Geography Reference
In-Depth Information
continent, however, local effects due to surface conditions create a unique situ-
ation that requires separate discussion. During the Northern Hemisphere summer,
intense surface heating over the Sahara generates a strong positive meridional tem-
perature gradient in the lower troposphere between the equator and about 25
◦
N.
The resulting easterly thermal wind is responsible for the existence of a strong
easterly jet core near 650 hPa centered near 16
◦
N as shown in Fig. 11.6. Synoptic-
scale disturbances are observed to form and propagate westward in the cyclonic
shear zone to the south of this jet core. Occasionally, such disturbances are pro-
genitors of tropical storms and hurricanes in the western Atlantic. The average
wavelength of observed African wave disturbances is about 2500 km, and the west-
ward propagation speed is about 8 ms
−
1
, implying a period of about 3.5 days. The
disturbances have horizontal velocity perturbations that reach maximum ampli-
tude at the 650-hPa level, as indicated in Fig. 11.7. Although there is considerable
organized convection associated with these waves, they do not appear to be driven
primarily by latent heat release, but depend, rather, on barotropic and baroclinic
conversions of energy from the easterly jet.
In Fig. 11.8 the absolute vorticity profile for the African easterly jet shown in
Fig. 11.6 is plotted. The shaded region indicates the area in which the vorticity
gradient is negative. Thus, it is clear that the African jet satisfies the necessary
100
-5
-5
16
0
-5
150
14
-10
200
E
12
5
W
250
10
300
-10
8
0
400
-5
6
500
E
4
700
-10
2
0
0
850
5
W
0
SFC
+12
+8
+4
0
-4
-8
-12
∆
Latitude
Fig. 11.6
Mean-zonal wind distribution in the North African region (30˚W to 10˚E longitude) for
the period August 23, 1974 to September 19, 1974. Latitude is shown relative to latitude of
maximum disturbance amplitude at 700 hPa (about 12˚N). The contour interval is 2.5ms
−
1
.
(After Reed et al., 1977. Reproduced with permission of the American Meteorological
Society.)
