Dry summers over northeastern South Africa and associated circulation anomalies

Climate Research

M. Rouault¹  C. J. C. Reason¹  H. M. Mulenga¹ 

关键词: Northeastern South African rainfall;    Circulation anomalies;    ENSO and non-ENSO droughts;    Composite analysis;   

DOI  :  10.3354/cr025029

来源: Inter-Research Science Publishing

【 摘 要 】

ABSTRACT: Circulation anomalies associated with dry summers over northeastern South Africa are considered. Dry summers occurring during El Niño/Southern Oscillation (ENSO) episodes are examined separately from those during non-ENSO years, for which agreater 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-bearingdisturbances 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 intothe 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 inthe 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 ofmaximum westerly shear is shifted westwards, and conditions are generally unfavourable for cloud-band occurrence across South Africa; hence, rainfall is reduced.

【 授权许可】

Unknown