Geoscience Reference
In-Depth Information
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 south-
ern 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 south-
easterly 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 700-
mb level (i.e. 200 to 3000 m) in flows associated with
easterly jets, often producing squall lines, belts of sum-
mer 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
