Studies On The Size Change Of Tropical Cyclones Over The Western North Pacific Under Global Warming And Its Mechanism

In this study,based on the tropical cyclone(TC)best-track data of Japan Meteorological Agency(JMA),the trend of TC size at its lifetime maximum intensity in the western North Pacific(WNP)during the past decades was investigated.Next,an objective method to evaluate the performance of climate models on simulating global TC activity was designed and used to evaluate the capability of multiple CMIP5 and CMIP6 models to simulate TC.Then,the three CMIP6 models with better simulation performance of WNP TC were selected,and the simulated TC data under different carbon emission levels from these three models was used to investigated the size change of TC under global warming.Finally,the climate simulation output of EC-Earth3 model was taken as the input of WRF model to conduct a series of dynamical downscaling TC case experiments with high resolution,and the output of WRF model was used to verify the climatological rule of TC size change and reveal the related physical mechanism.The conclusions were shown as follows.Based on the 30 knot wind radius(R30)data of JMA,it is found that in the past 39years(1980-2018),the annual average TC size at its lifetime maximum intensity has a significant decreasing trend with time.After removing natural variability [i.e.,ENSO,Pacific Decadal Oscillation(PDO),Interdecadal Pacific Oscillation(IPO)] from the TC size time series,the results show that the residual trend of TC size change becomes insignificant.Therefore,in the past 39 years,natural variability,especially IPO,is the main factor which causes the significant decrease of TC size,and the effect of global warming on the trend of TC size may not be significant.However,due to the short period of current high confidence observation data(only nearly 40 years),it is difficult to estimate the contribution of global warming to TC size change under the interference of natural variability based on only the observation data.Three aspects,i.e.,TC track density weighted by destructive potential(cubic of wind speed),TC track classification and monthly variation of TC frequency,are considered to evaluate the models’ simulation performance of TC.Results show that,there exists some difference for the simulation performance of CMIP5 models in terms of above three aspects and the simulation performance of each model also differ in different ocean regions.Moreover,compared with CMIP5,CMIP6 models perform better in simulating TC overall,but it is important to note that for a certain model,the development of its version does not necessarily improve the performance of this model in simulating TC.By evaluating the simulation performance of multiple CMIP6 models,it is found that MRI-ESM2-0 model performs the best in WNP,followed by Tai ESM1 model,EC-Earth3 model,and BCC-CSM2-MR model.Since Tai ESM1 model from CMIP6 has not provided the model output data under different carbon emission scenarios in the future which is required for this study,the data of MRI-ESM2-0,EC-Earth3 and BCC-CSM2-MR model are used to explore TC size change under global warming.The results show that,with the increase of carbon emission level,the TC size in BCC-CSM2-MR model change little,but the TC size in EC-Earth3 and MRI-ESM2-0 model show a significant trend of increase.As the climate simulation output of EC-Earth3 was used to make the input of WRF model,the dynamical downscaling TC case experiments were conducted based on WRF model.The results show that,the difference of sea surface temperature(SST)is the key reason of TC size change under different carbon emission scenarios,and the influence of large-scale circulation on TC size change is opposite to that of SST.Namely,the SST warming makes the TC size increase significantly,while the large-scale circulation under the increasing carbon emissions will inhibit the increase of TC size.The combined effect of the above two will eventually lead to the increase of TC size under global warming.Moreover,the change of TC size is mainly caused by two factors,i.e.,TC intensity(i.e.,maximum wind speed,MWS)and the slope of 10-m tangential wind speed decreasing along the radius of TC outside the eyewall.SST and the large-scale circulation make the TC size change by influencing these two factors.With SST increase,the air-sea temperature difference and moisture difference get larger in TC area(i.e.,eyewall area and the outer-spiral rainbands area),and the corresponding surface sensible heat and latent heat fluxes increase.Thereby the sea transfers more surface entropy flux(SEF,the sum of sensible heat flux and latent heat flux)to the air,the increase in SEF is beneficial to the development of convection,and makes the pressure drop in the region,which favors the increase of of inflow in the low layer(950-800 h Pa).The low-level inflow of the eyewall area makes the convection strength,thus increasing TC intensity.The low-level inflow of the outer-spiral rainbands also favors the convection development.Subsequently,the increased inflow accelerates the outer surface tangential wind speed,and makes the slope of 10-m tangential wind speed decreasing along the radius of TC decrease.The combined effect of SST warming under the TC eyewall region and the outer-spiral rainbands leads to the change of 10-m tangential wind profile of TC and eventually increases the TC size.The changed large-scale circulation under global warming make the air-sea temperature difference and moisture difference decrease,and inhibits the TC intensity and size by the opposite process of SST warming effect.Eventually,the increase amplitude of TC size influenced by both large-scale circulation and SST is smaller than TC size under only SST warming.