Mechanisms by which Gulf of Guinea and eastern North Atlantic sea surface temperature anomalies can influence African rainfall

The sensitivity of precipitation over West Africa to sea surface temperature anomalies (SSTAs) in the Gulf of Guinea and the eastern North Atlantic is studied using a GCM. Results from nine 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 SSTAs than to warm anomalies. Three contributing factors are identified as follows: 1) latitude of the SST forcing, 2) background flow, and 3) nonlinearity of the Clausius- Clapeyron equation (no more than a 20% effect). Despite the relative insensitivity to eastern North Atlantic SSTAs alone, a superposition of the individual responses to SSTAs is shown to be a poor predictor of the response to combined SSTAs, especially over central northern Africa. A comparison of the modeled moisture budget anomalies to the difference between the summer seasons of 1988 and 1994 from the satellite observations and the NCEP reanalysis is conducted. While there may be many causes of precipitation differences between two particular years, the moisture budget anomalies are similar in that enhanced precipitation along the Guinean coast is supported mainly by low- level wind convergence from the south. The role of advection is also similar in the model and the reanalysis. However, the precipitation decrease over the Congo Basin that is associated with the Kelvin wave response to Gulf of Guinea SSTs in the model is not evident in the observations for these 2 yr.