The 2002-2003 southern Africa Rainy Season Summary

The rainy season in southern Africa extends from October to April, with the largest amounts typically observed between December and March. In general, southern Africa rainfall tends to be drier than average during El Niño, and wetter than average during la Niña (Ropelewski and Halpert, 1987, 1989, and 1996; Hastenrath, 1995; Dai et al., 1997; and Thiaw et al., 1999). The overall rainfall pattern in 2002-2003 was much consistent with the warm ENSO episode, which resulted in below normal rainfall across much of southern Africa.

The 2002-2003 southern Africa rainy season was characterized by significant rainfall deficits across the southern half of the region, including South Africa, Namibia, Botswana, Zimbabwe, and the southern half of Mozambique (Fig.1a,b). Rainfall amounts in this region averaged between 100 and 700 mm, and only in the 5-20th percentile locally in northeastern South Africa, southern Mozambique, southern Zimbabwe, central Botswana, and western Namibia (Fig.1c). Overall, the rainy season was marked by a delay in the onset of the rains in October 2002 and an early withdrawal of the rains in April 2003 (Fig.1d). Southern Africa rainfall has consistently been drier than normal from October to February. For the season as a whole , rainfall totals above the climatological mean have been recorded for the month of March only. A time series of cumulative and daily precipitation for Pietersburg in northeastern South Africa (not shown) is typical of the dryness in southern Africa. Dryness persisted from November through mid-December, followed by a brief period of recovery and long dry spells through the reminder of the season. The exception to the dryness in southern Africa was the northern half of Mozambique and northern and southeastern Madagascar, which received rainfall totals in the 70-90th percentile. Near-normal rainfall was also observed over northern Zimbabwe and along a band stretching from southern Botswana southward into the Northern and Western Cape provinces of South Africa. Rainfall totals in these areas ranged between 200 and 400 mm and were in the 30-70th percentile.

The low-level atmospheric circulation for the 2002-2003 rainy season featured easterly winds that averaged about 4 ms^-1 from the east central Indian Ocean westward into Madagascar and portions of interior southern Africa (Fig.2). This easterly flow was found along the equatorward flank of the Mascarene high pressure system and was very close to the climatological mean wind in this region for the period October through April. The significant rainfall deficits were associated with a weakening of the Mascarene high pressure system, indicated by the presence of an anomalous cyclonic circulation centered over Madagascar and 2 ms^-1 westerly wind anomalies across much of southeastern Indian Ocean. This anomalous circulation was even more enhanced in November, with southerly wind anomalies that extended from South Africa into Mozambique (Fig.3). This pattern of low-level anomalous cyclonic circulation kept moisture from the Indian Ocean away from continental southern Africa. In contrast, the wet spell observed in March, was associated with a broad low-level anomalous cyclonic circulation that covered much of southern Africa and which favored the penetration of Atlantic moisture into continental southern Africa (Fig.4).

The pattern of dryness observed during the period November 2002 to February 2003 was followed by enhanced rainfall activity in March due partly to the impacts from Cyclone Japhet early in the month which brought heavy rains (250-500% of normal) in southeastern Zimbabwe and over most of Mozambique. However, as result of rainfall deficits less than 50% of normal, drought persisted from eastern and northern Botswana into western Zimbabwe, northern South Africa, southern Mozambique, and Swaziland.