Geoscience Reference
In-Depth Information
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 (see
Figure 11.45
), 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
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 500mb level, with axes tilted west-
ward with height, divergence ahead and
convergence in the rear, moving eastwards at
a speed of some 550km/day, having a period-
icity of two to eight days and with associated
cold fronts.
3
Cut-off low pressure centers. 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
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. 376
). This has an
ascending limb over southern Africa; a strength-
ening 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 (see
Figure 11.46
). 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 (
Figure 11.47
).
°
S in
winter and south of 15-18
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 850-700mb level (i.e., 200-3000m) 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 40mm 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.
°
