Geoscience Reference
In-Depth Information
Figure 11.40
The structure of the
circulation over North Africa in August.
(A) Surface airflow and easterly tropical
jet. (B) Vertical structure and resulting
precipitation zones over West Africa.
Note the high-level tropospheric east-
erly jet and the lower African easterly
jet.
Notes
: = thunderstorm activity; MT
= monsoon trough.
Sources
: (A) Reproduction from the
Geographical Magazine
, London; (B)
From Maley (1982), copyright ©
Elsevier Science; reproduced by per-
mission; and Musk (1983).
from the subtropical high-pressure centre (see Figure
11.41) and are concentrated between June and August
into two tropical easterly jet streams; the stronger TEJ
(>20 m -1) at about 15,000 to 20,000 m and the weaker
AEJ(>10 m s
-1
) at about 4000 to 5000 m (see Figure
11.40B). The lower jet occupies a broad band from 13°N
to 20°N, on the underside of which oscillations produce
easterly waves which may develop into squall lines. By
July, the southwesterly monsoon airflow has spread far
to the north and westward-moving convective systems
now determine much of the rainfall. The leading trough
reaches its extreme northern location, about 20°N, in
August. At this time, four major climatic belts can be
identified over West Africa (see Figure 11.40A):
jet axes, apparently associated with easterly wave
disturbances from east central Africa, causes insta-
bility in the monsoon air.
3
A broad zone underlying the easterly jet streams,
which help to activate disturbance lines and thun-
derstorms. North-south lines of deep cumulonimbus
cells may move westward steered by the jets. The
southern, wetter part of this zone is termed
the
Soudan
, the northern part
the Sahel
, but popular
usage assigns the name
Sahel
to the whole belt.
4
Just south of the monsoon trough, the shallow tongue
of humid air is overlain by drier subsiding air. Here
there are only isolated storms, scattered showers and
occasional thunderstorms.
1
A coastal belt of cloud and light rain related to
frictional convergence within the monsoon flow,
overlain by subsiding easterlies.
In contrast to winter conditions, August temperatures
are lowest (i.e. 24 to 25°C) along the cloudy southern
coasts and increase towards the north, where they
average 30°C in southern Algeria.
Both the summer airflows, the southwesterlies below
and the easterlies aloft, are subject to perturbations,
2
A quasi-stationary zone of disturbances associated
with deep stratiform cloud yielding prolonged light
rains. Low-level convergence south of the easterly
