Analysis Of The Key Factors Influencing Tropical Cyclone Intensity Climate Change Over The Western North Pacific

Climate change and attribution of tropical cyclone(TC)intensity over the western North Pacific basin(WNP)is a controversial scientific issue.At present,the global climate model has a low ability to simulate the long-term change of TC intensity on its basin-wide scale.Based on observation or theory,most studies understand climate change of TC intensity qualitatively from the perspective of large-scale environmental field,but the corresponding mechanism is still unclear.This study has improved a TC intensity model for climate change research in the WNP,and evaluated the relative reliability of TC intensity data and the difference of large-scale environmental factors among the multiple reanalisys datasets.Then the improved coupled model is allowed to move along the observed tracks and forced by large scale environmental conditions.Over the WNP,the changes in the proportion of intense typhoon and the mean latitude where TCs reach their lifetime maximum intensity(LMI)on their basin-wide scale.are derived dynamically with the improved TC intensity coupled model.Based on numerical experiments,this study quantifies the contributions of large-scale environmental factors and TC prevailing tracks to the increase of the proportion of intense typhoon and the northward migration in the mean TC LMI latitude.The main results are obtained as follows:(1)The improved TC intensity coupled model can well simulate the climate changes of TC intensity over the WNP.The basinwide TC intensity is derived dynamically with this model,based on the track data from the Joint Typhoon Warning Center(JTWC),the Regional Specialized Meteorological Center(RSMC)of Tokyo,and the Shanghai Typhoon Institute(STI)of the China Meteorological Administration.The intensity records in the JTWC best track dataset are best correlated with the simulated intensity.(2)The climatologic mean spatial distributions and trend changes of SST,the mixed layer depth(MLD),and the vertical wind shear(VWS)except the outflow temperature(OFT)are relatively consistent in different reanalysis data.In the main development region,the warmer SST,the thicker MLD,and the cooler OFT are favorable to TC intensity development.(3)The improved TC intensity coupled model can well reproduce the increase trend of the proportion of intense typhoon,which indicates the increase of the proportion of intense typhoons during 1980–2015 is consistent with the corresponding changes in the ocean/atmosphere environment.The proportion change resulted from the temporal variations of the environmental parameters and the shifts of tropical cyclone prevailing tracks.The nonuniform spatial distribution of environmental parameters makes the shifts of tropical cyclone prevailing tracks contribute at least half the increase of the proportion of intense typhoons.The deepening of the ocean mixed layer resulting from the temporal variations and track shifts plays a dominant role in the observed increase of the proportion of intense typhoons.This study suggests that the increase of the proportion of intense typhoons in the western North Pacific basin could be larger than the projection in previous studies.(4)The improved TC intensity coupled model can well reproduce the northward migration in the mean TC LMI latitude,which indicates the northward migration in the mean TC LMI latitude during 1980–2015 is physically consistent with changes in the large-scale ocean/atmosphere environment and TC track.The poleward migration of the mean LMI latitude is mainly due to the TC track shift,which results primarily from the change in the large-scale steering flow.(5)It is found that large spread existed in the simulated climatologic spatial distributions and trend changes of SST,MLD,OFT and VWS in CMIP5.CESM1-BGC,CESM1-CAM5,CNRM-CM5 and IPSL-CM5B-LR can simulate the warming characteristics of the observed SST.CCSM4,CESM1-BGC and CESM1-CAM5 can simulate the deepening characteristics of the observed MLD.No model can simulate the cooling characteristics of the observed OFT.The observed VWS with CCSM4,CESM1-BGC,CESM1-CAM5,CNRM-CM5 and MPIESM-MR shows no obvious trend.(6)With increasing TC influence over the subtropical East Asia and decreasing TC activity over the South China Sea,the observed TC track changes are linked to the SVD leading mode of global sea surface temperature(SST)warming and the associated changes in large-scale steering flows.The selected five IPCC models can generally simulate the leading mode in their ensemble control run and prediction,suggesting the possible persistence of the reported track changes by 2040.