Study On Thermal Influence Mechanism Of Concurrent Variations Of East Asian Upper-level Jet Streams In Summer

East Asian upper-level jet stream(EAJS)is an essential part of the East Asian monsoon system,it can be divided into two parts:the East Asian subtropical jet and the East Asia temperate jet.The concurrent variation of two jet streams is closely related to the weather and climate anomalies in East Asia.Based on the ERA-Interim and NCEP-DOE datasets,the principal modes of jet stream variations in summer are analyzed.The circulation anomalies and the accompanying thermal forcing anomalies,which correspond to the concurrent variation of the two jet streams are examined.In addition,numerical experiments are conducted to analyze the Tibetan Plateau's thermal effects on the concurrent variations of EAJS by using CAM5.3 model.The dynamic processes are diagnosed to examine how the Tibetan Plateau's thermal anomalies influence the concurrent variation of jets.The main results are as follows:Through the EOF analysis of the zonal winds in summer,it is found that the leading modes of summertime zonal wind variation are characterized by the out-of-phase variations in the intensity and meridional displacement of jets.Corresponding to the two principal modes of the jet stream variation,the atmospheric circulation fields over East Asia are significantly different.With the out-of-phase variations in the intensityof two jets,the anomalies of circulation over East Asia show "positive negative positive" pattern from high latitude to low latitude.The anomalous centers are located in Siberia,Mongolia,and the Qinghai-Tibet Plateau,respectively.Circulation anomalies are consistent below 300 hPa.It is also found that there is an intimate relationship between the surface thermal field and temperature field,which can be used to investigate thermodynamic mechanisms of the concurrent variation.Based on the observational analyses,thermal mechanism of concurrent variations is investigated by using the CAM5.3 model.Numerical sensitivity experiments show that,with the change of heating on the plateau,there is an out-of-phase variation of intensity of subtropical jet and temperate jet.The dynamic diagnosis of the numerical results shows that the anomalous surface heating on the plateau alters the meridional temperature gradient distribution,then changes the zonal wind in the key regions of jet streams.The anomalous distribution of the transient eddy activities indicates that the change of the Eady growth rate leads to the anomaly of the eddy activities,and changes the average westerly flow through the internal processes in the atmosphere.In addition,the vortex momentum transfer leads to the change of the zonal wind in the temperate jet.The non-uniformity of the westerly anomaly caused by the vortex momentum transport can change the vertical shear of the zonal wind,leading to the change of Eady growth rate,which will further affect the jet intensity.