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 Atlantic sea surface temperature (SST) variability in the dynamics of the west African monsoon. A hypothesis that a warm spring (April-June) SST results in a wet monsoon is explored using a moist, zonally symmetric model. A positive 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 three 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 found to be important for relating tropical Atlantic spring SST to west African rainfall. Furthermore, the positive feedback between rainfall and soil moisture 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 1994 was associated with abundant summer rainfall over west Africa. These empirical observations are consistent with the proposed physical mechanism.

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