Study On The Main Modes Of Interannual Variation Of Land Surface Thermal Anomalies In Spring In Western Eurasia And Their Cause

Under the background of global warming,the land surface warming over western Eurasia(20°N–70°N,0–70°E)in summer has attracted widespread attention.Recent studies have shown that the spring land thermal anomalies over western Eurasia may be a potential driver of the Asian summer climate.Moreover,the warming trend of land surface temperature(LST)in this region during spring is comparable to that of summer,and the interannual variability is much greater than that of summer.However,most existing studies focus on the interdecadal warming during summer over western Eurasia,and there is a lack of in-depth research on the basic characteristics of the interannual variability of springtime land thermal anomalies in this region and its relationship with external forcings such as sea surface temperature(SST)and sea ice(SIC),as well as their mechanisms.To address these gaps in knowledge,this study combines both observational analysis and numerical experiments,using LST as a proxy for land thermal conditions,to investigate the interannual variation of the spring land thermal anomalies over western Eurasia from 1958 to 2017 and their possible causes.The main conclusions are as follows:(1)The interannual variability of spring land surface thermal anomalies over western Eurasia is dominated by the first three empirical orthogonal decomposition(EOF)modes,which are the region-wide consistent mode centered on the area north of the Caspian Sea,the north-south anti-phase mode divided by 45°N,and the east-west anti-phase mode bounded by40°E.All the above three dominant modes of land surface thermal anomalies are closely related to the simultaneous spring large-scale atmospheric teleconnection modes.Specifically,the changes in the region-wide consistent mode are significantly negatively correlated with the Scandinavian(SCA)and East Atlantic/West Russia(EA/WR)teleconnection patterns,and the in-phase combination of SCA and EA/WR teleconnection patterns favors the occurrence of the related LST anomalies.The changes in the north-south anti-phase mode are significantly negatively correlated with the North Atlantic Oscillation,while the east-west anti-phase mode is significantly positively correlated with the Eurasian teleconnection pattern.Further diagnostic analysis reveals that the teleconnection pattern-controlled atmospheric circulation anomalies promote the formation of the dominant modes of spring LST anomalies over western Eurasia via their influence on clouds and water vapor.(2)The region-wide consistent variation of spring LST anomalies over western Eurasia is closely related to the North Atlantic tripole SST anomalies in the preceding February.When the North Atlantic tripole SST anomalies are in a positive phase in February,the mid-latitude North Atlantic SST tends to be warmer,while the SST south of Greenland and in the subtropical North Atlantic tends to be colder,and this pattern can persist from February to spring.Such SST anomalies configuration can generate a stronger meridional temperature gradient in the lower troposphere near 45°N in the North Atlantic,enhancing the low-level atmospheric baroclinicity,which in turn leads to enhanced synoptic-scale transient eddy activity over the mid-high latitudes of the North Atlantic.Synoptic-scale transient eddy activity is closely related to changes in the low-frequency mean flow.On the one hand,the cyclonic vorticity forcings corresponding to the northern part of the enhanced synoptic-scale transient eddy activity region can trigger cyclonic circulation anomalies over southern Greenland,which in turn stimulates anomalous waves trains propagating eastward to Eurasia and leads to the formation of anomalous anticyclone in the mid-high latitudes of western Eurasia,favoring surface warming.On the other hand,enhanced synoptic-scale transient eddy activity can also bring more warm and moist air flows from the North Atlantic to the Barents-Kara Seas via accelerating the polar front jet,resulting in the tropospheric warming in the Arctic region.The Arctic warming tends to increase the 1000-500 h Pa atmospheric thickness over the high latitudes,which weakens the meridional thickness gradient and ultimately leads to the weakening of the upper zonal winds over the mid-latitude region of Eurasia,which is conducive to the formation and maintenance of anomalous anticyclones in the mid-high latitudes of western Eurasia and provides favorable conditions for local warming.(3)The north-south anti-phase variation of spring LST anomalies over western Eurasia is closely related to the preceding early autumn(September-October)SIC in the Chukchi Sea area and the simultaneous spring tropical North Atlantic SST,and the Arctic sea ice and tropical SST anomalies are independent of each other.When the SIC in the Chukchi Sea region decreases or the SST in the tropical North Atlantic warms,the spatial distribution of spring LST anomalies over western Eurasia exhibits a "warm south-cold north" pattern.The reduction of SIC in the Chukchi Sea region during early autumn can affect the spring western Eurasian climate through the dynamic process of the stratosphere-troposphere coupling.Specifically,the weakening of the stratospheric polar vortex caused by the decline of sea ice may persist in the stratosphere for several months and propagate downward to the lower and middle troposphere during late winter(January-February),resulting in a deep anomalous anticyclone over the Greenland region and triggering a "+-+" type anomalous wave train that propagates southeastward through Europe and eventually reaches west Asia,which makes anomalous cyclone and anticyclonic circulation over Europe and west Asia respectively and facilitates the formation of north-south anti-phase LST anomalies over western Eurasia.The warming of SST in the tropical North Atlantic is associated with stronger local tropical convection and adiabatic heating.On the one hand,it can produce anomalous Rossby wave sources in the mid-high latitude of the North Atlantic by strengthening the meridional Hadley circulation,which in turn triggers anomalous atmospheric wave trains propagating southeastward and favors the formation of north-south anti-phase LST anomalies in western Eurasia;On the other hand,it can suppress convective activities and precipitation over west Asia by causing anomalous Walker circulation in the tropical region,which favors local land warming.