The Effect of East African Topography on Flow Driven by Zonally Symmetric Forcing

Abstract

The low-level circulation in summer over the western Indian Ocean is characterized by southeast trades that are channeled near the African coast into a concentrated southerly flow across the equator and thence north-eastward into the southwest monsoon over the Arabian Sea. It is widely accepted that deflection by the East African highlands is responsible for this flow configuration. Existing theoretical models to a greater or lesser extent, build in this deflection by imposing a western boundary extending all the way to the top of the fluid or by prescribing longitudinally dependent sources and sinks for driving the flow. The purpose of this study is to determine what flow configuration occurs when these constraints are removed. For this we use zonally symmetric forcing to drive a planetary boundary layer model formulated in a terrain-following coordinate system that permits fluid to flow over as well as along, a topographic barrier. The results support the conclusion that East African topography alone can channel incident flow into a pattern with most of the observed features. An analysis of the diurnal oscillation in the model suggests a mechanism for the diurnal variation of low-level wind observed in northeastern Somalia.