1) June-September 1998: Western Africa Rainy Season
The Sahel region [bounded by 8°-18°N, 17°W-20°E and indicated by the boxed region in Fig. 53] receives approximately 90% of its mean annual rainfall during the June-September period. This rainfall pattern is closely related to the north-south movement of the ITCZ, which starts its northward movement in March and reaches its northernmost position (near 15°N) in August. Rainfall typically varies widely across the region, with long-term average totals reaching 1300 mm in the southwest, 700 mm in the southeast, and 100-300 mm in the north. Overall, the Sahel region as a whole experienced a near-normal rainy season during 1998 (Fig. 53b ), which followed significantly below normal rainfall during the 1997 season (Bell and Halpert 1998).
During 1998 below normal rainfall was confined to the southwestern Sahel, with the largest deficits (exceeding 500 mm) observed in Guinea and northern Liberia. Other regions experiencing below-normal rainfall included southern Senegal, Guinea-Bissau, and southwestern Mali. In contrast, rainfall averaged 100-200 mm above normal (120-200% of normal) over much of Niger during the season. Rainfall totals were particularly large during September, which resulted in devastating floods over eastern Mali, western Niger, and southeastern Niger.
Despite near-normal rains for the season as a whole, the onset of the rains was delayed across central and northern Senegal, Mauritania, western Mali, and southwestern Chad. In June and July, rainfall totals across northern Senegal and Mauritania only reached 50% of normal. However, the dryness in these regions was alleviated by heavy rains during the second half of August and September.
2) October 1997-April 1998: Southern Africa Rainy Season
The southern Africa rainy season typically lasts from October to April and reaches maximum strength from December through March. Most locations receive more than 75% of their mean annual rainfall during the rainy season, with some parts in the northwest receiving 90% of their normal annual rainfall. The interannual variability of rainfall over southern Africa shows a strong relationship to the ENSO cycle (Ropelewski and Halpert 1987, 1989), with below-normal rains typically observed during El Niņo episodes and above-normal rains observed during La Niņa episodes.
Overall, southern Africa experienced a drier and shorter-than-normal rainy season during 1997/98 (Fig. 54), with substantial rainfall ending in March and exceptional dryness occurring during December 1997, February and April 1998 (Fig. 54d). These conditions were consistent with the ongoing strong 1997/98 El Niņo, and contrasted with the active and prolonged 1996-97 rainy season (Bell and Halpert 1998).
During 1998 the most abundant rains (Fig. 54a) covered the climatologically wet region of eastern South Africa (50-100 mm above normal) and Mozambique (100-500 mm below normal). Significantly below-normal seasonal rainfall (Fig. 54b) was observed in the climatologically dry regions of western South Africa, Namibia, and western Botswana, where totals in many locations (50-150 mm deficits) dropped below the 5th percentile (Fig. 54c).
Significantly drier than normal conditions (50%-65% of normal rainfall) were also observed in northeastern South Africa and southern Zimbabwe, where totals were also generally below the 5th percentile. Accumulated rainfall and daily rainfall totals during November 1997-April 1998 are shown for two reporting stations located within these dry regions: Pietersburg, South Africa (Fig. 55 ) and Bulawayo Airport, Zimbabwe (Fig. 56). In both regions, rainfall was suppressed (Figs. 55a, 56a) and episodic (Figs. 55b, 56b ) during the season, with prolonged periods of little to no rainfall recorded. In Pietersburg, exceptional dryness persisted from late November 1997 into late January (Fig. 55b), and again from mid-February through April when only two substantial (exceeding 25 mm) rain events occurred. At Bulawayo Airport, nearly all of the substantial precipitation occurred during January (Fig. 56b), and dryness at this station was particularly acute during February-April when only three days of measurable precipitation were recorded.
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