A NESTED MODEL STUDY OF THE SAHELIAN CLIMATE RESPONSE TO SEA-SURFACE TEMPERATURE ANOMALIES

A nested high resolution atmospheric model is used to investigate the sensitivity of the Sahelian climate to large-scale sea-surface temperature(SST) anomalies. The nested system has realistic vegetation and detailed bottom orography. Two separate sets of northern hemispheric summer (June, July and August) numerical integrations are performed; one corresponding to the SST anomalies in 1950 when the Sahelian region was relatively much wetter than the long-term average conditions and a second integration based on 1984 SST anomalies when one of the driest rain seasons in the last few decades was experienced. Although the low resolution (R15 almost-equal-to 4.5(0) by 7.5(0) latitude by longitude) stand-alone global climate model reasonably simulates the lower rainfall amounts in 1984 compared to 1950, the nested system yields more realistic regional climate because its forcing includes more detailed effects of topography, land-sea contrasts, and land surface processes. In particular, two distinct rainfall maxima primarily anchored to the regions of highest terrain are simulated by the model. One corresponding to the highlands in Cameroon over the Adamawa Plateau and a second maxima over Guinea and Sierra-Leone. Inspection of model circulation indicates that the weaker moist cross-equatorial monsoon flow in the 1984 is responsible for the lower amounts of the Sahelian rainfall compared to 1950. Our results are in agreement with several diagnostic and modeling studies performed in the recent years which show that deficient sub-Saharan rainy seasons tends to coincide with the southwesterly surface monsoon flow not extending as far north along the West African coast as in the wetter years (Lamb and Peppler,1990, and others).