Differences And Possible Causes Of Rapid Intensification Of Typhoon Activities In The Northwest Pacific During Two Types Of El Ni(?)o Years

Tropical cyclone is one of the most destructive natural disasters in the world.The rapid intensification(RI)of tropical cyclone is one of the difficulties in forecasting.In Different El Ni(?)o events(EPW and CPW years),the rapid intensification of typhoon activity in the Northwest Pacific shows obvious differences.Therefore,this paper studies the differences and possible causes of the rapid intensification of typhoon activities in the Northwest Pacific during the El Ni(?)o period under the background of climate warming.The TC rapid intensification ratio was highest in late autumn(10-12 months),but the total TC frequency and the maximum frequency of RI occurred in summer.The possible controlling factors for tropical cyclones are the average TC position,the relative vorticity of the environment and the vertical profile temperature of the atmosphere and ocean.The relative low latitude TC position and the apparent relative vorticity of the cyclone are favorable for the rapid intensification of RI ratio in late autumn.In addition,thermodynamic conditions contribute more to the variation of RI Ratio,and the subsurface ocean conditions(deep mixing layer and strong thermal stratification in subsurface layer)are favorable for the increase of RI Ratio in November and December.EPW and CPW,two different El Ni(?)o events,have significant influence on the seasonal variation of the RITC in the Northwest Pacific.For the EPW(CPW)years from July to October(November to December),the RITC ratio is higher.The interdecadal variations from1979 to 2016 were analyzed to find the reasons for the variation of typhoon frequency in the late season.The differences in the annual RITC ratio between EPW and CPW are due to the location of TC generation,the path of TC movement and large-scale environmental conditions.In the late season,the location of TC in EPW year is more northwest,but in the peak season,the location of TC in the northwest(southeast)direction is disadvantageous to the larger RITC ratio.The significant southward steering flow anomalies in the main RI area(MRIR)are favorable to the northward movement of TC in the CPW year and unfavorable to the rapid intensification of TC.Compared with CPW year,the environment in the Northwest Pacificsoutheast(northwest)in EPW year is(not)conducive to the increase of TC frequency,the prolongation of TC average life history and the occurrence of stronger RITC.