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to such 18% compared to those of BoB persistently till 1992 followed by very
high increasing trend of such ratio to 0.46 in 2000. It shows a very high number
of D and above formed over AS compared to BoB from their annual total in
this period which also till 2011 continued to be same. In other words, in %, it
is AS during 1995-2011, where about 46% of cyclonic systems of those totally
formed over BoB in a year has been formed. When we consider intense systems,
to find, cyclogenesis ratio of AS to BoB at decadal scale for CS and above and
SCS and above in Fig. 4g, it shows their values are of high fluctuation of 0.1.-
0.6 of very short period variability during 1891-1990. But during 1990-2011,
it increased from 0.27 to 0.94 and 0.65 respectively for number of CS and
above and for SCS and above, which may be classified as another most
significant climatic shifting for AS after decrease in monsoon's D and above
over BoB. But when we further break them season-wise in Fig. 4h for two
major depression seasons i.e. monsoon and post-monsoon, it shows consistent
increasing of cyclogenesis ratio of AS to BoB which has reached in monsoon
(post-monsoon) from 10% (25%) of those in BoB in 1940 (1925) to 70% (50%)
in 2011. It is interesting to note that such tremendous shifting in climate features
in cyclogenesis between these two major basins of NIO have been noticed for
both seasons. We have shown this relationship of BoB to AS from actual
frequencies of monsoon's D and above, for 1891-2011 in Fig. 4i, which also
shows significant decreasing trend of D and above over BoB in monsoon since
1940s while a constantly slow increasing trend over AS from 1920 till 2011.
5. Conclusions
Present study has brought out inter decadal and linear trends of frequencies of
various intense cyclonic systems of NIO covering inter-annual, sub-seasonal
and basin-wise scale in their frequencies, intensification ratio etc., using data
of 1891-2011. Some of the most important findings are the season-wise shifting
in the formation of these systems. It shows shift of the season in highest
formation of D and above to post-monsoon from monsoon and equal or more
SCS forming in pre-monsoon which occurred for the 1
st
time in recent decade
2001-2011, out of all 12 decades of their formations. Basin-wise response to
these climate shifting shows, it was the decreased genesis at BoB with specific
to monsoon in case of D and above which has been causing such prime and
unusual climatic shifting during 1980-2011. Analysis shows the decreasing
trend of monsoon's D and above of BoB since 1940s is unusual, as their numbers
have been consistently downward till 1997 when it was reduced from 7.6 to
1.7, a 78% decrease of their decadal frequencies in 1990-2000 compared to
1935-1950. It finds annual cyclogenesis for whole India of various intensities
have followed epochal variation of mainly two cycles of increasing and
decreasing trends at multi-decadal time scale with two maximum in 1940s and
1970s. Similar analysis, but basin-wise, shows same multi-decadal trends for
BoB with two major maximum during 1940s and 1970s respectively for systems
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