In this study the NCEP nested Regional Spectral Model (RSM) is used to investigate the physical mechanism of interannual variability of the Short Rains over East Africa. The model resolution is 80 km for the outer domain and 20 km for the inner domain with 19 vertical layers. The nested system has realistic vegetation and detailed bottom topography. The outputs from the ECHAM3 atmospheric climate model provide the large-scale circulation forcing for the nested system. The simulation period is September-January from 1970 to 1996. Three numerical integrations are performed, and the ensemble mean of the three simulations is validated by observations.
The nested system captures both the large-scale characteristics of the circulation and the terrain-induced local features such as local precipitation maxima. The model reproduced the observed interannual variability of the Short Rains and of the timing of the onset of rains in most of the years. The regional climate influence of Lake Victoria is also simulated. The model results show that the large-scale atmospheric circulation anomalies play a dominant role in determining the rainfall anomalies. A positive correlation between rainfall anomalies and warm ENSO events is evident over most of East Africa, by which enhanced moist westerly flow from the Atlantic Ocean and the mainly easterly flow from the Indian Ocean converge over East Africa. SST anomalies over the Indian Ocean also affect the interannual variability of rainfall over East Africa