Characteristics of Cyclogenesis over the Indian Region during 1891-2011 with Special Emphasis on Bay of Bengal vis-à-vis Arabian Sea - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 108

of respective intensities while for AS, it shows major downward trend in 1977
and then reversed from 1995.
Amplitude and periodicity of these inter-decadal variation shows the BoB
region have D and above fluctuates between -4 to 4 which are of very high
amplitude and follows very long period of 30-50 years periodicity e.g. 1920-
1976 when it was persistently above normal, a period when ENSO-ISMR were
strongly linked in contrast to AS which follows very low periodicity of 5-20
years with varied amplitude of -1 to 1. Frequency of SCS and above for whole
Indian region also shows that it follows lower epochal periodicity with low
amplitude of -1 to 1 of above normal and below normal of 5-20 years. Their
basin-wise splits for BoB and AS also show similar variation of lower epochal
periodicity and amplitude. Intensification ratio(IR) to SCS and above from
total system formed Indian region shows that it has been increased to double
from 1891 till 2011 while it is 3-times in 2011 since 1950s. Such IR mainly
attributes to BoB variation. Season-wise study of IR for main cyclone season
of post-monsoon and pre-monsoon shows it continued to be higher in pre-
monsoon for the whole period with recent decade having 0.6 in contrast to
post-monsoon reaching very low intensification stage of 0.1-0.2. Annual
cyclogenesis ratio for D and above of AS to BoB showed that it was stable for
a very long period of 1891-1970 with AS having 10-20% of formation during
such long period compared to those total annual cyclonic systems of D and
above formed over BoB. But, it has recently increased explosively to 46%
during 1999-2011. For CS and above and for SCS and above, it shows their
values are of high fluctuation of 10% to 60% of very short period variability
during 1891-1990. During 1990-2011, it increased from 27% to 94% and 85%
respectively for number of CS and above and for SCS and above which may
be considered the 3 rd most significant climatic shifting detected in this study
for AS.
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