The relationship between the summer typhoon genesis over the western North Pacific and South China Sea and the main climatic conditions in the preceding winter to spring

Based on the NCEP/NCAR reanalysis monthly data、OLR data and Tropical Cyclone data from 《Typhoon Annual and Tropical Cyclone Annual》edited by China Meteorological Administration, we studied the relationship between the climatic conditions and the number and frequency of tropical cyclones that engender over the western North Pacific and South China Sea in summer. The tropical cyclones that we investigate usually occur with maximum sustainedwinds of equal or greater than 17 m/s, and may be categorized as Tropical Storm, SevereTropical Storm and Typhoon. We simply call them Typhoon in this paper. Firstly, characteristics and differences of the climatic background between the years with more typhoons and those with less typhoons have been compared. The results show that the summer typhoon produce has a close relationship with SST and ITCZ anomalies in the preceding winter and spring. With a La Ni?a like SSTA pattern in the preceding winter and spring, the ITCZ migrates northwest ward and is enhanced around 160oE in the central equatorial Pacific. The Pacific ITCZ is more vigorous and its location is more north than usual, especially in typhoon genesis region in the western Pacific Ocean. The background is propitious to have a more typhoon summer. On the other hand, an El Ni?o like SSTA pattern in the preceding winter will be companied with weaker ITCZ activities, and its location is more south over the western equatorial Pacific Ocean from the preceding winter to spring. It is propitious to have a less typhoon summer. In the more typhoon year, the warm SSTA over the western Pacific in the preceding winter provides a favorable and important background conditionto the typhoon produce in the following summer. It enhances the convergence in the troposphereand increases the water vapor supply over warm SSTA regions. In the following spring,disturbances from the tropical ITCZ play a more important role. When the ITCZ moves northward in spring, anomalous convergence will appear over warm SSTA region and inspire the positive feedback between the large scale moisture flux at low levels and the latent heat release in the atmosphere, which is conducive to the typhoon occurrence in summer. Conversely, if a cold SSTA maintains over the western North Pacific during the preceding winter and spring, it impairs the convergence in the troposphere and decreases the water vapor supply over the cold SSTA region, which will bring on weaker ITCZ activities and the disturbances are lacking in the following spring. This results in a smaller number of summer typhoons.