Geoscience Reference
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3. Inter-decadal Climate Shifts of Frequencies and
Intensity of Cyclogenesis during 1891-2011 for Indian
Region, BoB and AS Using Annual Frequencies and
Seasonal Frequencies
Figure 1(a-f) shows trend of annual frequencies of various intense cyclonic
systems (D and above, CS and above, SCS and above) over Indian region and
then basin-wise differences of their frequencies for BoB, AS and main land
region of India from 11-years running means for 121 years (1891-2011). Figure
1a shows that total systems over the region has increased from nearly 10 in
1890-1910 to 16 in 1940s followed by slight decreasing trend in decadal
frequencies to nine in 1955 which was then followed by increasing frequencies
to 15-16 in 1970s. Thereafter, it has followed a consistently high decreasing
trend till 1990-2000 to eight when almost half of total systems have been formed.
During latest decade of 2000-2011, their frequencies have been persisting at
same lower number of 8-9 in a year. During the total period of 1891-2011, the
total linear trend of D and above has slight decreasing trend from 13 to 12.
Annual frequencies of CS and above in Fig. 1b show formation of 5-6 during
1891-1925 which then increased to seven in 1930. It decreased significantly to
3.5 in 1950 and then increased seven in 1970s. It has decreased to 3-4 in 1990
which now nearly stagnated till 2011. During the total period of 1891-2011,
the total linear trend of CS and above has decreasing trend from 6 to 4.
Figure 1b also shows that annual number of SCS and above remained
fluctuated in 1.5 to 2.5 for very long period of 1891-1960 in contrast to a
prominent decadal cycle of increasing and decreasing trends as was well noticed
in 1915-1950 as in Fig. 1a and Fig. 1b for D and above and CS and above. But
from 1960, SCS frequencies have followed similar decadal variation of systems
as by other systems discussed before. SCS has then increased from 2.5 in 1950s
to 4 in 1970 followed by decreasing trend to 2 in 1990. It has then remained
fluctuated 2-3 till 2011. Linear trend of SCS for the whole period of 1891-
2011 shows slight increasing trend of 2 to 3 during the period. Figures 1c and
1d analyzed for AS and BoB show that its frequencies over BoB has increasing
trend from 8 to 14 during 1911-1946 followed by decreasing to 6 till 1996 and
not of much change thereafter till 2011, in contrast to that of AS as increasing
trend during 1911-1976 from 0.5 to 3.3 and then decreased to 1.3 in 1991
followed by increasing to 2.5 till 2011. The decadal variation of annual
frequencies over land areas follows almost similar decadal trend as AS but it
has not shown much change during 1991-2011. Respective linear trends for
these frequencies both for AS and BoB have been shown in Figs 1c to 1f which
show dominant increasing trend of D and above (1 to 2.3) and SCS (0.4 to 0.7)
for AS while decrease in D and above (11 to 9) and increase in SCS (1.5 to 2.5)
over BoB.
Analysis of amplitude and epochal periodicity using anomalies of
frequencies of intense cyclonic systems of D and above and SCS and above at
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