Geoscience Reference
In-Depth Information
the Equatorial Trough is farthest north.
Figure
11.20
shows a maximum at Minicoy in June, with
a secondary peak in October as the Equatorial
Trough and its associated disturbances withdraw
southward. This double peak occurs in much of
interior peninsular India south of about 20
jet to the south over the Indian Ocean. The other
upper-air outflow to the north fuels the weaker
westerly jet there. Convective activity moves east
from the Indian Ocean to the cooler eastern
Pacific with an irregular periodicity (on average
40-50 days for strong waves; Note 1), finding
maximum expression at the 850mb level and
clearly being connected with the Walker circula-
tion. After the passage of an active convective wave
there is a more stable break in the summer
monsoon when the ITCZ shifts to the south. The
easterly jet now weakens and subsiding air is
forced to rise by the Himalayas along a break
trough located above the foothills (see
Figure
11.16
), which replaces the Monsoon Trough
during break periods. This circulation brings rain
to the foothills of the Himalayas and the
Brahmaputra valley at a time of generally low
rainfall elsewhere. The shift of the ITCZ to the
south of the subcontinent is associated with a
N and
in western Sri Lanka, although autumn is the
wettest period.
There is a variable pulse alternating between
active and break periods in the May to September
summer monsoon flow (see
Figure 11.16
) which,
particularly at times of its strongest expression
(e.g., 1971), produces periodic rainfall (
Figure
11.32
). During active periods, the convective
Monsoon Trough is located in a southerly
position, giving heavy rain over north and central
India and the west coast (see
Figure 11.16
).
Consequently, there is a strong upper-level
outflow to the south, which strengthens both the
easterly jet north of the equator and the westerly
°
70
60
50
40
30
20
10
0
16 21 25
31
5
10
15
20
25
30
5
10
15
20
25
31
510
15
2025
31
510
15
20
25
30
MAY
JUNE
JULY
AUGUST
SEPTEMBER
Figure 11.32
Mean daily rainfall (mm) along the west coast of India during the period 16 May to 30 September
1971, showing a pronounced burst of the monsoon followed by active periods and breaks of a periodic nature. All
years do not exhibit these features as clearly.
Source: After Webster (1987b). Copyright © 1987. Reproduced by permission of John Wiley & Sons, Inc.
