Geography Reference
In-Depth Information
18
0
0
100
0
2
15
-2
150
N
0
S
200
0
250
10
300
-2
-2
400
-2
500
5
S
N
700
-4
4
850
0
-2
0
2
0
SFC
R
N
T
S
R
Fig. 11.7
Vertical cross section along the reference latitude of Fig. 11.6 showing perturbation merid-
ional velocities in m s
−
1
. R, N, T, and S refer to ridge, northwind, trough, and southwind
sectors of the wave, respectively. (After Reed et al., 1977. Reproduced with permission of
the American Meteorological Society.)
condition for barotropic instability discussed in Section 8.4.2.
2
Baroclinic insta-
bility due to the strong easterly shear in the lower troposphere also appears to play
a role in these disturbances. Thus, both barotropic and baroclinic conversions from
the mean flow energy appear to be important for the generation of African wave
disturbances.
Because such disturbances continue to exist in the absence of strong mean wind
shears after they have propagated westward into the Atlantic, it is unlikely that
either baroclinic or barotropic instability continues to be the primary energy source
for their maintenance. Rather, diabatic heating through precipitating convective
systems appears to be the main energy source for such waves over the ocean.
2
It should be noted here that the profile shown in Fig. 11.6 is not a zonal mean. Rather, it is a time
mean for a limited longitudinal domain. Provided that the longitudinal scale of variation of this time
mean zonal flow is large compared to the scale of the disturbance, the time mean flow may be regarded
as a locally valid basic state for linear stability calculations.
