Does the Drakensberg dehydrate southwestern Africa?

Abstract

We have investigated the impacts of the high-elevated orography of the Drakensberg on the hydro-meteorological climate in the southwestern Africa, which is characterized by an annual aridity. The focus has been in the summer season (November to March), which contributes 70% of the annual rainfall of the region. A sensitivity experiment performed using a regional climate modeling, in which the Drakensberg is eliminated from the lower boundary condition, shows that precipitation is significantly enhanced over the southwestern Africa and its amplification exceeds 200% in a benchmark experiment. Without the Drakensberg, the lower-tropospheric easterly, associated with the subtropical anti-cyclone over the Indian Ocean, is allowed to penetrate and transport the moisture more westward. Correspondingly, a regime of the intense upward motion also shifts westward to Namibian coastal region and instead, the weak subsidence is formed around the Drakensberg in southeastern Africa. The westward-shifted upward motion is attributed to the enhanced rainfall, which is generated by local daily-anomaly circulation with the help of the enriched moisture. Consequently, the aridity over the southwestern Africa changes from severe arid/arid to semi-arid based on a traditional aridity index. We conclude that the Drakensberg partially contributes to the dry climate over the southwestern Africa.