Dry summers over northeastern South Africa and associated circulation anomalies

Abstract

Circulation anomalies associated with dry summers over northeastern South Africa are considered. Dry summers occurring during El Nino/Southern Oscillation (ENSO) episodes are examined separately from those during non-ENSO years, for which a greater mid-latitude influence on South African summer rainfall is evident. During the ENSO dry summers, anomalous upper tropospheric westerlies are dominant over most parts of the subcontinent and make the upper flow unfavourable for rain-bearing disturbances over northeastern South Africa. The barotropic instability criterion is not satisfied over most of tropical southern Africa and the maximum baroclinic shear south of Africa tends to be shifted west into the Southeast Atlantic; thus, conditions are unfavourable for tropical–extratropical cloud-band development and South African rainfall. High-pressure anomalies exist over southern Africa, suppressing convection, while the ascending branch of the Walker Circulation shifts offshore into the western Indian Ocean to lie over anomalously warm sea-surface temperatures (SSTs) there. During the non-ENSO dry summers, there is a less obvious indication of significant warming of SST over the southwest Indian Ocean or coherent offshore shift in the Walker Circulation. Instead, mid-latitude circulation anomalies tend to play a larger role such that increased advection of cool, dry South Atlantic air occurs over South Africa with an increased mid-latitude westerly influence. The location of maximum westerly shear is shifted westwards, and conditions are generally unfavourable for cloud-band occurrence across South Africa; hence, rainfall is reduced.