1.School of Atmosphere Science,Chengdu University of Information Technology,Plateau Atmosphere and environment Key Laboratory of Sichuan Province;2.School of Atmosphere Science,Lanzhou University;3.Hebei Province Weather Modification Office
Using the NCEP/NCAR reanalysis data from 1979 to 2019 and the temperature and precipitation of the daily data set of basic meteorological elements in China (V3.0), the indexes that objectively characterizes the changes of the northern and southern branch westerly flows around the Tibetan Plateau in winter are defined firstly. And then, the different characteristics of their changes are analyzed, and the influence mechanism of the abnormal changes of the two branch flows around Tibetan Plateau on the temperature and precipitation in China is preliminarily studied by means of correlation analysis and composite analysis. The main results are: (1) There are obvious different characteristics of changes between the northern and southern branch westerly flows around Tibetan Plateau in winter. When the northern branch (southern branch) flow is stronger and the southern branch (northern branch) flow is weaker, there is a large-scale anomalous anticyclone (cyclone) circulation system from the western Tibetan Plateau to the eastern coast of China in the middle-low latitudes of troposphere, and the central Tibetan Plateau is an anomalous anticyclone (cyclone) system center over 500hPa, which extends vertically to the height of 300hPa (inclining to the Northwest with the altitude). There is a significant negative correlation between the north and the south branch westerly flows of the troposphere. (2) The anomalous changes of the south/north branch westerly flows around Tibetan Plateau in winter have a significant impact on the winter weather and climate in China. When the northern branch flow of the Tibetan Plateau is stronger (weaker), the Hetao, Tibetan Plateau and the South China have relatively higher (lower) temperature and less (more) precipitation, but the lower (higher) temperature and more (less) precipitation in Northeast China. However, when the southern branch flow is stronger (weaker), the temperature is generally lower (higher) in the whole China, the precipitation is less (more) in northeastern China and northern Xinjiang, the more (less) precipitation in most parts of southwestern and southern China. (3) A comprehensive analysis the abnormal changes of the vertical structure of atmospheric elements over the East Asian troposphere influence on the China weather and climate anomalies shows: when the northern branch flow is stronger corresponding to the weaker southern branch flow, there are abnormal westerlies in the troposphere to the northern area of 35°N, and abnormal easterly winds above 850hPa to the southern area. The troposphere atmosphere over eastern China affected by the abnormal zonal flows around the Tibetan Plateau of the "Barotropic structure". The corresponding troposphere below 850hPa has a consistent anomalous southwest wind from southern to northern areas, the anticyclonic shear between the anomalous northwest wind and northeast wind above 850hPa in near 35 ° N, the abnormal descending motion and the warmer below 300hPa, these conditions are not conducive to the cold air southward movement and the anticyclone shear enhanced the effect of subsidence motion on temperature increase, as a result, the temperature is higher and the precipitation is less in most areas of eastern China. On the contrary, when the southern branch flow is stronger corresponding to the weaker northern branch flow, the abnormal zonal wind in the troposphere is an obvious "Baroclinic feature", and the anomalous westerly wind shows a slope zonal feature from lower to upper troposphere and from low latitude to high latitude. The Abnormal easterly wind of the lower troposphere is "triangle shaped 200hPa of North China, matching the meridional wind anomaly and the abnormal ascending movement from South China to North China, the "triangle shaped" abnormal cold air mass cuts southward into the South China Sea in the lower layer, the warmer southwest airflow climbs from south to north over the cold air mass to the middle and high latitudes in the upper layer, resulting in a large range of abnormally lower temperature and more than precipitation in China.