Application of the NCAR regional climate model to eastern Africa2. Simulation of interannual variability of short rains

Abstract

We have applied the NCAR RegCM2 to the simulation of the inter-annual variability of precipitation over eastern Africa for the short-rains season by performing a set of experiments for the years 1982 to 1993. The model reproduced the observed inter-annual variability of precipitation in most of the years. The results show that remote factors play a dominant role in determining the precipitation anomalies. Inter-annual variability of precipitation over Tanzania is closely related to E1 Nifio events in their mature phase and sea surface temperature (SST) anomalies over the Indian and Atlantic Oceans. The southward shift of the Arabian High results in a southward shift of the zonal Component to f the Inter-tropical Convergence Zone (ITCZ), which is responsible for early onset of the rainy season (e.g., 1982 and 1986). The enhanced St. Helena High and weaker Mascarene High lead to the eastward shift of the meridional branch of the ITCZ for the wet years. Model simulations confirmed a strong positive correlation between precipitation anomalies over Lake Victoria and the warm E1Nifio-Southern Oscillation events, by which enhanced moist westerly flow from the Atlantic Ocean and the mainly easterly flow from the Indian Ocean converge over Lake Victoria during wet years. The inter-annual variability of precipitation over Lake Victoria and the western Kenya Highlands (WKH) are strongly coupled. Positive precipitation anomalies over the WKH region are usually associated with weaker Arabian High and Mascarene High, which weaken the large-scale divergence over the WKH region and favour the development to f convection The inter-annual variability of precipitation over eastern K enya Highlands (EKH) is not directly related to the E1 Ni fi o events, but the association with a warm SST anomaly pattern over the western Indian Ocean is evident during wet years. An E1 Ni fio signal is, however, evident for wet years over the Turkana Channel, warm SST anomalies over the northern Indian Ocean contribute enhanced water vapor transport over the region.