摘要:利用1979～2019年NCEP/NCAR再分析资料和中国地面基本气象要素日值数据集（V3.0）的气温和降水资料，首先定义了客观表征冬季青藏高原南北两支绕流变化的指数，然后分析了其不同的变化特征，并采用相关分析、合成分析等方法初步研究了青藏高原南北两支绕流异常变化对中国气温和降水的影响机制。主要结果有：（1）青藏高原冬季北支绕流和南支绕流之间呈显著的负相关；北支（南支）绕流强、南支（北支）绕流弱时，对流层中低纬度地区从高原西部到我国东部沿岸为一个大范围的异常反气旋式（气旋式）环流系统，500 hPa高原的中部为一个异常反气旋（气旋）环流中心。（2）青藏高原冬季南北两支绕流的变化对中国冬季天气气候有显著影响。当青藏高原北支绕流强（弱）时，中国除东北是气温偏低（高）、降水偏多（少）外，河套、青藏高原及长江以南则是气温偏高（低）、降水偏少（多）；当南支绕流强（弱）时，中国气温普遍偏低（高），东北及新疆北部是降水偏少（多），南方大部分地区是降水偏多（少）。（3）分析高原绕流异常变化对中国天气气候的影响机制表明：当青藏高原北支绕流强、南支绕流弱时，中国东部35°N以北的对流层中都是异常西北风，35°N以南都是异常东北风，受高原异常纬向绕流影响，对流层大气为明显的“正压结构”；相应的对流层底层从南到北为一致的异常西南风，850 hPa以上35°N的之间为反气旋式切变和下沉运动异常，300 hPa以下异常偏暖，这些条件加强了下沉增温，导致中国东部气温偏高、降水偏少。当青藏高原南支绕流强、北支绕流弱时，对流层中的纬向风异常则为明显的“斜压特征”，异常西风呈现为从对流层低层到高层、低纬度到高纬度的倾斜的带状特征，其下方自华南近地面到华北200 hPa的“三角形”状异常东风，配合相应的经向风异常和华南到华北的异常上升运动，低层为“三角形”状的异常冷气团向南切入到中国南海，中上层为异常偏暖的西南气流在冷气团上自南向北爬升到中高纬度地区，导致中国大范围的气温异常偏低、降水偏多。
关键词:青藏高原动力作用/
绕流指数/
气温异常/
降水异常
Abstract: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.
Key words:Dynamics effect of the Tibetan Plateau/
Westerly detouring flow index/
Abnormal temperature/
Abnormal precipitation
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