Variation Characteristics of the Westerly Flow around the Tibetan Plateau in January and Its Impact on Climate in China

Using the National Centers for Environmental Prediction 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 indices that objectively characterize the changes of the northern and southern branch westerly flows around the Tibetan Plateau in winter are defined. The different characteristics of these changes are then analyzed, and the influence mechanism of the abnormal changes of the two branch flows around the 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 is a significant negative correlation between the northern and southern branch westerly flows around the 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 the troposphere. Moreover, the central Tibetan Plateau is an anomalous anticyclone (cyclone) system center over 500 hPa, which extends vertically to the height of 300 hPa (inclining to the Northwest China with the altitude). (2) The anomalous changes of the south/north branch westerly flows around the 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), there are relatively higher (lower) temperatures and less (more) precipitation in Hetao, the Tibetan Plateau, and south of the Yangtze River , but lower (higher) temperature and more (less) precipitation in Northeast China are observed. However, when the southern branch flow is stronger (weaker), the temperature is generally lower (higher) in the whole of China, the precipitation is less (more) in Northeast China and northern Xinjiang, and more (less) precipitation is observed in most parts of South China. (3) The influence mechanism of the anomalous flow around the Tibetan Plateau on the weather and climate in China shows that 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 850 hPa to the southern area. Influenced by the anomalous zonal flow around the plateau, the tropospheric atmosphere exhibits a “barotropic structure” over eastern China. The corresponding troposphere below 850hPa has a consistent anomalous southwest wind from the southern to northern areas. Conditions including the anticyclonic shear above 850 hPa near 35°N, the abnormal descending motion, and the warmer temperature below 300 hPa are not conducive to the cold air southward movement. Moreover, the anticyclone shear enhanced the effect of subsidence motion on the temperature increase. Therefore, 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 has an obvious “baroclinic feature,” and the anomalous westerly wind shows a slope zonal feature from the lower to the upper troposphere and from a low to a high latitude. The abnormal easterly wind of the lower troposphere is “triangle-shaped” from the surface of South China to 200 hPa 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, while 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 temperatures and greater precipitation in China.