Climate Change in Tropical Cyclones and Monsoon Depressions of North Indian Ocean - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 33

in the most recent decades (Fig. 1a). The annual frequency of TC during 1891
to 2009 had only a small decreasing trend (Fig. 1b). Both the TC and the MD
have a common feature namely a Four Decade Oscillation (FDO) in their annual
frequency, which is unique for the NIO basin. In the case of MD the FDO is
superposed on the strong linear decreasing trend. The FDO of TC and MD
however, has very little temporal phase difference as shown by the third
harmonic of their frequencies (linear trend removed) in Fig. 1c. Joseph and
Xavier (1999) had shown this earlier in their analysis of TC and MD of NIO.
FDO had large amplitude during 1965 to 2009 (Fig. 1a, 1b) with the years
around 1970 (1990) having high (low) frequency. This period is well covered
by satellite observations and reanalysis wind data. We have taken the years
1965 to 1974 as the high frequency period (HFP) and 1985 to 1994 as the low
frequency period (LFP).
Fig. 1: (a) Annual Monsoon depression frequency of the period from 1891 to 2009
along with five year running mean (thick line) and Linear trend (dashed line).
(b) Same as Fig. 1(a) but for tropical cyclones. (c) Third Harmonic with period of 39
years of Monsoon depression and Tropical cyclone frequencies.