EFFECTS OF INTRASEASONAL OSCILLATION OF EAST ASIAN ZONAL WIND ON TROPICAL CYCLONE LANDFALL IN THE MAINLAND OF CHINA DURING BOREAL SUMMER
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摘要: 使用1979—2015年欧洲长期天气预报中心所提供的ERA-Interim再分析资料和中国气象局上海台风研究所整编的西北太平洋热带气旋（TC）最佳路径资料，分析了7—8月东亚高空纬向风的季节内振荡（ISO）信号特征及其与登陆中国大陆热带气旋（TC）的关系。结果表明：（1）200 hPa纬向风在副热带、中纬度地区季节内振荡显著，尤其是在纬向西风带中，有两个南北分布的大值中心，方差贡献均超过50%。（2）基于东亚高空纬向风的ISO和EOF典型空间模定义的西风指数(EAWI)，可以用来描述东亚高空纬向西风在ISO尺度上的经向移动。（3）在西风指数的ISO负位相期间，登陆中国东南沿岸22 °N以北的TC增多；反之减少。在西风急流出口南侧的副热带区域，200 hPa ISO纬向风向北移动，使纬向西风位置偏北，出现东风异常，从而使西风减弱；TC引导气流为向西的异常，有利于TC登陆中国大陆偏北沿岸；同时有异常的ISO纬向异常东风切变，有利于TC登陆过程的维持。（4）在西风指数的ISO负位相期间，在对流层高层西风急流出口区向南输送的天气尺度的E矢量，在TC登陆地区，出现异常扰动涡度通量的辐合，引起了该区域的西风减弱。Abstract: Using ERA-Interim reanalysis datasets and the tropical cyclones (TC) best-track data from China Meteorological Administration during 1979—2015, this study analyzes the effects of intraseasonal oscillation (ISO) of East Asian zonal wind on tropical cyclone landfall in the mainland of China during boreal summer. The main conclusions are as follows: (1) The ISO of 200 hPa zonal wind in the subtropics and mid-latitudes is very strong, especially in the south and north of zonal westerlies, with their variance contributions all above 50%. (2) Based on the ISO of 200 hPa zonal wind and its EOF modes, we defined the East Asian westerly index (EAWI) to describe the meridional movement of the ISO of zonal westerlies. (3) When the ISO of EAWI is in negative anomaly, more TCs make landfall north of 22°N in the mainland of China. There is easterly anomaly of the ISO of 200hPa zonal wind over the exit region of the westerly jet, which makes it easy for the TC landfall. Meanwhile, an anomalous easterly wind shear and a zonal vertical wind shear are helpful to the TC landfall. (4) There is southerly anomaly anomalous transient eddy vorticity flux over the exit region of the westerly jet in the negatively anomalous ISO of EAWI, which resulted in the weakening of the westerly flow of the ISO of 200hPa.
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图 1 1979—2014年7—8月东亚纬向风的季节内振荡标准差（a，单位：m/s）及其方差对总方差贡献的分布（b，单位：%）
等值线为200 hPa纬向风平均场（单位：m/s）。
图 5 1979—2015年7—8月EAWI的季节内振荡的负位相(a)，正位相(b)和负位相减正位相(c)时200 hPa纬向风的季节内振荡合成异常分布（单位：m/s）
阴影区域表示通过0.05显著性水平检验。
图 6 1979—2015年7—8月EAWI的季节内振荡的负位相(a)，正位相(b)和负位相减正位相(c)时引导气流(850~300 hPa纬向风的质量权重平均)的季节内振荡合成异常分布(单位：m/s)
阴影区域表示通过0.05显著性水平检验。
图 7 1979—2015年7—8月EAWI的季节内振荡的负位相(a)，正位相(b)和负位相减正位相(c)纬向风垂直风切变（U200-U850）的季节内振荡合成异常分布
单位：m/s，阴影区域表示通过0.05显著性水平检验。
图 8 1979—2015年7—8月西风指数的季节内振荡负位相(a)，正位相(b)时10天以下天气尺度的E矢量(箭头，单位：m2/s2)合成异常分布
等值线为200 hPa纬向风的平均场（单位：m/s）。
表 1 1979—2015年7—8月EAWI的ISO不同位相登陆东南沿岸22 °N以北的TC频数和TC生成频数及其比值
ISO位相 TC登陆频数/次 TC生成频数/次 比例/% EAWI < -1σ 20 55 36.4 EAWI > 1σ 10 50 20.0 
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