Monthly characteristics of tropical cyclone in the North Indian Ocean

Based on the data of tropical cyclone in the North Indian Ocean from 1977 to 2008 from JTWC,the monthly SST and daily HGT from NOAA,and the global reanalysis data from 1982 to 2008 from NCEP,the double peak character of tropical cyclone over the North Indian Ocean is analyzed.The results show that:(1) the position of flow field in the 500 hPa Intertropical Convergence Zone(ITCZ) well explained seasonal changes of tropical cyclone,as well as the seasonally changes of South Asia High in the 500 hPa according to the changes of tropical cyclone;(2) the seasonal variation of SST and the outbreak of the Monsoon show their impacts on occurring frequency of tropical cyclone,which means that high SST and weak vertical wind shear were in favor of the tropical cyclone frequency;(3)When the Monsoon broke out,the strong wind vertical shear was not conducive to the formation of tropical cyclone.Considering all the factors of above,they are the factors that led to the double peak structure of the tropical cyclone in the North Indian Ocean.