Mechanistic Study Of Summer Warming In Eurasia And Its Impact On East Asian Climate

The surface air temperature has changed remarkably at both global and continental scale during the recent few decades.The significant warming trend over the Eurasian continent have received much attention,as the Eurasian continent is the largest continent in the world and its land surface thermal anomalies has nonnegligible effect on the climate system via land–atmosphere interactions.However,it is still difficult to detect and attribute heterogeneous warming over Eurasia on a regional scale,and to quantitatively analyze the contribution of different external forcing and internal variability to Eurasian warming.Based on multi-set observations and CMIP6 latest mode datasets,this study uses the optimal fingerprint method to detect and attribute to analyze the influence of the external forcing to the Eurasian warming key regions,and separate the dynamic contribution of the atmospheric circulation internal variability and the thermodynamic effect of radiative forcing by using the dynamical adjustment technique.On this basis,the impact of East Asian mid-latitude warming on mid-latitude cyclones and East Asian summer monsoon is discussed through observational analysis and numerical simulation,and the dynamic and thermal effects of East Asian mid-latitude abnormal warming on East Asian summer climate are analyzed.The main conclusions are as follows:(1)Through detection and attribution,it is found that the summer land surface warming in Northeast Asia(NEA)from 1960 to 2019 was mainly due to the greenhouse gas forcing,and the aerosol forcing also contributes to a certain extent.The summer land surface warming in Europe–West Asia and parts of the Northeast Africa is mainly affected by the greenhouse gas forcing,followed by the natural forcing.Based on regularization of the optimal fingerprint method,the results show that all forcing,anthropogenic forcing and greenhouse gas forcing are detected in the two key regions of Eurasian significant warming,and changes in the surface temperature can be successfully attributed to their effects.The aerosol forcing can be detected in the NEA and cannot be detected in EWA.The natural forcing is detectable in EWA and undetectable in NEA.(2)The interdecadal warming over the Eurasian continent in the middle and late 1990 s is mainly caused by the external forcing,while the internal variability causes regional differences in the warming of Eurasia.Among them,the internal variability has a certain contribution to the interdecadal warming in the NEA area,while the internal variability has a small contribution to the interdecadal warming in the EWA area.The thermal process plays a major role in the summer warming process of Eurasia from 1980 to 2019.The dynamic process of NEA also contributed to the interdecadal warming in the mid to late 1990 s in this area,and the effect of the dynamic process in EWA is weaker.The dynamic process caused by the internal variability contributes to the interdecadal warming of the NEA area.The dynamic effect caused by the internal variability of the EWA area is significantly weaker than that of the NEA area,and its contribution to the interdecadal warming of EWA is very small.(3)The internal variability and dynamic effect of the surface temperature in the NEA area are mainly affected by the North Atlantic Oscillation(NAO)and the Pacific Decadal Oscillation(PDO).The EWA is mainly affected by the North Atlantic Oscillation(NAO)and the Atlantic Multidecadal Oscillation(AMO).For the NEA area,PDO dominates the dynamic effect of surface temperature,and NAO plays a second role.When the NAO is in a negative phase,regardless of the PDO is in the positive or negative position,the response of the surface temperature of the NEA area is more intense.NAO's negative phase can inspire the power of PDO,which affects the effects of internal variability on the dynamic effect of surface temperature in the NEA area.The joint role of AMO and NAO in the EWA region did not have a significant effect on the surface temperature,since the two were weakened by mutual offset.(4)The land surface thermal anomaly over Northeast Asia has an important impact on the mid-latitude cyclones and the East Asian summer monsoon.The warming(cooling)in the EA key region over Northeast Asian will decrease(increase)the cyclone activity via the atmospheric baroclinity and energy conversion.The weakened(enhanced)cyclone activity will cause a positive(negative)anomaly of the synoptic transient dynamic forcing,so that the positional height field anomaly in the Baikal region is positive(negative).The above process enhances(weakened)the dynamic effect of weather-scale transient wave on time average stream,which is not conducive to(conducive to)the development of East Asian summer monsoon.(5)The thermal effect caused by the heating of radiative forcing is the main reason that affects the warming of Eurasia during 1979-2017.Although the dynamic effect of atmospheric circulation changes also has a certain warming effect in some areas of Eurasia,such as the NEA area,on the whole the contribution of the dynamic effect is not large.The decrease of the extratropical cyclone activity over the mid-latitude of East Asia from 1979 to 2017 was mainly due to the thermal effects.In the past few decades,the precipitation from northeast China to Mongolia has decreased significantly.The changes of the precipitation are mainly caused by the dynamic effects of atmospheric circulation.The contribution of thermal precipitation caused by radiative forcing is weaker than that of dynamic precipitation.