Geoscience Reference
In-Depth Information
(A)
June-Aug
(B)
Dec-Feb
Ferrel
Cell
W
Ferrel
Cell
W
J
H
J
H
30
°
TE
H
H
N Tropic
N Tropic
TE
North
Hadley
Cell
20
°
North
Hadley
Cell
TE
TE
TE
J
E
10
°
J
A
SW
MT
EW
0°
0°
EW
EW
South
Hadley
Cell
TE
EW
TE
10
°
TE
TE
South
Hadley
Cell
NW
H
H
20
°
S Tropic
S Tropic
J
J
30
°
W
Ferrel
Cell
W
Ferrel
Cell
Seasonal (monsoonal) wind reversals
Equatorial westerlies
Upwelling
Discontinuities
Figure 11.38
The major circulation in Africa in (A) June to August and (B) December to February. H: subtropical
high pressure cells; EW: equatorial westerlies (moist, unstable but containing the Congo high pressure ridge); NW:
the northwesterlies (summer extension of EW in the southern hemisphere); TE: tropical easterlies (Trades); SW:
southwesterly monsoonal flow in the Northern Hemisphere; W: extratropical westerlies; J: subtropical westerly jet
stream; JA and JE: the (easterly) African jet streams; and MT: Monsoon Trough.
Source: From Rossignol-Strick (1985). By permission of Elsevier Science Publishers B.V., Amsterdam.
25
established, but it involves a shift of the lower
tropospheric African Easterly Jet (AEJ) (see
Figure
11.40b
).
In winter, the southwesterly monsoon airflow
over the coasts of West Africa is very shallow
(i.e., 1000m) with 3000m of overriding easterly
winds, which are themselves overlain by strong
(>20m s
-1
) winds (see
Figure 11.41
). North of the
Monsoon Trough, the surface northeasterlies (
i.e.,
the 2000m deep Harmattan flow) blow clockwise
outward from the subtropical high pressure
center. They are compensated above 5000m by an
anticlockwise westerly airflow that, at about
12,000m and 20-30
20
15
10
5
J
FM
A
M
J J
Month
A
S
O
N
D
Figure 11.39
The daily position of the Monsoon
Trough at longitude 3°E during 1957. This year
experienced an exceptionally wide swing over
West Africa, with the trough reaching 2°N in
January and 25° N on 1 August. Within a few days
after the latter date, the strongly oscillating trough
had swung southward through 8° latitude.
Source: After Clackson (1957), from Hayward and
Oguntoyinho (1987). By permission of Hutchinson.
N, is concentrated into a
subtropical westerly jet stream of average speed
45m s
-1
. Mean January surface temperatures
decrease from about 26
°
C along the southern
coast to 14°C in southern Algeria.
°
