COMPARATIVE INFLUENCE OF LAND AND SEA SURFACES ON THE SAHELIAN DROUGHT - A NUMERICAL STUDY

The aim of this work is to compare the relative impact of land and sea surface anomalies on Sahel rainfall and to describe the associated an omalies in the atmospheric general circulation. This sensitivity study was done with the Meteo-France climate model: AR-PEGE. The sensitivit y to land surface conditions consists of changes in the management of water and heat exchanges by vegetation cover and bare soil. The sensit ivity to ocean surfaces consists in forcing the lower boundary of the model with worldwide composite sea surface temperature (SST) anomalies obtained from the difference between 4 dry Sahel years and 4 wet Sahe l years observed since 1970. For each case, the spatiotemporal variabi lity of the simulated rainfall anomaly and changes in the modelled tro pical easterly jet (TEJ) and African easterly jet (AEJ) are discussed. The global changes in land surface evaporation have caused a rainfall deficit over the Sahel and over the Guinea Coast. No significant chan ges in the simulated TEJ and an enhancement of the AEJ are found; at t he surface, the energy budget and the hydrological cycle are substanti ally modified. On the other hand, SST anomalies induce a negative rain fall anomaly over the Sahel and a positive rainfall anomaly to the sou th of this area. The rainfall deficit due to those anomalies is consis tent with previous diagnostic and sensitivity studies. The TEJ is weak er and the AEJ is stronger than in the reference. The composite impact of SST and land surfaces anomalies is also analyzed: the simulated ra infall anomaly is similar to the observed mean African drought pattern s. This work suggests that large-scale variations of surface condition s may have a substantial influence on Sahel rainfall and shows the imp ortance of land surface parameterization in climate change modelling. In addition, it points out the interest in accurately considering the land and sea surfaces conditions in sensitivity studies on Sahel rainf all.