The effect of soil moisture on circulation and rainfall in a tropical model

Abstract

The sensitivity of a tropical model to soil moisture content has been investigated using a simplified version of the UK Meteorological Office 11‐layer model. the model area represented a zonally‐symmetric version of West Africa. the results of one experiment (D) with a desert in the same latitudes as the Sahara were compared with those of another experiment (W) in which the desert was replaced by moist land. the initial fields contained an idealized easterly wave which developed during the experiments to form depressions whose movement, structure and associated rainfall were strongly influenced by the dryness of the underlying surface. the energetics of these disturbances were analysed and two distinct mechanisms for maintaining their eddy kinetic energy are proposed, depending on the soil moisture. Results from W suggest that once the land was moist it maintained itself in this state for at least several weeks. In D, however, the initial aridity north of 14°N was sustained, suggesting that ground dryness alone can cause deserts to persist. an analysis of observed rainfall data for the northern Sahel provides much stronger evidence for the persistence of rainfall anomalies from year to year than do similar analyses for the southern Sahel. the possibility that soil moisture anomalies could contribute to this persistence is briefly discussed.