2020年7月西北太平洋和南海“空台”环流特征分析

ABSENCE OF TROPICAL CYCLONES AND CIRCULATION CHARACTERISTICS IN JULY 2020 IN THE NORTHWESTERN PACIFIC AND THE SOUTH CHINA SEA

2020年7月西北太平洋和南海出现了史无前例的“空台”事件。利用NCEP再分析数据集、中国气象局(CMA)台风最佳路径等资料研究了此次“空台”现象的大尺度环流背景及动力和热力学特征。使用台风潜在生成指数(DGPI)分析发现2020年7月大尺度环流背景不利于台风生成,环流系统的异常通过影响对流层垂直风切变和垂直运动限制了台风的活动。2020年7月马斯克林高压较常年明显偏西偏弱,导致索马里急流强度减弱,越赤道气流不活跃,菲律宾以东洋面和南海海域盛行一致的偏东气流,历史同期活跃在该区域的季风槽无法建立,从而不利于热带扰动的生成。北半球极涡主体偏向西半球一侧,影响东半球冷空气势力较弱,副热带高压位置偏西;南亚高压较历史同期偏强且偏东,其东侧强盛的偏东气流将洋中槽截断,在西北太平洋区域出现反气旋性环流,该区域下沉气流增强,导致副热带高压强度增强,对流层中层强烈的下沉气流抑制了台风的生成和发展。此外,受中高层环流系统异常的影响,7月菲律宾吕宋岛以东洋面和南海地区环境垂直风切变较常年偏高2~4 m/s,南海部分海域偏高达4~8 m/s,同时该区域内异常偏强的下沉气流导致对流层低层相对湿度偏低,大气层结处于较为稳定的状态,动力和热力条件均不利于热带扰动的进一步发展。

The large-scale circulation, dynamic, and thermodynamic factors related to the lack of genesis of tropical cyclones in the Northwestern Pacific and the South China Sea in July 2020 were preliminarily analyzed by using the NCEP reanalysis datasets and tropical cyclone data created by the Chinese Meteorological Administration. By analyzing the Dynamic Genesis Potential Index(DGPI), we found that the anomalies of large-scale circulation limited typhoon activities by affecting tropospheric vertical wind shear and vertical movement. The Mascarene High was significantly weaker than usual, resulting in a weaker Somali jet stream. The cross-equatorial air flow was not active, and the easterlies consistently prevailed over the east of the Philippines and the South China Sea, which was not conducive to the establishment of monsoon trough over this region, thus affecting the formation of tropical disturbance. The center of the polar vortex in the Northern Hemisphere was closer to the Western Hemisphere in July 2020,and thus the cold air activities in the Eastern Hemisphere were not significantly affected, and the Subtropical High was more westward. The South Asian High was stronger and more eastward than before,and the oceanic trough was cut off by the strong easterlies of the South Asian High, which enhanced the downdraft over this region, and eventually intensified the Subtropical High. The downdraft in the middle of the troposphere limited the formation and development of tropical cyclones. Furthermore, due to the anomalous circulation at mid-and high-levels, the vertical wind shear over the east of Luzon Islands and the South China Sea was 2~4 m/s higher than normal in July, and 4~8 m/s higher over parts of the South China Sea. At the same time, the relative humidity in the lower troposphere was relatively low, and the atmospheric stratification was stable. Both dynamic and thermal conditions were not conducive to the further development of tropical disturbances.