Changes Of Tropical Cyclone Tracks In The Western North Pacific Over The Past 38 Years

In this study,the tropical cyclone(TC)trajectory model proposed by previous studies is improved first.Using several sets of TC best track data and reanalysis data,changes of the TC tracks and landfalls in China during the peak season(June-October)over 1979-2016 are studied.Changes of TC formation locations and the large-scale environmental steering flow,two facts of TC tracks change,are examined.And their contribution to the change of TC tracks is also discussed.The main result is as follows:1)The Gaussian weight interpolation method is used to improve the calculation of beta drift in the previous TC trajectory model.The improved trajectory model has a smaller difference between the simulation and the observation,especially on the coast of East Asia and at higher latitudes.Using the improved trajectory model,the simulation is more similar to the observation.Over the past 38 years,the influence of TCs over the South China Sea and the ocean to the east of the Philippines decrease,but increase around Taiwan and the ocean to the southeast of Japan.Under this TC tracks change,the number of TC that landfall in the central part of China increase,while that landfall in the southern part of China decrease.2)The change of large-scale steering flows is characterized by a cyclonic circulation centered over southeastern China.This steering flows change is caused by the eastward shift of the western Pacific subtropical high.TC formations decrease over the southern part of the South China Sea and to the east of the Philippines,but increase from the northern part of South China Sea to the northern Philippines.Using the potential genesis index,it is found that TC formation locations change is mainly caused by dynamic factors,especially by the change of vertical wind shear.3)Through numerical experiments,it is found that the change of TC formation locations makes a decrease of TC occurrences over the South China Sea,the ocean to the east of the Philippines and China,and an increase from southern China to the Philippines and over the ocean to the southeast of Japan.The change of steering flows makes the influence of TCs over the South China Sea decrease,and increase in subtropical regions.Therefore,the contribution of formation locations or steering flows in different regions is not same.4)Using the forecast field from June to October by CFSv2 model in early March of 2017,it is found that the occurrence of TCs increase over the South China Sea and decrease over subtropical ocean during the peak season by the trajectory model.This TC tracks change make an increase in the number of TCs macing landfall in the central and southern part of China,and a decrease in the northern part.The prediction is consistent with observation.