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 north Africa are investigated in a general-circulation model numerical experiment, together with the mechanisms involved. The model was integrated for four years with and without vegetation change over the Sahel region with different initial atmospheric 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 to the specified desertification area and the JAS season but is found also to the south of this area and extends into the October-November-December season. The changes in the annual 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 eastern Africa are also in line with the 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 analysed

Analysis of the vegetation trends using low resolution remote sensing data in Burkina Faso (1982-1999) for the monitoring of desertification

After about two decades of dramatic rainfall deficits that started in the late 1960s, the Sahel of West Africa has experienced increasing precipitation since the early 1990s. The implementation of the United Nations Convention to Combat Desertification (UNCCD) needs the identification of areas that record declining vegetation productivity over long-time periods. In this scope, we analyse the state of the vegetation productivity using long-term time series of NOAA AVHRR NDVI data and compare it to rainfall data. For this, 128 rain gauge data (RR) were compared with the integrated NDVI during the growing period (iNDVI) values in Burkina Faso from 1982 to 1999. During the analysed period, most of the studied stations in the country were stable for the iNDVI/RR (57.8%). However, 39.8% showed a weak to strong negative trend in the iNDVI/RR while only 2.4% showed a weak positive trend. These negative trends may reflect ongoing desertification processes in Burkina Faso and could be a starting point for the identification of hot-spots areas to determine where to take action to combat desertification