Construction and Quality of Best Tracks Parameters for Study of Climate Change Impact on Tropical Cyclones over the North Indian Ocean during Satellite Era - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 9

Table 4: Mean, standard deviation (SD) and coefficient of variation (CV)
of frequency of VSCS during pre-monsoon, monsoon and post-monsoon
seasons and year as a whole

Parameters Pre-monsoon Monsoon Post-monsoon Year as a whole

season season

Mean 0.5 0.3 1.2 2.0

SD 0.6 0.6 1.0 1.4

CV 108 206 88 70

4. Trends in Frequency in Satellite Era

4.1 Trend in Frequency of CD

Considering the short-lived disturbances (CDs with life period of less than one
day), the frequency of such systems was very high in the beginning of 19 th
century (Mohapatra et al., 2012). It gradually decreased to zero during 1961-
1970 (Fig. 1a). It decreased from 68 during 1901-1910 to 03 during 1951-
1960. It indicates over-estimation of genesis and hence frequency in the absence
of adequate data during pre-satellite era. Considering the impact of frequency
of short-lived systems on the long-term trends (Fig. 1b), there is significant
decreasing trend in the frequency of CD when short-lived systems are included.
Excluding the short-lived systems, the trend in annual frequency is insignificant.
Considering the trend in frequency of CD during satellite era (1961-2010),
there is significant (at 95% level of confidence) decreasing trend in frequency
of CDs over the BOB and NIO and no trend over the AS (Fig. 2a). The frequency
of CD has decreased at the rate of 1.6 per decade and 1.5 per decade over the
NIO and BOB respectively (Table 5). This trend is mainly due to the decreasing
trend in frequency of CDs during monsoon season (not shown).

Fig. 1(a): Frequency of CDs including and excluding the short-lived CDs during
1891-2009 (Mohapatra et al., 2012).