WEST AFRICAN RAINFALL DEFICITS AND SEA-SURFACE TEMPERATURES

Comparisons between years of below average rainfall over West Africa, sea-surface temperatures (SST) over the Atlantic Ocean and the world ocean, and latitudinal positions of the Inter Tropical Convergence Zone (ITCZ) over the Atlantic Ocean show that the relationships depend mainly on the rainfall anomaly patterns. The well-known SST dipole (cold northern ocean and warm equatorial and southern ocean) is only apparent during those August months with below average Sudano-Sahelian rainfall and above average Guinean rainfall (rainfall type'-/+') and an abnormal southward position of the ITCZ. In contrast, those August months that experience rainfall deficits over the whole of West Africa (rainfall type '-/-') are associated with warm SST anomalies over the eastern Pacific Ocean, cold persistant SST anomalies over the equatorial Atlantic Ocean and a more northward position of the ITCZ. Those patterns first appear in northern spring before the Sahelian rainy season. The composite SST differences for the '-/+' and '-/-' rainfall types computed with August or July-September amounts have a good resemblance with each other. Comparison of results related to the reverse July-September rainfall patterns (the '+ / -' and '+ / +' patterns) during the 1950s shows that the SST anomalies were globally colder when, on average, the Sahelian rainy seasons experienced significant excesses while the Guinean little dry seasons were more marked.