UCLA

Degradation of the land surface has been suggested as a cause of persistent drought in tropical north Africa. A general circulation model is used to assess the impact of degradation of five regions within tropical north Africa. Idealized degradation scenarios are used since existing observations are inadequate the determine the extent and severity of historical degradation. It is found that the impact of degradation varies between the regions. The greatest effects are found from degradation of the Sahel or West Africa, which result in substantial reduction of precipitation over the degraded area. Both surface evaporation and atmospheric moisture convergence are reduced. In the Sahelian case the precipitation reduction extends well to the south of the area of changed land surface. The occurrence of easterly wave disturbances is not altered by degradation, but the mean rainfall from each event is reduced. Degradation of an area in eastern north Africa results in smaller reductions of precipitation and moisture convergence. Finally, degradation of a southern area next to the Gulf of Guinea has little effect on precipitation because of a compensatory increase of moisture convergence. The simulated rainfall reduction following degradation of the Sahel is comparable to observed changes in recent decades, suggesting that degradation may have contributed to that change.