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recent decade of 2001-2011 when there has been higher than normal formation
of systems over AS. The BoB region had cyclogenesis anomalies amplitude of
D and above fluctuate between -4 to 4 which are of very high amplitude and
follows very long period of 30-50 years periodicity and persistently above
normal during 1920-1976, a period when ENSO-ISMR were strongly linked
(Ashok et al., 2004; Kumar et al., 1999; etc.) in contrast to AS which follows
very low periodicity of 5-20 years with varied amplitude of -1 to 1 (see Figs
2a and 2b). Hence, the persistently very lower number of D and above from
1980 till recent 2012 over Indian region is a part of very long period multi-
decadal variability of 30-50 years of similar variation mainly followed by BoB
and hence such formation has not been recovered as expected after such long
period of lower than normal formation of D and above during 1980-2012.
Figure 2c, in contrast, shows amplitude and periodicity of variation of intense
systems at SCS stage or more which are of lower values whether it is for AS or
BoB or for whole Indian region having very low periodicity of 5-20 years with
varied amplitude of -1 to 2.
Figure 2d shows 11-years running means of season-wise frequencies of D
and above for pre-monsoon, monsoon and post-monsoon seasons. It shows
that frequency of D and above in monsoon has followed well dominated decadal
shifting with highest of 9 in 1940s from 5 in 1910s. Thereafter, it almost followed
a high decreasing trend till 2000 when only around 2-3 systems formed—a
record reduction by nearly 66-80%, a highest reduction of their genesis among
all seasons. It then followed a pattern of not much change fluctuating at 2-4
during recent decade till 2011. This is almost same as was discussed with their
annual frequencies in Fig. 1c. However, frequency of D and above during
post-monsoon have consistently followed increasing trend from 2 in 1890 to 6
in 1977 followed by a slow decreasing trend in contrast to very high decreasing
trend in monsoon till recent 2011 to 3.5.
Season to season comparison of epochal variation and their amplitude in
Fig. 2d shows it was the monsoon season when the highest frequencies of D
and above, of 5-9, were formed for very long period of 111 years covering
1891-1980. But in post-monsoon, their frequencies were only restricted to 2-6
in same period of 1891-1980 while in pre-monsoon, there were only very few
systems of 1-2 formed during this period. However so well dominated
climatological seasonality in such cyclogenesis have changed from 1986 and
instead of monsoon having highest frequency of D and above, post-monsoons
have higher systems for the first time than monsoon in 121 years of data and
such trend still continued till 2001 with frequencies during both the seasons
being nearly same till 2011. Another interesting feature of seasonality
characteristic of intense cyclogensis is decadal variation of frequency of
monsoon cyclones. In monsoon, they normally form in June or 2
nd
fortnight of
Sept in the season while the month of July and August are free from such
formation because of strongest wind shear during these months, though these
latter months are highly favourable for depression which are less intense systems
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