Observational Analysis And Simulation Evaluation Of The Interannual Variation Of The Number Of Extremely High Temperature Days In Summer In Chin

In recent years,extreme high temperature events in summer have become more frequent in China,causing more and more harm to people’s health and life,industrial and agricultural production,infrastructure and ecosystem.Therefore,it is of great theoretical significance and social value to understand the temporal and spatial distribution characteristics of extremely high-temperature days(EHTD)in summer in China and to understand its occurrence law.Previous studies paid more attention to the variation characteristics of the number of summer EHTD in fixed areas,and rarely distinguished the variability characteristics on the interannual scale and the lower frequency scale(interdecadal and linear trend).Therefore,the understanding of the spatio-temporal distribution of the number of summer EHTD on the interannual scale in the whole country is limited.Based on the daily maximum temperature dataset(CN05.1)and JRA-55 reanalysis data from 1961 to 2017,this paper first reveals the dominant modes of interannual variability of summer EHTD in China through empirical orthogonal function decomposition(EOF),and explores the key influencing factors and related physical mechanisms that lead to the formation of each mode.The ability of global coupled climate models participating in the Sixth Coupled Model Comparison Program(CMIP6)to simulate the dominant modes of interannual variability of summer EHTD in China was further evaluated,and the models with better simulation ability were selected.On this basis,the future changes of the dominant modes of interannual variability of summer EHTD in China after global warming are analyzed by using the predicted results of the selected five better models.The main results obtained in this paper are as follows:(1)There are three dominant modes of interannual variability of EHTD over China in summer,namely,zonal mode,meridional dipole mode and Tibetan Plateau mode.In the zonal mode,the EHTD are concentrated in the latitudinal belt across China,which is mainly related to the Arctic oscillation(AO).The meridional dipole mode shows that EHTD variation appears seesaw pattern in a large area north and south of the Yangtze River.This mode is mainly influenced by Polar-Eurasia teleconnection wave train(POL)and the sea surface temperature(SST)anomaly in the equatorial warm pool.The increase of EHTD in the first two modes is related to the decrease of precipitation and the increase of incident solar shortwave radiation caused by local high-pressure anomalies.In the Tibetan Plateau mode,EHTD is mainly concentrated in the Tibetan Plateau,and the formation of this mode is mainly influenced by the zonal wave train propagating downstream from the Mediterranean Sea.On the one hand,the circulation anomaly corresponding to the wave train would lead to the divergence of water vapor and the weakening of upward motion,which would lead to the decrease of cloud cover and the increase of downward cloud-short wave radiation;on the other hand,it would lead to the warming of the atmosphere and the increase of downward long-wave radiation in the clear sky.Both of them jointly provided favorable conditions for the increase of EHTD.(2)CMIP6 coupled climate model has a certain ability to simulate the zonal and meridional dipole modes of summer EHTD interannual variation,but it cannot simulate the Tibetan Plateau mode.The simulation capability of each model for zonal and meridional dipole modes was evaluated by quantitative indexes,and five optimal models were selected,namely,AWI-CM-1-1-MR,EC-Earth3-Veg,EC-Earth3-Veg-LR,INM-CM4-8 and Nor ESM2-LM.The better model can simulate the spatial distribution characteristics of the local circulation field corresponding to the first two modes.The simulation ability of the local circulation field of the meridional dipole type is stronger than that of the zonal type,and the simulation ability of the spatial distribution of the surface temperature and geopotential height field is better than that of the precipitation.However,the optimal model has limited ability to simulate the large-scale circulation characteristics and SST anomalies corresponding to the two modes.(3)Based on the predicted results of five better models under the higher emission scenario(SSP5-8.5),the characteristics of summer mean land surface temperature in China and the future changes of the dominant modes of interannual variation of EHTD were analyzed.In the context of global warming,the average surface temperature in summer in China presents a uniform warming feature,and the spatial distribution of warming amplitude is uneven.The temperature increase amplitude is larger in northern China,but smaller in the Tibetan Plateau and southeast China.The dominant modes of the future interannual variation of EHTD in summer in China are consistent with the results of the historical simulation.The first mode shows consistent variation characteristics in the middle latitudes of China,and the second mode shows the spatial distribution of meridional dipole types in China,and the interannual variability of EHTD of the two modes will increase in the future.