Geoscience Reference
In-Depth Information
the rainfall during this season. Three types of
perturbation are particularly prevalent:
Annual rainfall decreases from 2000-3000mm
in the coastal belt (e.g., Conakry, Guinea) to about
1000m at latitude 20°N (
Figure 11.42
). Near the
coast, more than 300mm per day of rain may fall
during the rainy season but further north the
variability increases due to the irregular exten-
sion and movement of the Monsoon Trough.
Squall-lines and other disturbances give a zone
of maximum rainfall located 800-1000km south of
the surface position of the Monsoon Trough (see
1
Waves in the southwesterlies. These are
northward surges of the humid airflow with
periodicities of four to six days. They produce
bands of summer monsoon rain some 160km
broad and 50-80km in north-south extent,
which have the most marked effect 1100-
1400km south of the surface Monsoon Trough,
the position of which oscillates with the surges.
2
Waves in the easterlies. These develop on the
interface between the lower southwesterly and
the upper easterly airflows. These waves are
from 1500 to 4000km long from north to
south. They move westward across West
Africa between mid-June and October with a
periodicity of three to five days and sometimes
developing closed cyclonic circulations. Their
speed is about 5-10
Downpours in storms
Other showers
Light rain or drizzle
ATAR
300mm
60
CONAKRY
3500mm
(358mm)
40
6
300
20
4
2
280
0
0
260
BAMAKO
1082mm
60
240
longitude per day (
i.e.,
18-35km hr
-1
). At the height of the summer
monsoon, they produce most rainfall at around
latitude 14°N, between 300 and 1100km south
of the Monsoon Trough. On average, some 50
easterly waves per year cross Dakar. Some of
these carry on in the general circulation across
the Atlantic, and it has been estimated that 60
percent of West Indian hurricanes originate in
West Africa as easterly waves.
3
Squall-lines. Easterly waves vary greatly in
intensity. Some give rise to little cloud and rain,
whereas others have embedded squall-lines
when the wave extends down to the surface,
producing updrafts, heavy rain and thunder.
Squall-line formation is assisted where surface
topographic convergence of the easterly flow
occurs (e.g., the Air Mountains, the Fouta-
Jallon Plateau). These disturbance lines travel
at up to 60km hr
-1
from east to west across
southern West Africa for distances of up to
3000km (but averaging 600km) between June
and September, yielding 40-90mm of rain per
day. Some coastal locations suffer about 40
squall-lines per year, which account for more
than 50 percent of the annual rainfall.
°
40
220
6
20
4
200
2
0
0
180
ABIDJAN
2045mm
(274mm)
160
160
140
140
120
120
100
100
80
80
60
60
40
40
6
6
20
4
4
20
2
2
0
0
Mean monthly rainfall intensity
0
0
J
FM
A
M
J J
A S
N D
J
FM
A
M
J J
A S
N D
Figure 11.42
Mean number of hours of rain per month for
four West African stations. Also shown are types of rainfall,
mean annual totals (mm) and, in parentheses, maximum
recorded daily rainfalls (m) for Conakry (August) and Abidjan
(June). Dots show the mean monthly rainfall intensities (mm
hr
-1
). Note the pronounced Little Dry Season at Abidjan.
Station locations are marked on Figure 11.41.
Source: From Hayward and Oguntoyinbo (1987). By permission of
Rowman and Littlefield.
