Geoscience Reference
In-Depth Information
26.1.2
Monsoon Depression
The monsoon depressions (
Sikka 1977
) are systems that are intermediate in terms
of intensity between the relatively weak low-pressure systems that have wind
speeds less than
ms
1
and the tropical cyclones that have associated wind
8:5
ms
1
. The preferred region of formation of these monsoon
depressions over India is between
speeds exceeding
17
90
ı
E. Typically
the average number of monsoon depressions that can form over India during the
summer monsoon months of June to September is about 6 with the month of
August accounting for about two monsoon depressions. The maximum number
of monsoon depressions, that forms over India is however higher. While some
of the monsoon depressions owe their origin to weak easterly waves traveling
from the east, the other depressions can develop in situ over the North Bay of
Bengal. The monsoon depressions that form over North Bay of Bengal have a
horizontal radial extent of 1,000 km. Monsoon depressions are systems known to
be typically cold core below 700 hPa and warm core aloft. Because of the above
fact, the strongest winds associated with the monsoon depressions are observed
near 700 hPa. At higher levels, the cyclonic circulation associated with the monsoon
depression weakens and is absent at and above 300 hPa. The location of the center of
the monsoon depression slopes south-westward with height. The winds associated
with the monsoon depressions are asymmetric with stronger winds south of the
depression center at low levels. The maximum horizontal convergence of moist
air together with the associated maximum spatial precipitation pattern is found in
the south-west sector of the monsoon depression. The principal zone of heaviest
precipitation associated with a monsoon depression occurs at about 200-400 km
away from the center while a secondary zone of relatively lower rainfall is seen at
about 800 km to the west of the depression center.
Typically the monsoon depression moves in the west-to-west to north-west
direction (
Mooley and Shukla 1989
). During the months of June and September,
the movements of the monsoon depression can follow either the northerly direction
or they can recurve over the Bay of Bengal. However, during the other 2 months of
July and August, most of the monsoon depressions move in the west-north-westerly
direction over India. During the month of July, the average speed of a monsoon
depression is between 1.2 and
20
ı
30
ı
Nand
80
ı
and
and
ms
1
to the east of
85
ı
E while the average speed
2:4
85
ı
E. The average life
period of a monsoon depression is about 5 days for a depression which has formed
over the Bay of Bengal while the same is about 3 days for a depression that has
formed over the Arabian Sea and over land.
The monsoon depressions (
Shukla 1978
) which form during the Indian summer
monsoon season do not generally intensify into a tropical cyclone. The existence
of low level westerly winds together with strong upper level easterlies during the
Indian summer monsoon season is responsible for the existence of strong wind shear
in the vertical. Such strong vertical wind shears do not aid in the manifestation
of penetrative convection, the latter essential for the formation of large scale
ms
1
, to the west of
of the depression is between 4.8 and
9:6
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