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The occurrence and predictability of droughts over southern Africa

Mason, Simon J.; Tyson, Peter

Southern Africa lies within the subtropical high pressure belt of the Southern Hemisphere between about 15° and 35° S (Figure 8.1). Subsidence of air in large anticyclones predominates for most of the year, resulting in arid or semiarid conditions throughout much of the subcontinent. During the first half of summer (October-December), the southward migration of the locus of tropical convection provides early-season rainfall to the northern parts of the subcontinent, while the development of a subtropical trough within the quasi-permanent high pressure belt facilitates the occurrence of rainfall farther south from midlatitude systems. By January, the tropical atmosphere is usually dominant over most of the subcontinent, often bringing good rains, especially when links with westerly troughs form.
The interannual rainfall variability of the region is high, with the coefficient of variation exceeding 40 per cent in the drier western areas. Droughts are an inherent feature of the climate, and water resources are under growing pressure from population and industrial expansion, being particularly strained during drought years when demand for water increases. Given the high degree of interannual rainfall variability in the southern African region, skilful seasonal forecasts could greatly assist in water resource planning and the amelioration of drought and flood impacts. Since the 1991-2 drought, long-range seasonal forecasts for southern Africa using statistical methods have been produced by universities, the national meteorological services, and drought monitoring centres in South Africa and neighbouring countries. More recently, statistical forecasts have been supplemented by real-time general circulation model ensemble seasonal forecasts. In this chapter, progress in the understanding of droughts and predictability over southern Africa is reviewed.