Geoscience Reference
In-Depth Information
Climate Change in Tropical Cyclones
and Monsoon Depressions of
North Indian Ocean
K.S. Krishnamohan*, K. Mohanakumar and P.V. Joseph
Department of Atmospheric Sciences
Cochin University of Science and Technology
Fine Arts Avenue, Cochin - 682016, India
*e-mail: krishmet@gmail.com
1. Introduction
In the North Indian Ocean (NIO) bounded by longitudes 30 o E and 100 o E,
north of the equator, Tropical Cyclones (TC) with wind speed 34 knots or
more occur mainly during the pre-monsoon months April-May and the post-
monsoon months October-December. During the summer monsoon season June
to September, TC are few but we get Monsoon Depressions (MD) in large
numbers that differ from TC in their intensity, asymmetry of winds around the
centre, tilt of the circulation centre with height, vertical wind shear in the
environment etc. We have studied the climate change in TC and MD using the
data available for the 119 years 1891 to 2009 (IMD hard copy and e-atlases on
cyclones and annual cyclone reports published in the IMD quarterly journal
Mausam ). Gray (1979) using 20 years of data has shown that the global tropics
has seven ocean basins producing about 80 tropical cyclones in a year in which
only six are from NIO. It is well known that the cyclones of NIO have caused
the maximum loss of human life. The death toll in a single one, the Bangladesh
cyclone of November 1970, has been estimated to be about 300,000.
2. Climate Change in Annual Frequency of TC and MD
The annual frequency of MD (genesis during June to September) during the
period 1891 to 2009 had a large and statistically significant decreasing trend
with their frequency decreasing from about 12 in the earliest decade to about 4
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