OBJECTIVE CLASSIFICATION OF THE TRACKS OF TROPICAL STORMS IN THE BAY OF BENGAL

An objective analysis of tropical cyclone tracks is performed, with which the tracks of 131 tropical storms（TSs） in 1972-2011 are separated into three types that move west-, north- and northwestward, denoted as Types A, B and C, respectively. Type A（21 TSs and 16% of total） has the origin in the southwestern Bay of Bengal, with the TS in a unimodal distribution as its seasonal feature, occurring mainly in autumn; 18 of the 21 TSs（taking up 90%） land mostly on the western Bay coast（west of 85°E）; 5% of Type-A TSs attains the wind speed of 〉42.7 to 48.9 m/s. Type A has little or no effect on Tibet. Type B（74 TSs, 56.6% of the total） has its preferable origin in the central Bay of Bengal, with the TS in a bimodal distribution as its seasonal pattern. This type denotes the travel in the north in spring,with the landfall of 67 of the 74 TSs（accounting for 91%） mainly on the middle coast of the Bay（85° to 95°E）, and19% of the TSs reaching the wind velocity of 〉42.7 to 48.9 m/s, which exert great effect on Tibet and it is this TS track that gives strong precipitation on its way through this region. Type C（36 TSs, 27.5% of the total） has its main origin in the southern part of the bay, and these TSs are formed largely in autumn, moving in the northwest direction,and 23 of the 36 TSs（64%） land mostly on the western Bay coast, lasting for a longer time, with almost no impact upon Tibet.