Geoscience Reference
In-Depth Information
similar movement and strengthening of the
westerly jet to the north, weakening the Tibetan
anticyclone or displacing it northeastward. The
lack of rain over much of the subcontinent during
break periods may be due in part to the eastward
extension across India of the subtropical high
pressure cell centered over Arabia at this time.
It is important to realize that the monsoon
rains are highly variable from year to year,
emphasizing the role played by disturbances in
generating rainfall within the generally moist
southwesterly airflow. Droughts occur with some
regularity in the Indian subcontinent: between
1890 and 1975 there were nine years of extreme
drought (
Figure 11.33
) and at least five other years
of significant drought. These droughts are brought
about by a combination of a late burst of the
summer monsoon and an increase in the number
and length of the break periods. Breaks are most
common in August to September, lasting on
average five days, but they may occur at any time
during the summer and may last for up to three
weeks.
The strong surface heat source over the Tibetan
Plateau, which is most effective during the day,
gives rise to a 50-85 percent frequency of deep
cumulonimbus clouds over central and eastern
Tibet in July. Late afternoon rain or hail showers
are generally accompanied by thunder, but half or
more of the precipitation falls at night, accounting
for 70-80 percent of the total in south-central and
southeastern Tibet. This may be related to large-
scale plateau-induced local wind systems. How-
ever, the central and eastern plateau also has a
frequency maximum of shear lines and associated
weak lows at 500mb during May to September.
These plateau systems are shallow (2-2.5km) and
only 400-1000km in diameter, but they are
associated with cloud clusters on satellite imagery
in summer.
5 Autumn
Autumn sees the southward swing of the
Equatorial Trough and the zone of maximum
convection, which lies just to the north of the
weakening easterly jet (see
Figure 11.16
). The
breakup of the summer circulation systems is
associated with the withdrawal of the monsoon
rains, which is much less clearly defined than their
onset (
Figure 11.34
). By October, the easterly
trades of the Pacific affect the Bay of Bengal at the
500mb level and generate disturbances at their
confluence with the equatorial westerlies. This is
70
1918
60
50
40
30
20
10
0
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
Figure 11.33
The yearly drought area index for the Indian subcontinent for the period 1891-1988, based
upon the percentage of the total area experiencing moderate, extreme or severe drought. Years of extreme
drought are dated. The dashed line indicates the lower limit of major droughts.
Source: After Bhalme and Mooley (1980). Updated by courtesy of H. M. Bhalme. Reproduced by permission of the American
Meteorological Society.
