Periodicities in the series of cyclonic disturbances over India

The time series of yearly occurrence frequency of cyclonic disturbances over the Indian region for 1891–1970 is subjected to power spectrum analysis by the maximum entropy method (MEM) of Burg. Earlier analyses by the conventional Blackman-Tukey method had indicated periodicities atT= 2·3-2·5, 5 and 30–45 years at about 90–95% significance level. The present analysis shows peaks atT = 1·2, 2·4, 2·9, 3·7, 4·7, 6·4, 7·4, 9·2, 10·5, 20 and 40–60 years but the significance level may not be better than the earlier method. When data are analysed separately for first half and latter half, periodicities observed are quite different, the earlier half revealingT= 1· 2, 2·3, 2·9, 4·7, 9·0 and 23 years and the latter half revealingT= 1·2, 2·1, 2·4, 3·8, 6·7 and 21 years. Thus, a change of pattern before and after about 1920 is indicated. However, the power in the various peaks is small and a random origin cannot be ruled out. An analysis of several artificially produced samples of pseudo-Gaussian random white noise of the same mean, standard deviation and length (80 values) as the above physical samples indicated that similar peaks could occur by pure chance also. Physically, T=l·2 years corresponds to the Chandler’s wobble and other peaks could be harmonics of the sunspot cycleT= 11 years or double sunspot cycleT = 22 years. However, no peak is noticed at or nearT= 11 years whereasT near 22 years is noticed.