Interdecadal Variation Of The Atmospheric Heat Source Over The Tibetan Plateau: Impact On The Northern Hemisphere Summer Circulation

The thermal status over the Tibetan Plateau(TP)has a significant impact on the global circulation and precipitation.The non-adiabatic heating over the TP are one of the main driving forces affecting the intensity of the Asian summer monsoon and global atmospheric circulation anomaly.This paper mainly focuses on the inter-decadal variation of atmospheric heat source(Q₁)in summer over the TP,and analyzes its spatio-temporal distribution characteristics and possible influence mechanisms on circulation and precipitation in different regions in Northern Hemisphere(NH).In addition,in the context of increasing global warming from 1979 to present,the TP is a"sensitive area"of global change.The trend of atmospheric heat source over the TP and its potential relationship with the trend pattern of NH mid-latitude circulation are discussed.The main conclusions are as follows:(1)Spatial and temporal distribution characteristics of interdecadal variation of summer atmospheric heat sources over the Tibetan Plateau:During the summer of 1948-2018,the atmospheric heat source over the TP showed a significant"positive-negative-positive"interdecadal change.From 1948 to 1972,the heat source over the Plateau showed continuous heating anomaly.The TP Q₁ intensity was abnormally weakened from1973 to 2005.In the summer of 2006-2018,the TP Q₁ showed an increasing trend,and the heating intensity of the third stage was stronger than that of the first stage.There are some spatial differences in the interdecadal changes of atmospheric heating in different regions over the TP:the significant interdecadal changes of TP Q₁ mainly occur in the northwest and in the south of the Plateau.The northwest part of the TP Q₁ mainly depends on the contribution of surface sensible heat flux from the middle and lower troposphere(near TP surface to 300hpa).The southern margin of TP Q₁ mainly depends on the contribution of deep latent heat transport in the middle and upper troposphere(above 300hPa).(2)Characteristics of divergence circulation and kinetic energy conversion over the Tibetan Plateau in different Q₁ decadal periods:The inter-decadal variation of Q₁ over the Tibetan Plateau can lead to a global scale anomalous circulation driven by heat.A flow from the divergence center over the TP flows eastward,sinks over the tropical eastern Pacific Ocean and the western coast of North America,and flows back to the Asian heat source area with low-level east wind,thus a large-scale circulation circle driven by heat on a global scale is formed.In addition,as a response to the interdecadal variations of TP Q₁,this global circulation also has interdecadal variations.From the perspective of energy conversion,a large amount of effective potential energy is converted to divergent wind energy in the upper troposphere over the TP when the atmosphere is heated abnormally.This process is accompanied by abnormal high pressure and divergence flow at 100hPa over the Plateau.The transfer function C(K?,K?)between the rotating wind energy and the divergent wind significantly increased at 100 hPa in the upper troposphere,and a large amount of K?was converted to K?.When the heat source intensity over the TP is weak,the dynamic energy conversion C(K?,K?)between divergent wind and un-divergent wind is negative from the lower stratosphere to 200 hPa.(3)The influence of interdecadal variation in TP Q₁ on Northern Hemisphere circulation:When the TP Q₁ increases,the corresponding 200 hPa geopotential height pattern(Z200)in the Northern Hemisphere is similar to the negative phase of the circumglobal teleconnection(CGT).However,the influence of TP Q₁ on mid-latitude circulation is different from that of traditional CGT.The TP heat source and CGT have superimposed influence on the mid-upper troposphere circulation along the Northern Atlantic,through Europe to the western TP.Over eastern Asia,the Northern Atlantic and Northern America,the influence of the TP Q₁ and CGT on the circulation is quite different.In addition,through the inter-decadal thermal anomaly circulation caused by the TP Q₁,a large-scale subsidence and warming movement occurs over the central European continent and the Okhotsk Sea.The subsidence movement area is accompanied by convective suppression and significantly less total cloud cover,which helps to intensify the occurrence of dry and high-temperature weather in the above two regions.The plateau heat source will have a significant impact on the summer precipitation in eastern China.When the TP Q₁ increases,the precipitation in northern China increases significantly,the rainfall in Jianghuai regions decreases,and in the coastal areas of South China increases.Compared with traditional CGT,the inter-decadal of TP Q₁corresponds to an abnormal rain belt moving from Jianghuai region to North China in summer.When the TP Q₁ is abnormally strong,a strong Rossby wave source(RWS)appeared at 300hPa over the western plateau,which was related to the strong sensible heat in the middle and lower troposphere.At 100hPa,the RWS region appeared in the eastern and southwestern margin of the plateau.Among them,the RWS in the eastern TP is caused by the strong upper tropospheric divergence caused by deep convection in this area;The strong RWS in the southwest edge of the TP originated from a northward transportation of absolute vorticity caused by strong convection in the northeast Arabian Sea.(4)The trend of TP Q₁ under the background of climate warming and its relationship with the trend of mid-latitude circulation in the Northern Hemisphere:In the summer of 1979-2018,the TP Q₁ showed a continuous enhancement trend.The abnormal low pressure and high pressure centers in the western and eastern TP have increased significantly in recent 40 years,which is conducive to an abnormal convergence and upward movement in the middle and upper troposphere.These will help to capture water vapor from the Arabian Sea to TP.In the NH mid-latitudes where the TP is located,the 200hPa circulation in summer in recent 40years presents an enhanced zonal wave train pattern along the westerly jet(?45°n).Based on the CESM1 Large ensemble simulations,CMIP5 and CMIP6 models,the mid-latitude circulation and SST/precipitation trend in the northern hemisphere consistent with the observation can not be described.Through the fingerprint diagnosis of CESM PI control simulations and analysis of CESM-LE historical simulations,we find that the amplification pattern of mid-latitude wave train in the NH is significantly related to the cooling trend of sea surface temperature in the tropical Central-Eastern Pacific.The tropical convection anomaly(the convective suppression zone along ITCZ and the rainfall enhancement region at tropical Maritime Continent)can weaken the meridional temperature gradient from equator to pole and facilitate the formation of CGT,which plays a key role in the enhancement trend of mid-latitude wave train in the northern hemisphere in summer in recent 40 years.