大气环状模对夏季中国大陆登陆热带气旋频数气候变化的影响

采用1949—2016年7—8月美国国家环境预报中心及大气研究中心(NCEP/NCAR)再分析资料与中国气象局(CMA)上海台风所整编的热带气旋最佳路径数据集,研究大气环状模(Circumglobal Teleconnection,CGT)与中国大陆地区登陆热带气旋(Tropical Cyclone,TC)频数气候变化的关系。定义200 hPa经向风经验正交展开(EOF)的第一模态为CGT,其时间系数为环状模指数(Circumgolbal Teleconnection Index,CGTI)。CGT在北半球中纬度地区有5个异常中心,CGTI在1949—2016年呈明显的下降趋势,且存在一个2~3 a的周期振荡。CGT与大尺度环流异常存在密切的联系。研究表明:CGTI与中国大陆登陆TC频数气候变率具有显著的正相关,即CGTI表现为正异常时,登陆中国大陆的TC频数增加,反之减少。当CGT表现为正位相时,东亚副热带西风急流增强,急流南侧的反气旋切变增强,使TC登陆过程的活动区域200 hPa的辐散增强,此外,对流层高层Rossby波能量向南传播增强,形成波通量辐合,导致东风异常,产生了东风异常的引导气流和纬向风垂直切变,东风切变使得切变减小,增加了TC登陆中国大陆的可能性。

Impacts of atmospheric circumglobal teleconnection variation on the frequency of landfall tropical cyclone on the mainland China during summer

The NCEP/NCAR reanalysis data and the tropical cyclone best-track data which is compiled by Shanghai Typhoon Institute of China Meteorological Administration from 1949 to 2016 are used to study the relationship between atmospheric Circumglobal Teleconnection (CGT) and the frequency of landfall Tropical Cyclone (TC) on the mainland China during summer. Based on the Empirical Orthogonal Function (EOF) of 200 hPa meridional wind, CGT is defined by the first EOF pattern and its time factor is defined as Circumglobal Teleconnection Index (CGTI). CGT has five anomalous centers in middle latitudes of the Northern Hemisphere. CGTI shows a downward trend from 1949 to 2016 and has a periodic variation lasting two to three years. CGT is closely related to large-scale circulation anomalies. The result shows that there is negative correlation between CGTI and the TC frequency. In other words, the TC frequency is increasing during negative anomalous years, and vice versa. When CGT shows positive phase, the East Asian subtropical westerly jet intensifies, and the enhancement of anticyclonic shear on the southern side of the jet stream strengthens the divergence at 200 hPa around TC activity area during its landing process. Moreover, the intensification of Rossby wave energy propagating southward in troposphere leads to the convergence of wave flux, and therefore easterly anomaly appears in the corresponding region. Both of the above results in the easterly anomalous steering flow and vertical shear of zonal wind, thus increasing the possibility of TC landing in mainland China.