Impact Of Tropical Cyclone Activities Over The Western North Pacific On East Asian Summer Climate

The NCEP/NCAR reanalysis data,the tropical cyclone(TC)Best Track dataset from Regional Specialized Meteorological Ceneter-Tokyo Typhoon Ceneter and observed datasets collected at 2474 stations in mainland China are used for statistical diagnosing the impact of summertime TC activities over the western North Pacific on East Asian climate.Moreover,we carry out two sensitivity numerical experiments to suppress the generation and development of TCs over the integration period and to remove the TC votex from the first-guess large-scale field at the initial time by the use of the “modeling surgery” methodology.Those simulations are used for verifying the statistical diagnosis and revealing the corresponding physical mechanisms.The main conclusions are as follows:In activie western North Pacific TC years,the intensity of the East Asian summer monsoon is stronger,and the southerly monsoon flow in the lower troposophere over the East Asia could reach higher latitudes.Correspondingly,the East Asian summer monsoon rainbelt could also shift further north and maintained in the middle and lower reaches of the Yangtzi River Valley for a relatively short period in active TC years,leading to less precipitation there.Meanwhile,the large-scale atmospheric circulation in the middle and upper troposphere in active TC years is significantly different from that in inactive TC years.At 500 hPa,the western Pacific subtropical high weakens and eastward retreats,corresponding to the northward shift of its ridge in active TC years.At 200 hPa,the East Asian subtropical upper-level jet axis and the South Aisan high both exhibit a northward displacement.Therefore,the frequent TC activities over the western North Pacific amplify the features of the already strong East Asian summer monsoon.The feedback of frequent TC activities over the western North Pacific on the western Pacific subtropical high leads to an abnormal anticyclonic circulation band that dominates a large area from mid-eastern mainland China to the south of Japan in the middle and lower troposphere.As a result,there is less precipitation in the lower and middle reaches of Yangtze River with longer sunshine duration,corresponding to higher surface air temperature increases and more hot days in the active TC years than in the inactive TC years.Moreover,there is more precipitation in the coastal region of southern China due to more TCs landing there in active TC years,leading to lower surface air temperature increases and less hot days.Therefore,to a certain degree,TC activities over the western North Pacific contributes to the interannual variability of the summertime precipitation and hot risk in the east China.WRF can well reproduce TC activities over the western North Pacific and the corresponding large-scale circulation.Moveover,TCs are removed from the sensitivity experiment output by the use of the “modeling surgery” methodology.The impact of TC activities over the western North Pacific on the East Asian summer climate is investigated by comparing the simulation with or without TC.It shows that TC activities over the western North Pacific release release large amounts of latent heat,resulting in triggering a northward wave train throughout the summer.As a consequence,the wave train superposes on the Pacific-Japan teleconnection wave train and subsequently exerts an influence on the large-scale circulation in middle latitude,leading to an obviously change in the summer climate.The latent heat released by TCs could directly warm the air around and thus change large-scale meridional temperature gradient in the entire troposphere and the lower stratosphere,leading to a change in position and intensity of large-scale systems,such as the East Asian subtropical upperlevel jet,the western Pacific subtropical high,the South Asian high and so on.Moreover,TC can influence the wind field directly through the dynamic process,leading to the change of weather systems.TC activities significantly affect the precipation by changing the distribution of water vapor content in the atmosphere and the physical conditions of precipitation formation.The changes in atmospheric circulation and precipitation can further influence the distribution of surface air temperature.Therefore,TC activities can significantly affect the East Asian summer climate at interannual time scale.