Study On The Interannual Variability And Long-term Trend Of Tropical Cyclone Movement And Intensity In The Northwest Pacifi

The detection and attribution of changes in tropical cyclone(TC)intensity under global warming is an important scientific issue.Few studies have analyzed the relative importance of environment parameters and track changes to TC intensity and the influences of changes in large-scale environmental flows on β drift.Using the TC best-track data and reanalysis data during 1958-2020 and a downscaling intensity model,this study evaluates the influence of TC vortices on the large-scale environment that affecting TC activities.Based on the evaluation,the linkage between large-scale environment and the interannual variation and trend of β drift is examined,and contributions of changes in β drift to TC motion are confirmed.Meanwhile,a TC intensity model,which is forced by TC tracks and environmental parameters,is used to simulate TC intensity in the western North Pacific basin.Contributions of TC tracks and environmental parameters to changes in TC intensity are quantified by numerical experiments.Changes in TC intensity caused by changes in environmental parameters in the future are also forecasted.The main results are obtained as follows:(1)The influence of TC vortices can be ignored when studying the interannual variation and long-term trend of TC motion and intensity.Although TC vortices influence the magnitude of the estimated monthly and climatological low-level relative vorticity in the TC active region,they have little influence on the estimated wind patterns,the magnitude of vertical wind shear and environmental steering,and so is the interannual variation and long-term trend of the estimated environmental parameters.(2)The interannual variation of β drift in the western North Pacific basin is more important to TC motion than environmental steering.The westward component of β drift in the environment with anticyclonic relative vorticity and equator-ward relative vorticity gradient is slower while the northward component is faster in the environment with anticyclonic relative vorticity.(3)The significant increasing trend of northward component of TC motion in the western North Pacific basin during 1965-2019 is due to the significant increase of the northward component ofβdrift,which is accompanied by the decrease 200-h Pa easterly wind and significant decrease easterly vertical shear.Based on previous studies,the weakening of the easterly shear makes TCs to drift to the left of the anormal westerly vertical shear,resulting in the increase of the northward component ofβdrift.(4)The intensity model can well reproduce the increasing trend of the frequency and proportion of intense TC in the western North Pacific basin during 1958-2020.Both changes in environmental parameters and TC tracks contribute about 50% to the increase of the proportion of intense TC.Changes in vertical wind shear and sea surface temperature play the considerable role,while changes in ocean mixed layer depth and outflow temperature are not important.Changes in TC motion,which are caused by changes in β drift,increase the northward TC tracks,which give TCs more chances to develop into the intense TC.(5)Large differences exist in the simulated four environment parameters in CMIP6 historical run.Using the TC intensity model,the trend of intense typhoons frequency simulated by the output of CMCC-ESM2,EC-Earth3-VEG,INM-CM4-8 and MPI-ESM1-2-HR are the most consistent with that simulated by the reanalysis.Changes in four environmental parameters in SSP585 are relatively consistent,and the forecasted proportion of intense TC based on TC intensity model may increase in the future.The results of multimodal averaging show that the proportion of intense TC may increase from 26.1% during 2020-2039 to 31.4%during 2060-2079,and then change slowly thereafter.The increase of forecasted TC intensity in the selected models is weaker than that in the multimodal averaging.