Characteristics And Mechanisms Of Interannual And Interdecadal Variations Of Summer Surface Temperature Over Central Asia

Based on NCEP / NCAR reanalysis data,CRU surface air temperature(SAT),snow cover and global sea surface temperature(SST)data,the dissertation investigates the interannual and interdecadal variation of summer SAT over Central Asia,including.Its relationships with SST in the North Atlantic and snow cover in the Tibetan Plateau(TP)are also investigated through relevant statistical analysis and atmospheric dynamic diagnosis.The results are summarized as the following:The results of EOF analysis show that the leading mode of the summer SAT over Central Asia features a same-sign temperature anomalies over Central Asia and explains62% of the total variance.When the SAT is anomalously high in summer,there is a pronounced high-pressure system dominating the Central Asia.The high-pressure system is accompanied by warming advection as well as descending motion over Central Asia,favoring the warming of the SAT over Central Asia.Further analysis explores the relationship between summer SAT over Central Asia and the snow cover extent(SCE)of the TP on the interannual variation.The interannual variation of summer SAT over Central Asia is significantly and positively correlated with the SCE over the central-eastern TP in April.In April,the SCE over the centraleastern TP is higher than normal,which causes a significant cooling effect on the atmosphere above the TP.The effect of anomalously high SCE can persist into the early summer and produce vorticity disturbance in the core area of the subtropical westerly jet(SWJ)over the north of TP.This vorticity perturbation propagates downstream,forming a Rossby wave train-like pattern along the wave guide of the SWJ.In the following summer,Central Asia is controlled by a significantly high-pressure system,which is conducive to the warming of summer SAT.This work analyzes the abrupt change of summer SAT over Central Asia and its relationship with SST in North Atlantic and SCE of the TP.The results show that the summer SAT over Central Asia has a significant climate shift at 2005.The standardized regional average index of summer SAT over Central Asia has changed from previous negative phase to positive phase.The analysis of the anomalous atmospheric circulations related to the interdecadal changes of summer SAT over Central Asia shows that after 2005,the anticyclonic circulation system in the west of Central Asia is abnormally enhanced.The atmospheric subsidence associated with the anomalous anticyclone can cause warming.Meanwhile,the reduction of cloud amount caused by the enhancement of this anticyclone anomaly leads to the increase of downward shortwave radiation and therefore is favorable for increase of summer SAT over CA.The composite analysis results show that the abrupt change of summer SAT over Central Asia in 2005 is closely related to the warming of SST in the middle and high latitudes of the North Atlantic and the reduction of the SCE in the west of TP.The anomalous positive SST in the middle and high latitudes of the North Atlantic can stimulate a Rossby wave propagating downstream.The anomalous negative SCE over the western TP can also increase the temperature of the atmosphere over Central Asia through the albedo effect of snow cover.Both of the anomalous SST and SCE can enhance the anticyclone over Central Asia,which is conducive to the abnormally high summer SAT over Central Asia after 2005.