Abstract: Global warming and its impact on severe weather events have been one of the most popular topics of scientific interest during the recent decades. Several studies have examined impact of global warming over the frequency and intensity of tropical cyclones (TCs) forming over different oceanic basins. While some studies have reported a decrease in TC frequency and an increase in severe TC frequency during the recent years, some others are uncertain on whether changes in TC activity have exceeded the variability expected from natural causes. The present study analyses variability in TC activity over the North Indian Ocean (NIO) basin [which comprises the Bay of Bengal (BOB) and the Arabian Sea (AS)] in the context of changing environmental conditions under global warming scenario based on data of 1951-2010. Significant trends in cyclonic disturbances (CD), TC and severe TC activity over NIO are noted and environmental features known to be associated with cyclonic activity, viz., sea surface temperature (SST), vertical wind shear (VWS), low level vorticity and mid-tropospheric humidity are examined in relation to the trends in the CD/TC/severe TC activity over the NIO. The trends are also analysed from the point of view of ENSO forcing. Significant decreasing trends are observed in CD frequency and their further intensification into severe TCs over BOB during the post-monsoon chief cyclone season and the monsoon season despite increasing SST. Concomitant with this decrease, there is a significant decreasing trend in mid-tropospheric humidity over BOB during both the seasons. Over the AS, no significant trends in frequency of CD, TC and severe TC are observed during the post-monsoon and monsoon seasons. However, probability of intensification into TCs and severe TCs over AS has increased in association with decrease in VWS over AS during the two seasons. There are no significant trends in the frequency of CDs, TCs and severe TCs over BOB and AS during the pre-monsoon season. However, frequency of CDs intensifying into TCs and TCs intensifying into severe TCs have increased significantly over AS and BOB respectively in association with decreased VWS over AS and increased low level cyclonic vorticity over BOB. Concomitant with the decreasing CD frequency, in general, a decrease in frequency of cyclogenesis over the southern parts of BOB leading to a northward shift of mean latitude of formation is also observed. Examination of role of ENSO on the CD, TC and severe TC trends indicates that the impact of ENSO has decreased in recent years.
