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Expert Assessments

Climate Assessment Table of Contents

Regional Climate Highlights - Africa

1) The 1998-99 Southern African Rainy Season

Southern Africa receives the bulk of its mean annual rainfall during October to April, with the largest totals typically observed between December and March. The climatologically wet region of southeastern Africa tends to be drier than average during El Niņo episodes, and wetter-than-average during La Niņa episodes (Ropelewski and Halpert, 1987, 1989, and 1996; Janowiak 1988; Hastenrath, 1995; and Dai et al., 1997). Overall, the 1998–99 rainy season was characterized by significant rains across much of southeastern Africa (Figs. 45a, b), and by drier-than-normal conditions over much of interior southern Africa. The season also featured an early onset of the rains in October and November 1998, and an early withdrawal of the rains in March and April 1999 (Fig. 45d). The overall wetness over southeastern Africa was consistent with the ongoing La Niņa episode, and is in contrast to the below-normal to near-normal rainfall observed in this region during the 1997–98 rainy season in association with El Niņo conditions.

Regionally, 1998–99 seasonal rainfall totals were near-normal to above- normal over much of Mozambique, southern and central Malawi, eastern Zimbabwe, the Northern Transvaal, Kwazulu/Natal, and Eastern Cape provinces of South Africa. Rainfall totals in these areas averaged between 500–900 mm. Accumulated rainfall and daily rainfall totals during the season are shown for two reporting stations within these wetter-than-average regions: Pietersburg, South Africa (Fig. 46a) and Harare, Zimbabwe (Fig. 47a). Rainfall at both stations occurred almost on a daily basis from November 1998 through January 1999 (Figs. 46b, 47b). Harare also experienced substantial rainfall during most of February, and again in mid-March. In contrast, the rains at Pietersburg ended in late January, with only episodic, light rain observed after that. Only minimal rainfall was observed at both stations during late March and April.

The climatologically dry regions of southwestern and south-central Africa received near-normal to significantly below-normal precipitation during the 1998–99 rainy season (Figs. 45b, c). For example, near-normal rainfall was observed over Swaziland and Lesotho, and below-normal rainfall was recorded over portions of the Free State and North-West Province of South Africa, and across southern Botswana westward into Namibia. Elsewhere, exceptionally dry conditions were observed in the desert regions of central Namibia and southwestern Botswana, where rainfall totals less than 100 mm were only in the 5–10 percentile (Fig. 45c).

Overall, the low-level atmospheric circulation for the 1998–99 rainy season featured easterly winds which averaged 4–8 m s-1 from the eastern Indian Ocean westward to Madagascar and across portions of interior southern Africa (Fig. 48a). Over the Indian Ocean, this easterly flow was found along the poleward flank of the Mascarene High pressure system. A notable break in this easterly flow was evident in the region between southern Africa and Madagascar, which experienced only weak low-level easterlies during the season.

The primary anomalous circulation during the season occurred during DJF (Fig. 48b), in association with a pronounced amplification and poleward shift of the low-level Mascarene High across the western and central Indian Ocean. This anomaly feature was accompanied by enhanced easterlies between 10°–20°S across the central and western Indian Ocean, and with anomalous poleward flow in the region between Madagascar and Mozambique. Farther west, an anomalous cyclonic circulation was evident over interior southern Africa during the period, with anomalous westerly flow found near 15°S along the northern flank of this circulation. Overall, these conditions were associated with confluence extending southwestward along the Mozambique coast, and with anomalous large-scale convergence across southeastern Africa and the Mozambique Channel. These circulation features were accompanied by an anomalous anticyclonic circulation at upper levels in association with an amplification/ southward shift of subtropical ridge. Collectively, these conditions represented an enhanced monsoonal circulation over southeastern Africa, and were consistent with the above-average rainfall observed during the period. At upper levels this anomaly pattern was linked to the much large-scale pattern of anticyclonic streamfunction anomalies previously noted in the lower and middle latitudes of both hemispheres [see section 3e(2)].

2) June–September 1999: West African Rainy Season

The Sahel region [bounded by 8°–18°N, 18°W–20°E and indicated by the boxed region in Fig. 49] receives approximately 90% of its mean annual rainfall during the June–September period. This rainfall pattern is closely related to the Intertropical Convergence Zone (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 experienced an exceptionally wet rainy season during 1999 (Fig. 49), with area-averaged totals reaching the highest levels since 1967 (Fig. 50). These plentiful rains followed a near-average 1998 rainy season and suppressed 1997 rainy season. Wetter-than-normal precipitation was observed primarily across the northern portion of the Sahel during 1999 (Fig. 49b) and near-average totals were observed in the South. The only part of western Africa that experienced below-average rainfall during the season was the southwestern Sahel area, which recorded totals that were approximately 400 mm below the 1961–90 mean. However, this area still recorded more than 900 mm of rain during the season.

The atmospheric circulation during the 1999 season featured a strong southerly and southwesterly flow of tropical moisture at 925-hPa into the Sahel region (Fig. 51), as well as a significant penetration of moist, southwesterly flow into the northern part of the Sahel. The largest wind anomalies at 925 hPa were observed over the western Sahel (Fig. 52a), where westerly anomalies averaging 1–3 m s-1 extended well inland from the eastern tropical Atlantic Ocean. This enhanced monsoonal flow was even more prominent at 850-hPa (Fig. 52b), where westerly wind anomalies averaging 2–4 m s-1 extended from the eastern tropical Atlantic eastward to the southwestern Sahel and across the entire Gulf of Guinea. This anomalous flow was associated with enhanced cyclonic vorticity along the equatorward flank of the African Easterly jet (AEJ) [see section 4a(2), Fig. 31d], and with enhanced low-level convergence over much of the Sahel.

These low-level circulation features were associated with an overall poleward shift of the AEJ, and with a strengthening of the upper-level jet or Tropical easterly jet (TEJ) from the eastern Pacific eastward to Indonesia (Figs. 17c, 30a). All of these conditions typify other wet Sahel years as well.

During 1999, the regional conditions which contributed to abundant Sahel rains were linked to a global-scale atmospheric circulation pattern characterized by an amplification and poleward extension/ shift of the subtropical ridges in both hemispheres from the eastern Pacific eastward across the Atlantic and Africa to Australasia (Fig. 23c). As discussed in section 3e(2), this global-scale mode of atmospheric variability was associated with the La Niņa-related pattern of tropical rainfall shown in Figs. 17 and 19.