The impact of mechanisms of the land surface degradation over the Sahel area on seasonal variations of atmospheric and hydrological components over tropical norht Africa are investigated in this general circulation model (GCM) numerical experiment study. The model was integrated for four years with and without vegetation change over the Sahel region with different initial atmosheric conditions. The results demonstrate that the degradation of the land surface can have a significant impact on the Sahelian regional climate. It increases the surface air temperature, and reduces the precipitation, runoff and soil moisture over the Sahel region during the July-August-September (JAS) season. The impact is not only limited within the specified desertification area and during the JAS season but also to the south of this area and during the October-November-December (OND) season. The changes in the reainfall annual cycle and the JAS mean surface temperature over the Sahel area are consistent with the observed climate anomalies of the past forty years. The changes in rainfall to the south of Sahel including eastern Africa are also in line with observed anomalies.
The reduction in total diabatic heating rate and relative increase in subsidence motion in the upper troposphere are consistent with the rainfall anomalies. The variations in convective heating rate, which were caused by changes in latent heat flux from the land surface and moisture flux convergence in the atmosphere, are the dominating factors in this process. The radiative cooling is a secondary effect. The influence of the initial conditions on the simulation of the soil water balance is also analyzed.
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Center for Ocean-Land-Atmosphere Studies
last update: 1 July 1996
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