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seasons of pre-monsoon and post-monsoon with further break up of their
frequencies to AS and BoB for these seasons. Figure 4a shows such
intensification ratio for whole Indian region being of least fluctuated between
0.1 and 0.25 i.e. 10-25% of total systems in a year intensified to SCS and
above, for a very long period of 1891-1960 i.e. for 71 years, irrespective of
very high formation of total systems of D and above in 1930s and 1940s in
Fig. 1a. It was mainly because 1930s and 1940s too had very high number of
SCS and above, as observed in Fig. 1b. But it has then increased to 0.3 only in
1965 after which, it has remained stable for longer period of 1965-2012 of 48
years, which is interesting to note. This is because annually, when total systems
formation of D and above over India reached at its 2
nd
highest peak in 1960s,
higher number of systems were also intensified to SCS and above, while by
1990 and 2010, though their total systems decreased to almost half to one
third, number of D and above intensified to SCS and above, has decreased at
very slow rate (see Figs 1a and 1b). Hence, relative ratios of those intensified
to SCS to total systems remain same or increased in 1990 or 2010 compared to
that of 1960 (see Fig. 4a) with same intensification trends still being persisting
till 2011. Figure 4b shows basin-wise intensification ratios over BoB and AS
which shows that it has followed same climatic variation of intensification rate
for BoB as over whole Indian region as in Fig. 4a, except BoB has slightly
high intensification ratio in 1960-1990 compared to India in contrast to slight
low intensification in 1990-2011 for BoB. This was because climatic trends
difference in genesis of frequencies of total systems before 1990 compared to
after 1990 between BoB and AS.
It was 1990-2010, when number of SCS and above reduced to nearly one
third to half in BoB compared to 1960-1990 when depression only reduced to
half (see Fig. 1 and Fig. 4b) and intensification ratio had not much changed for
whole period of 1960-2010, while for AS, both have increased significantly
from 1995 resulting intensification factors of D to SCS and above (Fig. 4b)
reaching to 0.53 by 1996-2010. It is further interesting to note from Fig. 4c
about intensification to SCS and above, from total systems in main cyclone
season of post-monsoon and pre-monsoon. It continued to be higher in pre-
monsoon for the whole period with recent decade having 0.6 in contrast to
post-monsoon intensification rate of 0.1-0.2. We have further analyzed this
intensification ratio with respect to pre-monsoon and post-monsoon separately
both for AS and BoB (Figs 4d and 4e). It was the BoB where the intensification
rate has been constantly higher, though for post-monsoon, there are not much
differences in their intensification ratio from 1995 to 2010. The pre-monsoon
intensification rate over BoB continued to be higher.
Figure 4(f-g) shows cyclogenesis ratio of AS to BoB defined as portion or
ratio of systems of respective intensities formed in AS in a year or season to
those formed in BoB from annual frequencies of D and above, CS and above,
and SCS and above. Figure 4h shows their ratio for D and above in two main
seasons of monsoon and post-monsoon followed while Fig. 4i showing such
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