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(A)
30°
60°
90°
120°E
45°N
80
40
0
J W
30°
-40
J E
15°
-8 0
J E
-40
-40
(B)
30°
60°
90°
120°E
45°N
30°
DRY
15°
Figure 11.25 The easterly tropical jet stream. A: The location of the easterly jet streams at 200mb on 25 July 1955.
Streamlines are shown in solid lines and isotachs (wind speed) dashed. Wind speeds are given in knots (westerly
components positive, easterly negative). B: The average July rainfall (shaded areas receive more than 25cm) in
relation to the location of the easterly jet streams.
Source: From Koteswaram (1958). By permission of Tellus.
pressure cell over the plateau. The southwest
monsoon in South Asia is overlain by strong
upper easterlies (see Figure 11.19 ) with a
pronounced jet at 150mb (about 15km), which
extends westward across Saudi Arabia and Africa
( Figure 11.25 ). No easterly jets have so far been
observed over the tropical Atlantic or Pacific. The
jet is related to a steep lateral temperature
gradient, with the upper air getting progressively
colder to the south.
An important characteristic of the tropical
easterly jet is the location of the main belt of
summer rainfall on the right ( i.e., north) side of
the axis upstream of the wind maximum and on
the left side downstream, except for areas where
the orographic effect is predominant (see Figure
11.25 ). The mean jet maximum is located at about
15 °N, 50-80°E.
The monsoon current does not give rise to a
simple pattern of weather over India, despite the
fact that much of the country receives 80 percent
or more of its annual precipitation during the
monsoon season ( Figure 11.26 ). In the northwest,
a thin wedge of monsoon air is overlain by
subsiding continental air. The inversion prevents
convection and consequently little or no rain falls
in the summer months in the arid northwest of the
subcontinent (e.g., Bikaner and Kalat: Figure
11.20 ). This is similar to the Sahel zone in West
Africa, discussed below.
Around the head of the Bay of Bengal and
along the Ganges valley the main weather
mechanisms in summer are the 'monsoon
depressions' ( Figure 11.27 ), which usually move
westward or northwestward across India, steered
by the upper easterlies ( Figure 11.28 ), mainly in
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