalous warm water deepened the thermocline throughout the eastern Gulf of Guinea in early 1984, and gradually spread southward and back into the interior basin throughout that year. Secondary factors in elevating equatorial SST were the local advection of warm surface water from the north and a reduction of advection of cool coastal water from the east. In contrast with 1984, the anomalous warming of 1988 seems to have been largely the result of changes in the equatorial winds during spring of the same year. These wind anomalies are likely, themselves, to have resulted from the increase in SST to the east. During both years anomalous deepening of the thermocline in the east (however it was caused) prevented the normal seasonal cooling of the equatorial waters and thus led to elevated SSTs. The eastward shift of heat also had important consequences for the coastal regions of southern Africa.