Study on recurving of tropical cyclones in the Bay of Bengal during 1991-2015

Abstract

Tropical cyclone (TC) is a very intense low-pressure system formed over tropical oceans, and its translational motion is largely modulated by large-scale wind flow. Due to the position in the tropical region and the funnel shaped structure, the Bay of Bengal (BoB) is favorable for the formation of the strongest and deadliest TCs in the world. The BoB is located in the northeastern part of the Indian Ocean between 5° N to 22°N and 80° E to 100° E. Usually, TC moves west or northwest direction but sometimes it changes its track from the original path which is called recurvature of TC. In this study, the recurvature of TCs formed over the BoB during 1991-2015 is determined using data from the Joint Typhoon Warning Center. For environmental analysis 6-hourly ERA-Interim reanalysis data with 0.75° × 0.75° resolution are also used in this study. The BoB experiences 79 cyclones during the study period, among them 58.23% (46 cyclones) are found to be recurving in nature. Within the recurving TCs, 16 cyclones are developed in the pre-monsoon (March-May) whereas 30 cyclones are formed during post-monsoon (October-December). The nature of the recurving of TCs is divided into four types which shows that 13.04 %, 41.30 %, 8.7 % and 36.96 % TCs are representing left, right, looping and meandering curvature characteristics, respectively. During study period eight cyclones have diverted more than 100 degree angle from the original track within 24 hours and their wind speed above 65 knots, which are very severe or super cyclonic storms. Among these 8 cyclones, 4 cyclones i.e. cyclone (1998), cyclone (2004), Mala (2006), Nargis (2008) are developed in the pre-monsoon and another 4 cyclones i.e. cyclone (1996), cyclone (1999), Giri (2010), Madi (2013) are formed during post-monsoon. The analysis of environmental wind shows that the upper level wind flow at 300 hPa is responsible for the movement and recurvature of TCs in pre-monsoon, whereas, 500 hPa mid-level flows causes the movement of TCs in post-monsoon. Cyclones in the pre-monsoon normally recurved but landed to the east coast of the BoB, whereas, that in the post-monsoon finally moved to the west coast. The environmental condition before and after recurvature of TCs suggest that cyclones in pre-monsoon become intensified increases from 19% to 63% according to the magnitude of horizontal wind at 850 hPa level around the TC center. The intensification is also accompanied by increasing the values of low and mid-level relative vorticities, low level moisture flux and by lowering the values of surface pressure and SST. But all these values that have been increasing or decreasing during curvature show uncertainty to the change in intensity for the TCs in the post-monsoon season.