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pre-monsoon does not change much for 1891-2011 with their frequencies
fluctuating between 0.5 to 1. Comparison of frequencies of SCS and above
between both cyclone seasons of pre- and post-monsoon in Fig. 2f shows them
to be formed a little higher number in post-monsoon till 1955 followed by
post-monsoon frequencies shooting to 3-5 times of those formed in pre-monsoon
during 1960-1999. Thereafter, it is interesting to note that during last decade of
2001-2011, number of SCS formation during both of these main storm seasons
have been equalized.
Figure 3a shows relative contribution of each season's frequencies of D
and above to total annual systems while Fig. 3b shows for SCS and above for
relative contribution of two main cyclone seasons of pre- and post-monsoon to
total annual SCS systems, in terms of % of annual systems formed, in these
seasons corresponding to respective system's intensities. It shows during 1891-
1980, monsoon alone account upto 50-60% of D and above followed by very
less contribution to their annual formation reaching to 30% till 2000 and then
a small recovery to 38% by 2011. But in other season, it remained around 30-
35% for post-monsoon from 1891 till 1960 followed by increased relative %
formation by 2000-2011 to 40-45% in post-monsoon while in pre-monsoon it
remained around 10% till 1980 from 1891, which then increased to 20% during
1991-2010. During such decadal shifting of their % of formation, it is interesting
to note a higher % of formation of these systems from annual totals that have
been formed in post-monsoon during 1987-2011 compared to monsoon with
highest of 66% of total annual systems of depressions formed in post-monsoon
and 31% in monsoon during mid 1990s. But when relative % of formation of
system of SCS and above from two main cyclone seasons of pre- and post-
monsoon of a year to total number of SCS and above in that year are analyzed
in Fig. 3b, it shows that % contribution to annual cyclone remains major and
constant to nearly 80% SCS in post-monsoon with 20% pre-monsoon from
1891 till 1995 followed by decrease in formation in post-monsoon to 60% till
end of the period by 2011 with increase in pre-monsoon to 40% in 2011. It
may be noted that during the latter period, the total annual SCS and above
formed, also were less compared to those in 1990s. This analysis shows season-
wise, post-monsoon has become most favourable for depression formations
from mid 1990s for Indian region among all seasons in contrast to monsoon as
was conventional before 1990s, with pre-monsoon as most favourable among
all seasons for SCS formation in contrast to post-monsoon as was before
year 2000.
Figure 3 (c-f) shows basin-wise (Arabian Sea, BoB, Indian land areas)
vis-à-vis season-wise decadal variation and trend of various cyclonic systems
over Indian region. It is interesting to note from Figs 3c and 3d that frequencies
of D and above during pre-monsoon season over both ocean basins have not
shown any noted variability like their annual frequencies over all India or BoB
for such long period of 121 years with their frequencies almost persisting in 0
to 0.5 and 1-2 respectively. The comparison between each season for BoB in
Fig. 3d shows most systems formed in monsoon till 1983 followed by post-
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