Tropical Cyclone Activity over the Indian Ocean in the Warmer Climate - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 72

Table 1: Group-wise distribution of storm frequency and storm days during 1977-
1991 and 1992-2006.

Basin Group Storm frequency (%) Storm days (%)

1977-1991 1992-2006 1977-1991 1992-2006

NAO Class 1 17 16 14 7

Class 2 36 28 27 24

Class 3 47 56 59 69

WNP Class 1 10 17 6 14

Class 2 20 18 15 14

Class 3 70 65 79 72

ENP Class 1 12 15 7% 9

Class 2 28 33 22 29

Class 3 60 52 79 62

SWP Class 1 18 14 12 8

Class 2 28 24 23 16

Class 3 54 62 65 76

NIO Class 1 25 31.34 22 23

Class 2 47 34.33 40 36

Class 3 28 34.33 38 41

SIO Class 1 20.7 21.50 16 16

Class 2 29.6 22.00 25 20

Class 3 49.7 56.50 59 64

Class 1 : Cyclonic storm
Class 2 : Severe cyclonic storm
Class 3 : Very severe cyclonic storm or higher intensity

is found in the extreme west with expansion of cyclogenesis region in the
north-east direction. These changes in cyclogenesis are in confirmation with
the Gray's Parameters like SST, vertical shear etc. for NIO basin as seen in
Fig. 3 but not very well represented for the SIO basin (figure not shown for
SIO).

3.3 Duration of TC Season over NIO

Analysis of the duration of the season for pre- and post-monsoon cyclones
shows increase in the pre-monsoon cyclone season length and an insignificant
decrease in the post-monsoon cyclone season length (Fig. 4). Detailed analysis
shows that the increase is due to the start date decreasing faster. In short, pre-
monsoon cyclones are formed more earlier now and commencement of the
cyclone season is ahead of time. The total season length shows an increasing
trend, mainly due to increase in pre-monsoon season length indicating expansion
of cyclone season (Fig. 4).