THE INFLUENCE OF LAND-SURFACE PROPERTIES ON SAHEL CLIMATE .1. DESERTIFICATION

This is a general circulation model sensitivity study of the physical mechanisms of the effects of desertification on the Sahel drought. The model vegetation types were changed in the prescribed desertification area, which led to changes in the surface characteristics. The model was integrated for three months (June, July, August) with climatologic al surface conditions (control) and desertification conditions (anomal y) to examine the summer season response to the changed surface condit ions. The control and anomaly experiments consisted of five pairs of i ntegrations with different initial conditions and/or sea surface tempe rature boundary conditions. In the desertification experiment, the moi sture flux convergence and rainfall were reduced in the test area and increased to the immediate south of this area. The simulated anomaly d ipole pattern was similar to the observed African drought patterns in which the axis ofthe maximum rainfall shifts to the south. The circula tion changes in the desertification experiment were consistent with th ose observed during sub-Saharan dry years. The tropical easterly jet w as weaker and the African easterly jet was stronger than normal. Furth er, in agreement with the observations, the easterly wave disturbances were reduced in intensity but not in number. Descending motion domina ted the desertification area. The surface energy budget and hydrologic al cycle were also changed substantially in the anomaly experiment.