Geoscience Reference
In-Depth Information
ACE:
y
= 0.4004
x
+ 10.556 and
PDI:
y
= 0.4011
x
+ 5.7698
Lastly Net TC (NTC) activity for Indian Ocean is defined. The storm
frequency (SF) and number of storms days (NSD) of TCs having crossed (i)
Class 1 threshold (Class 1+2+3), then (ii) Class 2 threshold (Class 2+3) and
finally (iii) Class 3 threshold (Class 3) are obtained. NTC is then worked out
for each year as:
NTC = avg {SF
3
/ (avg SF
3
), SF
2+3
/ (avg SF
2+3
), SF
1+2+3
/ (avg SF
1+2+3
), NSD
3
/ (avg NSD
3
), NSD
2+3
/(avg NSD
2+3
), NSD
1+2+3
/ (avg NSD
1+2+3
)}
× 100
Here, subscripts 1, 2 and 3 represent different classes considered and 'avg'
is an average function. The averages for all the six parameters in the above
expression over the considered period are:
avg SF
3
= 1.1,
avg SF
2+3
= 2.2,
avg SF
1+2+3
= 4.8,
avg NSD
3
= 7.8,
avg NSD
2+3
= 15.3 and
avg NSD
1+2+3
= 27.9.
NTC is expressed in percentage and its average value is 100. It is seen
from Fig. 5 that NTC parameter has increasing trend with trend coefficient
0.96 which is significant at 99%. This suggests the considerable increase in the
cyclonic activity over NIO. It is also seen that high cyclonic activity occurred
in the decade 1991-2000. Extrapolating the plot, it is anticipated that NTC
activity will be normal (near average NTC i.e. 100%) in near future.
4. Conclusion
This cogitation investigates TC activity over Indian Ocean in relation to other
ocean basins. Detailed examination for the period 1977-2006 shows increase
in intense cyclonic activity over north and south Indian Ocean in recent 15
years along with north Atlantic and south west Pacific ocean basin. Cyclones
over South Indian Ocean are becoming more and more intense with the highest
rate. The study reveals that cyclogenesis events over the Arabian Sea have
increased whereas Bay of Bengal witnesses decrease in these events. These
changes are in confirmation with Gray's parameters. Further length of the
cyclone season in NIO for both pre- and post-monsoon seasons shows increase
in total cyclone season length mainly due to increase in pre-monsoon cyclone
season length indicating expansion of cyclone season over NIO. Energy metrics
like ACE and PDI show significantly increasing trend over NIO throughout
the period. NTC parameter formulated for NIO shows high cyclonic activity
in 1991-2000 but thereafter declining and approaching to normal in the recent
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