The Role of Orography in Determining the Sahelian Climate

Abstract

We have examined numerical modelling evidence based on the NASA GEOS-1 general circulation model in order to investigate the role of orography in determining the West African summer climate anomalies. Orographic climatic control and the influence of sea-surface temperature (SST) anomaly forcing were investigated by systematic suppression or inclusion of these effects in the numerical experiments. Our main finding is that the orography of Africa plays a major role in determining the climate of the Sahel and the coastal region of West Africa during the northern hemispheric summer season. These orographic effects are comprised of: (i) a quasi-stationary orographic ridge–trough dipole generated by the passage of the low-level prevailing easterlies over the Atlas–Ahaggar mountain complex over northern Africa, and (ii) an elongated zonal windward orographic ridge generated as the cross-equatorial summer monsoons from the South Atlantic Ocean basin ascends over the elevated landmass of West Africa. Over the Sahel, the moist south-westerly flow emanating from the Atlas–Ahaggar orographic trough over the Atlantic Ocean and the dry hot north-easterlies originating from the corresponding orographic windward ridge tend to converge along a zonal axis thus reinforcing the Inter-Tropical Convergence Zone. This results in wetter conditions over the Sahel than would have been the case in the absence of orography. The model simulations show that further to the south along the coastal region of West Africa, the zonal orographic ridge results in drier conditions than in the model runs when orography is suppressed. The net result is a permanent orographic-induced rainfall dipole pattern over West Africa. Further model simulations were conducted in which the 1973 SST anomaly conditions were described with or without the orography of Africa. The 1973 SST anomaly pattern is characterized by positive anomalies in the Southern Hemisphere and negative anomalies in the Northern Hemisphere. The model results show a weaker orographic-induced rainfall dipole pattern across West Africa in response to the 1973 SST anomaly pattern. The net result is wetter conditions along the coastal region and rainfall deficits over the Sahelian zonal strip. © 1997 by the Royal Meteorological Society.