Effects Of Intra-seasonal Variability Of Soil Moisture On Extreme High Temperature In Eurasia

With the continuous intensification of global warming,soil presents a drying trend in many areas.The aridification of soil moisture,induced by the land-atmosphere coupling process,causes further increases in surface air temperature and exacerbates the intensity,frequency,and duration of extreme high-temperature.Although many scholars have conducted extensive research on the characteristics of soil moisture changes and their climate effects,they mainly concentrate on interannual,decadal,or long-term climatic changes.However,the changes in the intra-seasonal variability of soil moisture can also influence the likelihood of extreme high-temperature probably.Therefore,this paper analyses the characteristics of intra-seasonal variability of soil moisture in spring and summer over Eurasian using ERA5 reanalysis data and the Coupled Model Intercomparison Project Phase 6(CMIP6)model simulation data.Based on correlation analysis and joint probability density distribution methods to investigate the plausible effects of intra-seasonal variability of soil moisture caused by land-atmosphere coupling on the frequency of extreme high-temperatures over Eurasian landmass in spring and summer and explore possible physical mechanisms.The main conclusions are as follows:(1)The ERA5 reanalysis datasets and CMIP6 models daily data were used to investigate the intra-seasonal variability characteristics of shallow soil moisture in Eurasia.The results indicate that the ERA5 reanalysis data is consistent with the multi-model ensemble average of CMIP6,and the intraseasonal variability of soil moisture is small in West Asia and mid-latitudes of Asia during both spring and summer.The enormous intra-seasonal variability of soil moisture is observed in Europe and Indo-China Peninsula during spring and in Indian Peninsula and mid-and high-latitudes during summer.Additionally,the intra-seasonal variability of soil moisture in the Indian Peninsula displays a once small-to-large transition in both spring and summer,possibly related to the onset of the Indian monsoon.From a modeling perspective,the MPI-ESM1-2-LR model is one of the models closest to the results of ERA5 reanalysis data.(2)Based on the CMIP6 land-atmosphere coupling experiments,we found that the land-atmosphere coupling effect significantly increases the intraseasonal variability of summer soil moisture in most areas of Eurasia and further increase the frequency of summer extreme high temperature,especially in Europe,Siberia,Northeast Asia,and Indo-China Peninsula.Specifically,On the intra-seasonal timescale,the land-atmosphere coupling increases the variability of surface sensible heat flux and net longwave radiation heating the atmosphere by intensifying the soil moisture variability,which amplifies the variability of surface air temperature and thereby leads to the enhancement of the extreme high-temperature days.This highlights the importance of the changes in the intra-seasonal variability of soil moisture on the increasing likelihood of heat extremes in summer.(3)In spring,the land-atmosphere coupling effect also increases the intraseasonal variability of soil moisture in most parts of Eurasia.In Europe,Northeast Asia,and East Asia,the increased intra-seasonal variability of soil moisture leads to drier soil and higher surface air temperature,increasing the frequency of extreme high-temperature events.On the contrary,in West Asia and Central Asia,the increase in soil moisture variability leads to wetter soil and lower surface air temperature,reducing extreme high-temperature events.Further analysis indicates that this may be related to abnormal spring precipitation in the region.Increasing soil moisture intra-seasonal variability leads to decreased surface latent heat flux,and more energy is transferred to sensible heat flux.In addition,the increased variability of soil moisture can decrease cloud cover,increasing the shortwave radiation reaching the surface.The combined effects of sensible heat and net shortwave radiation increase surface temperature,leading to more extreme heat events.