While there are disparities in model simulations of precipitation trends in West Africa, there is evidence of an upward trend in rainfall over the wet Cameroon highlands and southern Nigeria. Over eastern Guinea and the Grain Coast, there are indications for a drying trend. Under greenhouse gas forcing the selected models project an increased severity of extremely dry events over the Sahel and the Sahara, with less severe droughts over the Gulf of Guniea coast. The model predictions show an increase in 10-year high rainfall events along the Gulf of Guinea coast and vice versa further north in the Sahel. West Africa is characterised by very steep rainfall gradients. The climatology obtained from the Climate Research Unit (CRU) TS 2.1 dataset show two summer precipitation maxima along the Guinean Coast and near the Cameroon highlands. Along the zonal band there is a relative precipitation minima between 5°W and 3°E, while the northern parts of the West Africa (i.e. the Sahel; between 9°N and 15°N, and the Sahara) are amongst the world's driest places.

Owing to the oberved spatial inhomogeneities in precipitation patterns, regions which experience coherent rainfall anomalies in a given season (defined from April to November) have been demarcated. The regionalization (using cluster analysis) has been based on the observed precipitation for the period 1931-1990. The changes in mean as well as extreme precipitation are not uniform across the whole of West Africa.

With repect to changes in extremely wet events, the models give an indication for increase in 10-year highest rainfall events. Over the Sahel there are projections for lesser extreme wet events in future climate. Though the MIUB ECHO G model gives an opposite indication to that of the other models, even in this model there is a dipole pattern in the expected changes between the Gulf and the Sahel. With sea-surface temperatures in southeastern Atlantic Ocean projected to be warmer in the future climate, wetter extremes in the Guinean coast seem more plausible than in the Sahel.

With exception of the MIUB ECHO G model, the selected models project more severe dry extremes in the Sahel. A decrease by more than 20% is predicted in the 10-year driest event. As is the case with the wet extreme, there is a dipole pattern of change with driest extremes along the Gulf of Guinea coast likely to become less severe.