Application of the NCAR Regional Climate Model to eastern Africa: 2. Simulation of interannual variability of short rains

Abstract

We have applied the NCAR RegCM2 to the simulation of the interannual 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 interannual variability of precipitation in most of the years. The results show that remote factors play a dominant role in determining the precipitation anomalies. Interannual variability of precipitation over Tanzania is closely related to El Niño 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 of the Intertropical 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 El Niño‐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 interannual 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 favor the development of convection. The interannual variability of precipitation over eastern Kenya Highlands (EKH) is not directly related to the El Niño events, but the association with a warm SST anomaly pattern over the western Indian Ocean is evident during wet years. An El Niño 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.