2019年11月西北太平洋热带气旋生成频数异常偏多的成因分析

Study of the causation of anomaly higher tropical cyclone genesis frequency over the western North Pacific in November 2019

基于1979—2019年NCEP-DOE再分析资料、中国气象局发布的热带气旋(TC)最佳路径数据集和HadISST全球海温资料等,研究了2019年11月西北太平洋TC生成频数异常偏多的可能原因。结果表明:2019年11月西太平洋副热带高压(WPSH)强度偏强、脊线偏北,其南侧偏东气流与越赤道气流交汇形成的西北太平洋热带辐合带偏强偏北、向东延伸,为TC生成创造了低层强的辐合、高层强的辐散、小的风速垂直切变以及对流层中层充足的水汽等有利的大尺度环境条件,导致TC生成频次异常偏多、生成位置偏北偏东。采用EOF方法进行分析,发现11月东亚大槽年际变化的第二模态为南北反位相型分布,对应的第二特征向量(PC2)与WPSH脊线指数具有高度一致性,且与同期西北太平洋上空TC生成频数呈显著的正相关关系。即当11月东亚大槽北部加深而南部变浅时,对应WPSH脊线偏北,TC生成频数偏多,表明深秋季节西风带槽脊活动对西北太平洋TC生成有一定的调制作用。进一步的诊断分析揭示了北太平洋海温异常可能是造成东亚大槽经向偶极子型变异的重要因子。

Based on the tropical cyclone(TC)data from CMA,NCEP-DOE reanalysis and HadISST datasets,this study investigates the possible reason for the above-normal tropical cyclogenesis frequency over the western North Pacific(WNP)in November 2019.The intensity of the western Pacific subtropical high(WPSH)is found to be much stronger with the ridge line shifting northward in November 2019.The related WNP monsoon trough tends to be intensified,move northward and extend eastward,accompanied by the enhanced lower-level convergence,stronger upper-level divergence,weaker vertical zonal wind shear and a positive mid-level relative humidity anomaly in the tropical western-central Pacific,which favors the increased tropical cyclogenesis frequency in the WNP.The EOF analysis reveals that the second mode of the annual variations of East Asian trough(EAT)is a meridional dipole distribution.The corresponding second eigenvectors are in high consistency with the WPSH ridge index and in significantly positive correlation with the TC genesis frequency during November of 1979—2019,suggesting the modulation effect of the extra-tropical circulation on the WNP TC genesis.Further examinations indicate that the meridional dipole pattern of the EAT may be attributed to the anomalous SST over the North Pacific.