The 2000-2001 southern Africa Rainy Season

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). However, the overall rainfall pattern in 2000-2001 was much accounted for by internal variability in the atmosphere, and not quite consistent with the prolonged cold ENSO episode that lingered from the fall of 1998 into the fall of 2001, and which resulted in above normal rainfall across much of southern Africa during the 1998-99 and 1999-2000 seasons.

The 2000-2001 southern Africa rainy season was characterized by significant rainfall deficits along the east coast from northeastern South Africa northward into Mozambique (Fig.1a,b). Rainfall amounts in this region averaged only between 300 and 800 mm, and only in the 5-20th percentile locally in northeastern South Africa (Fig. 1c). Overall, the rainy season was marked by large intraseasonal variability, with a delay in the onset of the rains in October 2000, a late withdrawal of the rains in April 2001, and alternate periods of dry and wet spells in between (Fig. 1d). A time series of cumulative and daily precipitation for Nampula in northeastern Mozambique shown in Fig. 2 is typical of the dryness in the east coast of southern Africa. Rainfall was well below normal in December and January, followed by a gradual recovery in February and March 2001. The exception to the dryness in the east coast of southern Africa was southern Zimbabwe, which received rainfall totals in the 70-90th percentile. Near to above normal rainfall was also observed over much of central and western southern Africa, which is climatologically drier than the east coast. Rainfall totals in central South Africa, Botswana, and Namibia, ranged between 200 and 600 mm and were in the 70-90th percentile in some areas.

The low-level atmospheric circulation for the 2000-2001 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. 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 (Fig. 3a). The significant rains observed early in November 2000 were associated with an amplification of the Mascarene high pressure system indicated by 2-6 ms^-1 easterly wind anomalies across much of southern and central Indian Ocean and the presence of a cyclonic circulation centered over southern Angola. This pattern of cyclonic/anticyclonic couplet resulted in strong low-level convergence and enhanced convection across much of northeastern southern Africa. In contrast, the dry spell observed in December and January, when the season was at its prime was associated with a weakening of the Mascarene high pressure system, indicated by 2-4 ms^-1 westerly wind anomalies across much of central Indian Ocean and eastern southern Africa. This anomalous circulation was even more enhanced in January, with broad westerly wind anomalies that extended from the Gulf of Guinea, crossed central and southern Africa into central Indian Ocean (Fig. 3b). The pattern of dryness observed in December and January 2000-2001 was followed by enhanced rainfall activity in February. Extreme heavy rainfall in the range 300-500 mm (not shown) inundated the central provinces of Mozambique, southern Malawi, north-central Zimbabwe, and southern Zambia. This pattern was consistent with an enhancement of convection over Indonesia, but also with the establishment of a circulation pattern quite similar to that in place in November 2000. However, consistent with the overall pattern of atmospheric circulation for the season, tropical cyclone activity was weak. We note however, that tropical cyclone Dera, which formed over the Mozambique Channel on March 9 brought very heavy rains and flooding in northern Mozambique and the west coast of Madagascar.

Fig. Safr-1: Oct 2000-Apr 2001 (a) total precipitation (mm), (b) precipitation anomalies (mm), (c) precipitation percentiles based on a gamma distribution fit to the 1961-90 base period. Panel (d) shows a monthly time series of precipitation percentiles, based on precipitation totals averaged over the boxed region depicted in panels (a)-(c).

Fig. Safr-2: Daily and accumulated precipitation (mm) at Nampula, Mozambique, during October 2000-June 2001. Accumulation of climatological precipitation also shown. Base period 1971-2000.

Fig. Safr-3: 850 hPa total wind for (a) Oct 2000-Apr 2001 and (b) Jan 2001 wind anomalies. Anomalies are departures from the 1979-95 base period means.