Sensitivity of the West African Monsoon to Atlantic SST Anomalies

The sensitivity of precipitation over West Africa to SSTAs in the Gulf of Guinea and the eastern North Atlantic is studied using a GCM. Results from 9 perpetual July simulations with various imposed SSTAs are presented and analyzed to reveal associations between the precipitation and SST fields via large-scale circulation and atmospheric moisture anomalies.

Rainfall increases over the Guinean Coast and decreases over the Congo Basin when warm SSTAs are present in the Gulf of Guinea. These precipitation perturbations are related to the forcing of a Kelvin and a Rossby wave. The former is associated with a weakening of the Walker circulation, while the latter strengthens the West African monsoon. Rainfall over West Africa is less sensitive to cold anomalies than to warm anomalies due to the non-linearity of the Clausius-Clapeyron equation. It is also less sensitive to eastern North Atlantic SSTAs than to Gulf of Guinean SSTAs due to the latitude difference of the forcing and the background flow. A superposition of the individual responses to SSTAs is shown to be a poor prediction of the response to combined SSTAs.

A comparison of the model results to the summer seasons of 1988 and 1994 from the satellite observations and the NCEP reanalysis is conducted. The model results compare favorably at the process level, suggesting that this kind of analysis could be used to improve our prediction capabilities for precipitation over West Africa.