东亚高层纬向风季节内振荡与盛夏登陆中国大陆热带气旋频数年际变化的联系

Relationship between Intraseasonal Oscillation of East Asian upper-level zonal wind and interannual variation in the frequency of tropical cyclone landfall over mainland China in midsummer

摘要:

利用1979—2017年欧洲中期天气预报中心提供的ERA-Interim再分析数据与中国气象局-上海台风研究所（China Meteorological Administration-Shanghai Typhoon Research Institute，CMA-STI）、美国联合台风警报中心（Joint Typhoon Warning Center，JTWC）整编的西北太平洋热带气旋（Tropical Cyclone，TC）资料集，分析东亚高层（200 hPa）纬向风季节内振荡（Intraseasonal Oscillation，ISO）与7—8月登陆中国大陆TC频数年际变化的联系。结果表明：7—8月中国大陆登陆TC频数与西风急流出口区南侧（北侧）纬向风为显著负（正）相关，南侧显著相关区与北侧的差定义的东亚西风急流指数（East Asian Westerly Jet Index，EAWJI）可定量描述急流经向移动，EAWJI负异常时急流北移、登陆TC偏多，反之急流南移、登陆TC偏少。急流北移，TC活动区域对流层高层呈偏东风异常，产生异常东风切变，有利于TC登陆过程的维持，使登陆中国大陆TC频数增多。东亚高层纬向风ISO与年际变化的标准差场、EOF模态的高度相似性说明两者由同一空间主导模态所控制，表明若其ISO偏北偏南振荡发生频率为非正态分布，剩余偏差将改变其季节平均。TC登陆多年，东亚西风急流指数ISO呈更高频率偏北移动，引起急流出口区南侧ISO尺度扰动涡度通量辐合，使季节平均西风减小，急流位置北移，说明高层纬向风ISO可通过间接调制影响TC登陆的大尺度环流进而与登陆TC频数的年际变化相联系。

Abstract:

Using the ERA-Interim reanalysis database and the tropical cyclone (TC) best-track dataset from both China Meteorological Administration (CMA) and Joint Typhoon Warning Center (JTWC) during the period of 1979-2017, this study analyzes the relationship between Intraseasonal Oscillation (ISO) of East Asian upper-level (200 hPa) zonal wind and the interannual variation in the frequency of tropical cyclone landfall over mainland China during boreal summer.
The TC frequencies of landfall in July-August present a significant interannual variation.In addition, an East Asian Westerly Jet Index (EAWJI), which represents the meridional migration of the westerlies, is defined based on the two regions where significant correlations exist between TC landfall frequencies and 200 hPa zonal wind.The EAWJI and TC landfall exhibit a significant negative correlation.That signifies that when a negative (positive) anomaly appears, westerlies appear in the northern (southern) position, and the TC landfall frequency may increase (decrease).When the westerly jet moves to the north, the troposphere in the TC active region presents anomalous easterly winds, thereby resulting in abnormal easterly shear, which is conducive to maintaining the TC landfall process and increasing the frequency of TC landfall in mainland China.
The high similarity in both standard deviation and EOF modes regarding the East Asian upper-level zonal wind's ISO variation and interannual variation indicates that they are governed by a common mode of spatial variability.This signifies that the asymmetric frequency of north (south) conditions would generate residual deviations and alter the seasonal mean.In Landfall High-Frequency years, ISO-filtered EAWJI exhibits a higher frequency of north conditions, which cause the convergence of ISO-scale transient eddy flux in the south of the westerly jet exit, and lead the westerly jet to drift northward.Therefore, by modulating the large-scale circulation which affects the process of TC landfall, the ISO of the 200 hPa zonal wind can be indirectly connected with the interannual variation of the TC landfall frequency.