Modeling the impact of land surface degradation on the climate of tropicalnorth Africa

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 a ssess the impact of degradation of five regions within tropical north Afric a. Idealized degradation scenarios are used since existing observations are inadequate to 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, w hich result in substantial reduction of precipitation over the degraded are a. Both surface evaporation and atmospheric moisture convergence are reduce d. In the Sahelian case the precipitation reduction extends well to the sou th of the area of changed land surface. The occurrence of easterly wave dis turbances is not altered by degradation, but the mean rainfall from each ev ent is reduced. Degradation of an area in eastern north Africa results in s maller reductions of precipitation and moisture convergence. Finally, degra dation of a southern area next to the Gulf of Guinea has little effect on p recipitation because of a compensatory increase of moisture convergence. Th e simulated rainfall reduction following degradation of the Sahel is compar able to observed changes in recent decades, suggesting that degradation may have contributed to that change.