1) October-December 1997: Equatorial East Africa rainy season
Tropical eastern Africa has two rainy seasons which occur during March-May and October-December (OND). The year-to-year variability of rainfall during the OND season shows a strong relationship to the ENSO (Ropelewski and Halpert 1987, 1989), with above-normal rainfall observed during Pacific warm episodes and below-normal rainfall observed during Pacific cold episodes.
During OND 1997, record rainfall (in many areas averaging 5 to 10 times the normal) was observed throughout the region in association with the ongoing strong El Niño conditions. Estimated rainfall totals during the period averaged more than 600 mm throughout Kenya, southern Somalia, the Ethiopian Highlands, and extreme northeastern Tanzania, with local maxima exceeding 1100 mm in northwestern Kenya and 900 mm in extreme southern Kenya (Fig. 44a , Fig. 22). Estimated precipitation anomalies averaged more than 500 mm across southern Somalia and the eastern half of Kenya, with the largest anomalies exceeding 700 mm observed in northern Kenya (Fig. 44b). The seasonal precipitation totals in both regions reached the 99th percentile, suggesting record amounts for the period (Fig. 44c ). For the region as a whole, area-averaged rainfall totals exceeded the 90th percentile in all three months (Fig. 44d ). This excessive rainfall is in marked contrast to the drier-than-normal conditions that prevailed during the OND 1996 wet season.
Accumulated rainfall and daily rainfall totals during OND 1997 are shown for two reporting stations in Kenya (Fig. 45). Mombasa [located in extreme southeastern Kenya], and Meru [located in central Kenya] each experienced precipitation totals of more than 1500 mm during the period (Figs. 45a, b, respectively), with measurable precipitation observed almost every day at both stations (Figs. 45c, d). This compares to normal seasonal totals of approximately 250 mm at Mombasa and 450 mm at Meru. In Mombasa, the heaviest rains fell during 17-22 October, when 6-day totals exceeded 500 mm. A second period of heavy rains occurred from 31 October-4 November, when totals reached 250 mm. In addition to these extreme events, daily totals exceeded 12.5 mm on many days during OND, which further exacerbated conditions in the region. In Meru, totals exceeded 12.5 mm on 42 of the 92 days during OND. Heavier totals of 25 mm were observed on 26 days during the period, while totals exceeding 50 mm were recorded on six days.
Throughout equatorial eastern Africa, this repetitive and heavy rainfall resulted in disastrous flooding. In some areas, these conditions resulted in mass migration and destruction of property. According to the World Meteorological Organization, flooding in southern Somalia along the Juba and Shabelle Rivers claimed an estimated 2000 lives and forced hundreds of thousands of inhabitants from their homes. Extensive flooding along the Tana River in eastern Kenya also left thousands homeless and caused extensive property damage.
2) October 1996-April 1997: Southern Africa rainy season
The southern Africa rainy season typically lasts from October to April and reaches maximum strength between November and March. Most locations receive more than 75% of their annual rainfall during this 7-month period, with some parts in the northwest receiving more than 90% of their normal annual rainfall. The year-to-year variability in rainfall over southern Africa shows a strong relationship to the ENSO cycle (Ropelewski and Halpert 1987, 1989), with below-normal rainfall normally observed during Pacific warm episodes and above-normal rainfall observed during Pacific cold episodes.
Overall, southern Africa experienced an active and prolonged 1996/97 rainy season, with substantial rains beginning in October 1996 and continuing through July 1997. The most abundant rains (800-1000 mm) fell across the climatologically wet region of eastern South Africa (not shown), with departures of 160-230 mm observed across the region. Approximately 100-150 mm of this surplus fell during the normal rainy season (Fig. 46a) with the rest (60-80 mm) falling during May-July (Fig. 46b). Southern and eastern Botswana also experienced a good rainy season, with totals averaging 500-600 mm throughout the region. The totals were 100-150 mm above normal in both areas during October-April (Fig. 46a), and 60-80 mm above normal in southeastern Botswana during May-July (Fig. 46b).
Elsewhere, western Zimbabwe and northeastern Namibia observed well above-normal rainfall during
their normal wet season, and near-normal rainfall during May-July. In contrast, precipitation was below normal
across southern and eastern Mozambique during both periods, with totals averaging more than 200 mm below
normal during October-April, and 40-80 mm during May-July. For the period as a whole, most of southern
Mozambique recorded approximately 75% of normal rainfall, while most of the eastern region reported 50-75% of
average
rainfall.
Three tropical storms contributed to the abundant rains over southeastern Africa during the 1996/97 wet season. The first of these systems, Tropical Storm Gretelle, brought heavy rainfall during January to eastern South Africa and southern Mozambique. Subsequently, tropical cyclones Josie and Lisette brought torrential downpours to southeastern Africa, and triggered extensive flooding in central and northern Mozambique.
The time series of area-averaged monthly mean rainfall for the boxed region (20°-35°E, 18°-33°S) shown in Figs. 46a-b indicates that totals were above the 70th percentile in all months between October 1996-July 1997, except for December and February (Fig. 47). In fact, February was the only month in which rainfall was substantially below normal during the season, as indicated by a drop in the area-mean value to the 15th percentile.
During the subsequent 1997/98 wet season, rainfall was normal to above-normal during October and November. However, totals were substantially below-normal during December 1997 (Fig. 47), with almost no precipitation observed in parts of southern Mozambique, Zimbabwe and eastern South Africa. This dryness is consistent with strong El Niño conditions
3) June-September 1997: Western Africa rainy season
The Sahel region (bounded by 8°-18°N, 17°W-20°E and indicated by the boxed region in Fig. 48) receives approximately 90% of its mean annual rainfall during the June-September period. This rainfall is closely related to the north-south movement of the ITCZ, which begins its northward movement in March and reaches its northernmost position (near 15°N) in August. Rainfall during the wet season varies widely throughout the region, with the lowest totals of 100-300 mm observed in the north and the highest totals of 1300 mm and 800 mm observed in the southwestern and southeastern quadrants, respectively. Overall, the Sahel experienced a shortened and substantially below-normal rainy season during 1997 (Fig. 48), with most of the rainfall deficits observed across the southern half of the region during July-September (Fig. 48b).
The season began with heavy rains during June across the northern half of the Sahel and across the Gulf of Guinea region (not shown). However, this beneficial rain was short-lived, with July-September totals falling significantly below-normal across the southern two-thirds of the Sahel (Fig. 48b). During July-September, the largest rainfall totals were observed in the climatologically wet region of the southwest, where totals ranged from 500-700 mm (Fig. 48a ). However, these totals were 300-700 mm below normal (Fig. 48b), and in some areas were less than one-half the climatological mean. Farther east, July-September totals averaged 300-500 mm across the heavy rainfall regions of the south-central and southeastern Sahel. These amounts were also significantly below normal, with deficits ranging from 100-200 mm in the central and south-central Sahel sectors to 200-400 mm in the southeast.
