Interaction of the Atlantic equatorial cold tongue and African monsoon

Seasonal cycle of sea surface temperature (SST) in the equatorial Atlantic is characterized by a rapid cooling from April to July. With the onset of summer monsoon over West Africa, enhanced cross-equatorial southeasterly winds cool the equatorial ocean through Ekman upwelling and thermocline shoaling in the east. Previous studies suggest that the ocean dynamics plays more important role in this Atlantic seasonal cooling than in its Pacific counterpart. Surface winds over the ocean, on the other hand, are strongly influenced by the surrounding continents. Our GCM experiments show that the summer easterly acceleration is largely forced by the continental monsoon in the Gulf of Guinea while the air-sea interaction is essential in the central and western basin, much like in the Pacific (Okumura and Xie, J. Climate, 2004).

Whereas the annual harmonic is dominant in equatorial Atlantic SST, the easterly wind and thermocline depth show significant semiannual signals in the east. The easterlies accelerate in October-November, resulting in a shoaling of the thermocline. Using high-resolution satellite data, we show that the central Atlantic SST decreases from late November to early December in response to the accelerated easterlies and the shoaling thermocline. This secondary cooling has been captured well in some widely used climatologies because of their low monthly resolution. The seven-year PIRATA observations support the existence of a secondary seasonal cooling in November-December, suggesting a stronger thermocline feedback on SST than previously thought. Further studies will be needed to elucidate the mechanism for the November-December easterly reacceleration and its influence on the ocean.