A mechanism relating tropical Atlantic spring sea surface temperature and west African rainfall

In this paper, we describe a mechanistic study on the role of tropical Atla ntic sea surface temperature (SST) variability in the dynamics of the west African monsoon. A hypothesis that a warm spring (April-June) SST results i n a wet monsoon is explored using a moist, zonally symmetric model. A posit ive spring rainfall anomaly has been simulated over the ocean, in response to the warm SST, which then propagates onto the land and persists two to th ree months, even after the SST anomaly vanishes. While the ocean-atmosphere interaction is crucial for the initial development of the rainfall anomaly over land, the interactions between the ocean, land, and atmosphere are fo und to be important for relating tropical Atlantic spring SST to west Afric an rainfall. Furthermore, the positive feedback between rainfall and soil m oisture is responsible for some of the persistence in the rainfall anomaly. We present a case-study for the wettest (1994) and driest (1992) years of the 1990s so far. The observations show that a warm spring SST anomaly in 1 994 was associated with abundant summer rainfall over west Africa. These em pirical observations are consistent with the proposed physical mechanism.