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.
