BIOSPHERE FEEDBACK ON REGIONAL CLIMATE IN TROPICAL NORTH-AFRICA

The impact of land-surface degradation over the Sahel area on seasonal variations of atmospheric and hydrological components over tropical n orth Africa are investigated in a general-circulation model numerical experiment, together with the mechanisms involved. The model was integ rated for four years with and without vegetation change over the Sahel region with different initial atmospheric conditions. The results dem onstrate that the degradation of the land surface can have a significa nt impact on the Sahelian regional climate. It increases the surface a ir 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 to the specified desertification area a nd the JAS season but is found also to the south of this area and exte nds into the October-November-December season. The changes in the annu al rainfall cycle and the JAS mean surface temperature over the Sahel area are consistent with the observed climate anomalies of the past 40 years. The changes in rainfall to the south of the Sahel including ea stern Africa are also in line with the observed anomalies. The reducti on in total diabatic heating rate and relative increase in subsidence motion in the upper troposphere are consistent with the rainfall anoma lies. The variations in convective heating rate, which were caused by changes in latent-heat flux from the land surface and moisture flux co nvergence in the atmosphere, are the dominating factors in this proces s. The radiative cooling is a secondary effect. The influence of the i nitial conditions on the simulation of the soil water balance is also analysed.