Geography Reference
In-Depth Information
100
4
3
2
1
16
150
14
4
200
12
250
0
10
300
8
400
500
6
4
4
5
700
2
850
6
5
4
3
2
1
0
SFC
-8
+12
+8
+4
0
-4
-12
∆
Latitude
Absolute vorticity (units 10
−
5
s
−
1
) corresponding to the mean wind field of Fig. 11.6.
Shading shows region where β
−
∂
2
Fig. 11.8
u/∂y
2
is negative. (After Reed et al., 1977. Reproduced
with permission of the American Meteorological Society.)
11.1.4
Tropical Monsoons
The term “monsoon” is commonly used in a rather general sense to designate any
seasonally reversing circulation system. The basic drive for a monsoon circulation
is provided by the contrast in the thermal properties of the land and sea surfaces.
Because the thin layer of soil that responds to the seasonal changes in surface
temperature has a small heat capacity compared to the heat capacity of the upper
layer of the ocean that responds on a similar time scale, the absorption of solar
radiation raises the surface temperature over land much more rapidly than over the
ocean. The warming of the land relative to the ocean leads to enhanced cumulus
convection, and hence to latent heat release, which produces warm temperatures
throughout the troposphere.
Much of the tropics is influenced by monsoons. The most extensive monsoon
circulation by far is the Asian monsoon. This monsoon completely dominates the
climate of the Indian subcontinent, producing warm wet summers and cool dry
winters. An idealized model of the structure of the Asian summer monsoon is indi-
cated in Fig. 11.9. As indicated in Fig. 11.9, the 1000-200-hPa thickness is larger
over the land than over the ocean. As a result there is a pressure gradient force at the
upper levels directed from the land to the ocean. The divergent wind that develops
in response to this pressure gradient (shown by the arrows in Fig. 11.9) causes a
net mass transport out of the air column above the continent and thereby generates
a surface low over the continent (often called a
thermal low
). A compensating
