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 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...

Interannual variability in rainy season characteristics over the Limpopo region of southern Africa

“…The mean rainfall rate for DJF is 3 mm/day with a gradient from about 3.5 mm/day in the east to around 2.5 mm/day in the west. Anomalous wet and dry summers during this period agree with those derived by Mulenga et al (2003) for a slightly larger region of northern South Africa using monthly gridded data from the South African Weather Service. In addition, these authors showed that the region experienced significantly below average DJF rainfall for nine out of ten strong mature phase El Niño summers in the 1921-2001 period, with 1957/58 being the single exception.…”

“…Figure 3). This increase in advection of South Atlantic air was identified by Mulenga et al (2003) as being a fundamental characteristic associated with anomalously dry seasons during neutral seasons as opposed to the El Niño-induced droughts where there is relative subsidence over southeastern Africa and an associated offshore shift of the tropical convection. In addition to the weakening of the South Atlantic anticyclone and increased advection of South Atlantic air, Figure 3 indicates that conditions are unfavourable for tropical-extratropical cloud bands to align themselves NW-SE across the country as is needed for good rains.…”

“…Rainfall (mm/day) 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 Composite seasons On the basis of the more recent pentad data plotted in Figure 1, the most pronounced dry summers occurred during the 1982/3, 1991/2 and 1997/8 El Niños, closely followed by the neutral 2001/2 summer. As pointed out by Mulenga et al (2003), it needs to be appreciated that not all significantly dry summers over the region correspond to El Niño events. Of the five La Niña summers during the period, only 1995/6 and 1999/2000 correspond to very wet conditions over Limpopo with 1980/81 (a neutral year) being the third wettest summer, and another neutral season, 1987/88, the fourth wettest.…”

Stochastic simulation of rainfall in the semi‐arid Limpopo basin, Botswana

“…Neither temperature nor pressure was significant in the amounts model. Although the ENSO indices were expected to be important based on the literature (Richard et al, 2000;Mulenga et al, 2003;Reason et al, 2005;Manatsa et al, 2008), none of these indices were within the eight most statistically significant external predictors.…”

Tropical systems from the southwest Indian Ocean making landfall over the Limpopo River Basin, southern Africa: a historical perspective

“…During this time of the year, tropical weather systems invade southern Africa in the form of tropical cyclones, tropical lows and easterly waves (Karoly and Vincent, 1998). Also, wet spells during summer are pulsed at frequencies that are consistent with the passage of tropical waves over the southeast African and SWIO region (Hayashi and Golder, 1992) while anomalous easterly flow in the 5-20°S band in the region of Madagascar leads to increased rainfall over southeast Africa (Mulenga et al, 2003).…”