Assessment on historical cyclone tracks in the Bay of Bengal, east coast of India

Abstract

The Bay of Bengal (BoB) basin in the North Indian Ocean reports the highest number of tropical cyclones, and the frequency is much higher almost five times compared with the Arabian Sea (AS). Semi-enclosed nature of this basin in conjunction with its funnel shape steers the cyclone pathway striking the land. Historical reports signify that most deadly cyclones with highest catastrophe and death tolls occurred in this basin. The four maritime states located in the east coast of India are highly vulnerable to tropical cyclones. Hence, there is a need to understand the risk factors attributed from extreme winds, storm surge, and associated coastal inundation. Historical data signify higher frequency of post-monsoon cyclones compared with pre-monsoon cyclones in the BoB. This study aims to develop a synthetic track or the most probable cyclone track for each state located along the east coast of India. The synthetic track is a general guidance for numerical models, with utility to evaluate and assess the risk factors along coastal belts. This study also deals with aspects on frequency and cyclogenesis locations in the BoB based on analysis from historical data. Results for decadal scale variability signify that in the present decade, the eastern BoB is quite conducive for cyclogenesis. In this region, oceanic thermal field exhibits a warming trend, and found to extend up to water depth of 600 m. In addition, this study also evaluates the energy metrics such as Power Dissipation Index (PDI), and Accumulated Cyclone Energy (ACE) for tropical cyclones that occurred during past four decades in the BoB. The results signify that PDI for tropical cyclones in the present decade is six times higher when compared with the past. The study has practical applications for mapping coastal vulnerability in a changing climate. Tropical cyclones are associated with devastation resulting in enormous loss of life and property during its landfall. In a broad perspective, lack of disaster management pre- paredness would contribute to high vulnerability. The gen- esis of a cyclone usually referred as 'cyclogenesis' is the development stage or strengthening of a cyclonic circula- tion in the atmosphere during the initial stage of its forma- tion. Tropical cyclones form over warm ocean surface, and favourable conditions are sea-surface temperature (SST) greater than 26 ∘ C; low magnitude of vertical wind shear; larger low-level vorticity; and higher mid-troposphere rel- ative humidity. In global oceans, the western part of the North Pacific reported the maximum number of tropical cyclones (average of 26 per year) followed by eastern North Pacific (average of 17 per year). The South Indian Ocean and North Atlantic has an average of 10 per year, while regions in the North Indian Ocean (NIO) have an average of five cyclones per year (Niyas et al., 2009). Notably, the NIO alone accounts for about 7% of global tropical cyclone activity, with a frequency about five