O.S.R.U. Bhanu Kumar, P Suneetha and S Ramalingeswara Rao
Keywords: Tropical cyclones, SSTs, MJOI, SOI
Abstract: Analysis of 118 years of data relating to tropical storms highlights that there is a significant change in frequency of tropical storms with rising Sea Surface Temperatures (SSTs) environment over the North Indian Ocean (NIO) on annual, seasonal and sub-seasonal scales. Decadal variability study pointed out that time-series of annual frequency of tropical storms and SST datasets reveal conspicuous turning points in the years, 1970 and 1966 respectively, which coincide with the starting of global warming period. Further there is a significant decreasing trend in the former, while a clear cut increasing tendency is observed in the SSTs field and both trends are statistically significant at 1% level over the NIO. The relationship between them is inverse and the coefficient of correlation between them is -0.52. Next, tropical severe cyclones, which appear maximum in the month of November over NIO have a direct relationship with rising SSTs and a coefficient of correlation between them is to a tune of 0.89 for the above study period. Secondly in the context of recent global warming scenario, datasets (1981-2008) relating to frequency of the tropical systems over the BOB are analyzed in Pre and Post-monsoon seasons; it is observed that there is a clear cut decreasing trend. To find out the possible reason for the decreasing trend, the authors also examined the possible relationships among frequency of the tropical systems, SSTs over the BOB, Madden & Julian Oscillation (MJO) and Southern Oscillation (SO) Indices separately. Total number of systems over the BOB is highly influenced by above SSTs only in the Pre-monsoon season, while they are significantly correlated with MJO and SO Indices in the Post-monsoon season. Above relationships are very robust in the month of November due to high frequency of tropical cyclones in every year. Finally there is a contrasting difference in the frequency of total number of tropical cyclones in the years of El Nino and La Nina episodes
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