Influence of Summer Monsoon on Asymmetric Bimodal Pattern of Tropical Cyclogenesis Frequency over the Bay of Bengal  

作　　者：XING Wen HUANG Fei 

机构地区：[1]Physical Oceanography Laboratory and Key Laboratory of Ocean-Atmosphere Interaction and Climate in Universities of Shandong,Ocean University of China

出　　处：《Journal of Ocean University of China》2013年第2期279-286,共8页中国海洋大学学报（英文版）

基　　金：supported by the National Basic Research Program of China(973Program:2012CB955604);National Natural Science Foundation of China(No.40975038,40830106);the CMA Program(GYHY200906008)

摘　　要：The influence of summer monsoon on tropical cyclone (TC) genesis over the Bay of Bengal (BoB) is explored using an empirical genesis potential (GP) index. The annual cycle of cyclogenesis frequency over the BoB shows an asymmetric bimodal pattern with the maximum genesis number appearing in late October and the second largest in early May. The two peaks correspond to the withdrawal and onset of the BoB summer monsoon, respectively. The semimonthly GP index calculated without TC days over the BoB is consistent with TC genesis frequency, indicating that the index captures the monsoon-induced changes in the environment that are responsible for the seasonal variation of TC genesis frequency. Of the four environmental variables (i.e., low-level vorticity, mid-level relative humidity, potential intensity, and vertical wind shear) that enter into the GP index, the potential intensity makes the largest contribution to the bimodal distribution, followed by vertical wind shear due to small wind speed during the summer monsoon onset and withdrawal. The difference in TC genesis frequency between autumn and late spring is mainly owing to the relative humid-ity difference because a divergence (convergence) of horizontal moisture flux associated with cold dry northerlies (warm wet wester-lies) dominates the BoB in late spring (autumn).The influence of summer monsoon on tropical cyclone （TC） genesis over the Bay of Bengal （BOB） is explored using an empirical genesis potential （GP） index. The annual cycle of cyclogenesis frequency over the BoB shows an asymmetric bimodal pattern with the maximum genesis number appearing in late October and the second largest in early May. The two peaks correspond to the withdrawal and onset of the BoB summer monsoon, respectively. The semimonthly GP index calculated without TC days over the BoB is consistent with TC genesis frequency, indicating that the index captures the monsoon-induced changes in the environment that are responsible for the seasonal variation of TC genesis frequency. Of the four environmental variables （i.e., low-level vorticity, mid-level relative humidity, potential intensity, and vertical wind shear） that enter into the GP index, the potential intensity makes the largest contribution to the bimodal distribution, followed by vertical wind shear due to small wind speed during the summer monsoon onset and withdrawal. The difference in TC genesis frequency between autumn and late spring is mainly owing to the relative humidity difference because a divergence （convergence） of horizontal moisture flux associated with cold dry northerlies （warm wet westerlies） dominates the BoB in late spring （autumn）.

关 键 词：tropical cyclone genesis Bay of Bengal summer monsoon asymmetric bimodal pattern GP index 

分 类 号：P444[天文地球—大气科学及气象学] TQ325.12[化学工程—合成树脂塑料工业]