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 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.
KEY WORDS: Northeastern South African rainfall · Circulation anomalies · ENSO and non-ENSO droughts · Composite analysis
Resale or republication not permitted without written consent of the publisherClim Res 25: 29-41, 20031981, Miron & Tyson 1984, van Heerden et al. 1988, Barclay et al. 1993, D'Abreton et al. 1993, quasibiennial oscillations (Mason & Lindesay 1993, Jury et al. 1994, regional SST anomalies (Walker 1990, Goddard & Graham 1999, Landman & Mason 1999, Reason 1999, 2001, Reason & Mulenga 1999, Jury et al. 2000 and ENSO (van Heerden et al. 1988, Jury et al. 1994, Rocha & Simmonds 1997, Reason et al. 2000, Richard et al. 2000.In this paper, the circulation changes associated with strong ENSO-related droughts in northeastern South Africa as well as those significant droughts in this region that do not occur during ENSO events are investigated using NCEP/NCAR Reanalysis data (Kalnay et al. 1996). The purpose of the study is to use these data to investigate, in greater detail than previously, the circulation patterns associated with droughts during ENSO events and also to see how such patterns may differ from those found during droughts that occur in other years (non-ENSO droughts). Since ENSO is primarily a tropical Indo-Pacific phenomenon, it is natural to consider whether ENSO droughts over southeastern Africa essentially arise from modulations to the local Walker Circ...