| Under global warming,extreme climate events are becoming more frequent and severe in China,causing tremendous impacts on people's lives and socio-economic development.To provide richer and more reliable projections of climate change,the Coupled Model Intercomparison Project Phase 6(CMIP6)presented new scenarios combining Representative Concentration Pathways(RCPs)and Shared Socioeconomic Pathways(SSPs).Based on the latest global model CESM2,MPI-ESM1-2-HR(later referred to as CESM and MPI)results from CMIP6 archive,this study conducted present climate simulations(1980-2014)and future climate projections(2016-2060)under high emission scenarios(SSP5-8.5)in China with a horizontal resolution of 30 km using the state-of-the-art regional climate model CWRF,focusing on the future changes of extreme climate and compound extreme events under high emission scenarios.Combined with the population and cropland information,the future risk of compound extreme events in China was also predicted quantitatively.In addition,this study also made an investigation into the mechanism of Tibetan Plateau remote impacts on East China.The main results obtained in this paper are as follows:(1)The added value of CWRF downscaling was clarified.CWRF significantly outperformes the global models for China's climate simulations,especially in the densely populated and relatively economically developed region.CWRF reduces the CESM and MPI bias in simulating the annual mean temperature over eastern China from 0.94°C and 0.58°C to0.32°C and 0.02°C,respectively.Such significant improvements support CWRF to more accurately simulate the threshold,frequency and duration of extreme temperature events(heat wave and cold wave)in eastern China.In addition,compared with the global models,CWRF more accurately reproduces the spatial distribution structure of the monsoon rain bands in China during the flood season,and well corrects the misreporting of the precipitation centers by CESM and MPI in the southwest and east of the Qinghai-Tibet Plateau during spring,summer and autumn.The more accurate precipitation simulation also enables CWRF to greatly improve the underestimation(overestimation)of extreme summer precipitation in eastern China by MPI(CESM).Moreover,CWRF reduces the simulated difference in annual mean precipitation between global models from 0.44 mm/day to 0.09 mm/day,and this reduction is meaningful for mitigating future climate prediction uncertainty caused by the difference in climate sensitivity between global models.(2)The process and mechanism of Tibetan Plateau heating effect in the downstream region were revealed.Statistical analysis based on the observed data showed a significant correlation between the spring surface temperature anomalies on the Tibetan Plateau and the variability of summer precipitation in eastern China.The results of sensitivity experiments using the CWRF model indicate that the deep soil of the plateau can preserve the positive temperature forcing applied to the surface and soil of the initial field in the sensitivity experiments for several months,acting as an energy storage and continuously transporting heat to the surface.The consequent enhanced local wet convection sends disturbance energy into the middle and upper troposphere and propagates downstream with the Rossby wave train.The diagnostic analysis revealed that the intensifying of South Asian high,the shift of the East Asian jet and the resulting change in the secondary circulation in the jet exit area are the core reasons for the change in temperature and precipitation over the eastern China.(3)Extreme climate changes are predicted for different warming periods(1.5°C,2°C,and 3°C)in China under high emission scenarios(2016-2060).The CESM,MPI and CWRF all predicted significant warming in China,and the wet zone tends to expand to the northwest.There are some differences between CWRF downscaling and global models in predicting precipitation changes in eastern China during the flood season.MPI and CESM predicted increased precipitation over most of the eastern China,while CESM also predicted decreased precipitation over northern China.Unlike the MPI,MPI?CWRF predicted a decrease in precipitation along the southeast coast,and CESM?CWRF enhanced the magnitude of precipitation changes predicted by CESM.These variances are a continuation of the global model correction effect of the CWRF downscaling in the present climate simulations,with correlations above 0.9,but since there is no clear linear relationship between simulation bias and future changes,it is not possible to improve the confidence in the predictions by removing the bias from the future projections.The extreme climate event changes predicted by models are mainly in the enhancement of heat wave events and weakening of cold wave duration,but the changes are weaker during the 1.5°C warming period,and the cold wave events even show an increasing trend in some regions,after which they gradually become larger.When the warming reaches 3°C,the frequency and duration of heat wave(cold wave)events will be much higher(lower)than the current level.In addition,the four compound extreme events discussed in this paper(dry heat wave,humid heat wave,dry cold wave,and humid cold wave)generally follow the trends of corresponding extreme temperature events,and are also characterized by different trends in different warming periods(4)The risk of population and cropland in the eastern China arising from four compound extreme events in the future was quantified.The population exposure to wet and dry heat waves calculated based on the population and the total duration of different compound extreme events will reach several times or even tens of times the current level in different regions,while the population exposure to wet and dry cold waves will be reduced by more than 90%.The duration of cropland affected by compound heat wave events in the four major cropland regions will be elevated by a factor of 2 to 12 depending on the region,and the duration affected by compound cold wave events will be reduced by more than 50% in most regions. |
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