6. Inter-annual Variation and Trends in Tropical Cyclones and Monsoon Depressions Over the North Indian Ocean

Indian coastal regions are vulnerable to the destructive effects of land-falling tropical cyclones (TCs) that form over the North Indian Ocean (NIO). In the view of global climate variability and global warming, it is important to analyse the trends in frequencies of cyclonic disturbances (CDs) forming over the NIO. CDs include depressions (maximum sustained surface wind speed (MSW) of 17–33 knots) and TCs (MSW ≥ 34 knots). In this chapter, long-term trends in seasonal and annual frequency of CDs, TCs and severe TCs (MSW of 48 Knots or more) over the NIO are presented based on the data of 1901–2010. The trends are also analysed with the perspective of climate forcing such as the El-Nino/Southern Oscillation (ENSO). Over the Bay of Bengal (BOB) and the NIO, significant decreasing trends in the frequencies of CDs as well as TCs are observed during the monsoon season (June–Sept) during the period 1901–2010. Significant increasing trend in severe TCs during the post-monsoon season (October–December) was also observed during the same period. Over the Arabian Sea (AS), there is a significant increase in the frequency of CDs during the monsoon and the post-monsoon seasons. However, during the satellite period (1961–2010), CDs, TCs and severe TCs over the NIO and BOB show significant decreasing trends for the monsoon and post-monsoon seasons and the year as a whole. No significant trend is observed over the AS during the same period. Analysis of role of ENSO on the frequency of CDs/TCs/severe TCs indicates that the impact of ENSO has decreased in the recent years. However, in the case of the post-monsoon TCs and severe TCs, significant negative relation has emerged since 1995 with Nino 3.4 SST in concurrent and antecedent (monsoon season) modes. Significant positive relationship that existed between the monsoon rainfall over meteorological sub-divisions of Central India and CDs frequency during the period 1940–1990 weakened drastically during the later years. Also, significant positive relationship that existed with the north-east monsoon rainfall (October–December) over south-eastern sub-divisions of India up to 1980 has weakened subsequently.