Southern Africa

Southern Africa lies between the South Atlantic and Indian Ocean subtropical high-pressure cells in a region subject to the interaction of tropical easterly and extratropical westerly airflows. Both of these high-pressure cells shift west and intensify (see Figure 7.10) in the southern winter. Because the South Atlantic cell always extends 3° latitude further north than the Indian Ocean cell, it brings low-level westerlies to Angola and Zaire at all seasons and high-level westerlies to central Angola in the southern summer. The seasonal longitudinal shifts of the subtropical high-pressure cells are especially significant to the climate of southern Africa in respect of the Indian Ocean cell. Whereas the 7 to 13° longitudinal shift of the South Atlantic cell has relatively little effect, the westward movement of 24 to 30° during the southern winter by the Indian Ocean cell brings an easterly flow at all levels to most of southern Africa. The seasonal airflows and convergence zones are shown in Figure 11.45.

In summer (i.e. January), low-level westerlies over Angola and Zaire meet the northeast monsoon of East Africa along the intertropical convergence zone (ITCZ), which extends east as the boundary between the

Figure 11.44 Extent of precipitation systems affecting western and central North Africa and typical tracks of Soudano-Sahelian depressions.

Source: After Dubief and Yacono; from Barry (1991).

Mammal Teeth Types Diagram

Figure 11.44 Extent of precipitation systems affecting western and central North Africa and typical tracks of Soudano-Sahelian depressions.

Source: After Dubief and Yacono; from Barry (1991).

recurved (westerly) winds from the Indian Ocean and the deep tropical easterlies further south. To the west, these easterlies impinge on the Atlantic westerlies along the Zaire air boundary (ZAB). The ZAB is subject to daily fluctuations and low-pressure systems form along it, either being stationary or moving slowly westward. When these are deep and associated with southward-extending troughs they may produce significant rainfall. It should be noted that the complex structure of the ITCZ and ZAB means that the major surface troughs and centres of low pressure do not coincide with them but are situated some distance upwind in the low-level airflow, particularly in the easterlies. This low-level summer circulation is dominated by a combination of these frontal lows and convectional heat lows. By March, a unified high-pressure system has been established, giving a northerly flow of moist air, which produces autumn rains in western regions. In winter (i.e. July), the ZAB separates the low-level westerly and easterly airflows from the Atlantic and Indian Oceans, although both are overlain by a high-level easterly flow. At this time, the northerly displacement of the general circulation brings low- and high-level westerlies with rain to the southern Cape.

Thus tropical easterly airflows affect much of southern Africa throughout the year. A deep easterly flow dominates south of about 10°S in winter and south of 15 to 18°S in summer. Over East Africa, a northeasterly monsoonal flow occurs in summer, replaced by a southeasterly flow in winter. Easterly waves form in these airflows, similar to, but less mobile than, those in other tropical easterlies. These waves form at the 850 to 700mb level (i.e. 200 to 3000 m) in flows associated with easterly jets, often producing squall lines, belts of summer thunder cells and heavy rainfall. These waves are most common between December and February, when they may produce at least 40 mm of rain per day, but are rare between April and October. Tropical cyclones in the South Indian Ocean occur particularly around February (see Figure 11.8 and Table 11.1), when the ITCZ lies at its extreme southerly position. These storms recurve south along the east coast of Tanzania and Mozambique, but their influence is limited mainly to the coastal belt.

With few exceptions, deep westerly airflows are limited to the most southerly locations of southern Africa, especially in winter. As in northern mid-latitudes, disturbances in the westerlies involve:

1 Quasi-stationary Rossby waves.

2 Travelling waves, particularly marked at and above the 500-mb level, with axes tilted westward with height, divergence ahead and convergence in the rear, moving eastward at a speed of some 550 km/day, having a periodicity of two to eight days and with associated cold fronts.

3 Cut-off low-pressure centres. These are intense, cold-cored depressions, most frequent during March to May and September to November, and rare during December to February.

A feature of the climate of southern Africa is the prevalence of wet and dry spells, associated with

Figure 11.45 Mean SLP (mb) over the sea, based on daily ECWMF analysis for 1985 to 1992 (A) summer (JJA) and (B) winter (DJF) mean flow lines on sea and land. Thick solid lines in summer represent the ITCZ and Zambian air boundary (ZAB). Broken thick line in winter is the mean ZAB between the dry continental southeast trade winds and the moist southwest monsoon air.

Source: Van Heerden and Taljaard (1998).

Figure 11.45 Mean SLP (mb) over the sea, based on daily ECWMF analysis for 1985 to 1992 (A) summer (JJA) and (B) winter (DJF) mean flow lines on sea and land. Thick solid lines in summer represent the ITCZ and Zambian air boundary (ZAB). Broken thick line in winter is the mean ZAB between the dry continental southeast trade winds and the moist southwest monsoon air.

Source: Van Heerden and Taljaard (1998).

broader features of the global circulation. Above-normal rainfall, occurring as a north-south belt over the region, is associated with a high-phase Walker circulation (see p. 302). This has an ascending limb over southern Africa; a strengthening of the ITCZ; an intensification of tropical lows and easterly waves, often in conjunction with a westerly wave aloft to the south; and a strengthening of the South Atlantic subtropical high-pressure cell. Such a wet spell may occur particularly during the spring to autumn period. Below-normal rainfall is associated with a low-phase Walker circulation having a descending limb over southern Africa; a weakening of the ITCZ; a tendency to high pressure with a diminished occurrence of tropical lows and easterly waves; and weakening of the South Atlantic subtropical high-pressure cell. At the same time, there is a belt of cloud and rain lying to the east in the western Indian Ocean associated with a rising Walker limb and enhanced easterly disturbances in conjunction with a westerly wave aloft south of Madagascar.

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  • bisrat
    How westerlies affect southern africa weather?
    9 years ago
  • Fatima
    What is weather like south central africa?
    9 years ago
  • mewael
    How do tropical easterlies affect weather over southern Africa?
    1 year ago
  • belle scott
    How upper converge affect weather in southern africa?
    11 months ago

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