Geoscience Reference
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et al., 2012 at global scale confirmed existence of increasing trends of intense
systems over global ocean basins. Emanuel et al. (2005) shows substantial
increase in the power of tropical cyclones (denoted by the integral of the cube
of the maximum winds over time) for the West Pacific and Atlantic basins
during the last 50 years.
Webster et al. (2005) show there has been a substantial global increase
(nearly 100%) in the proportion of the most severe tropical cyclones (categories
4 and 5 on the Saffir-Simpson scale), from the period 1975 to 2004, which has
been accompanied by a similar decrease in weaker systems. Some of these
studies have also analyzed trends of various ocean-atmospheric parameters
termed as genesis parameters to find out the cause of such trends over Indian
region e.g. Dash et al., 2004; Hoarau et al., 2011; Evan et al., 2011, etc. Most
of these studies have either emphasized north Indian Ocean as one domain of
cyclogenesis activities or frequencies restricted to monsoon or whole year while
finding various results. However, analysis of annual frequencies of cyclogenesis
of intensities of depressions (D) and above when performed for Arabian Sea
(AS) and Bay of Bengal (BoB) for 1941-1950 vis-à-vis 2001-2010, it shows in
1941-1950, 90% of total systems of NIO were formed over BoB with remaining
10% formed over AS while for 2001-2011, it decreased to 65% for BoB and
increase to 35% for AS, a significant shift in respective formation proportion.
Similarly, there has been trends of more formation of number of severe systems
e.g. severe cyclonic storms (SCS) and above over AS during recent decade of
2001-2010 touching 42% of total severe systems from those total system formed
in NIO which was just about 8.9% of the total intense systems formed during
1961-1970 while for BoB, the formation decreased from 91% of the total severe
systems of NIO to 57%. Because of higher frequencies of formation of SCS
over AS, much of the systems also moved to extreme west off Arabia coast
affecting Oman and other Arabia coast during recent years. Hence, there is a
need to study both linear and decadal trends of cyclonic disturbances of various
intensities i.e. intensity of D and above, Cyclonic Storms (CS) and above, and
SCS and above, using long period data by considering AS region and BoB
region separately and NIO as one domain. Normally, around 65% of total annual
systems were formed in monsoon, till 1970s while in recent years from 1990s,
such higher formation have shifted to post-monsoon in most years. Further,
frequency of monsoon depressions also have been continued to decrease and
there were not a single monsoon depressions formed in the recent years of
2002, 2010 and 2012, while post-monsoon has no such high trend. Hence, it is
necessary to understand how epochally, decadal trends between these three
prominent cyclonic seasons are linked to each other including intra-seasonal
climatic shift and basin-wise shift in their genesis, if any.
In the present study, an attempt has been made to find climate variability
of various characteristics of cyclogenesis with respect to their intensity,
frequencies, area of formation etc. at interdecadal time scale using their long
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