青藏高原夏季高层纬向风季节内振荡特征

Intraseasonal oscillation characteristics of zonal wind at upper troposphere over Tibetan Plateau in summer

利用1979—2019年ECMWF提供的ERA-Interim逐日再分析资料,采用Morlet小波分析、滤波及合成分析等方法,探究了青藏高原夏季对流层高层纬向风季节内振荡(IntraSeasonal Oscillation,ISO)的主要周期及其传播特征。结果表明:青藏高原夏季高层纬向风季节内振荡的主要周期为10~30 d,其强度存在明显的年际差异。在纬向风ISO强年,振荡过程持续时间长、振幅强,ISO方差中心从对流层高层向下影响到对流层中层,表现为相当正压结构。其传播在纬向上主要表现为ISO中心从高原东部3次向东传,可达西太平洋地区;经向上分别有4次自中高纬向南传播的10~30 d ISO中心与来自低纬地区的ISO中心在高原南侧汇合,其强度在高原南侧有所加强,强振荡中心可向南传播到达低纬地区。ISO的位相演变主要表现为低频反气旋和低频气旋中心在高原东部交替出现,引起东部地区上空低频东风和低频西风的强度变化。在ISO极端活跃位相,高原东部低频西风达最强。

Based on the ERA-Interim reanalysis data four times a day provided by ECMWF from 1979 to 2019,the main period of the intraseasonal oscillation and its propagation characteristics of zonal wind at upper troposphere over the eastern Tibetan Plateau in summer are studied by the Morlet wavelet,filtering and composite analysis methods.Results show that the 10—30 d is the main period of intraseasonal oscillation of zonal wind at upper troposphere over the eastern Tibetan Plateau in summer and its intensity has significant interannual difference.In the strong intraseasonal oscillation years,the low frequency oscillation process lasts for a long time and has a large amplitude.The center of variance of intraseasonal oscillation influences the mid-troposphere from the upper troposphere,which shows an equivalent barotropic structure.In zonal propagation,the intraseasonal oscillation centers mainly propagate eastward from the eastern Tibetan Plateau for three times and reach the Western Pacific region.In meridional propagation,10—30 d intraseasonal oscillation centers propagating southward from the mid and higher latitudes converge with centers from the lower latitude on the south side of the Plateau for four times from June to August,and their intensities are strengthened.The strong oscillation centers can propagate southward to the lower latitude areas.The phase evolution of intraseasonal oscillation is mainly manifested in the alternation of the low frequency anticyclone and the low frequency cyclone center over the eastern Tibetan Plateau,which leads to the intensity change of low-frequency easterly wind and low-frequency westerly wind over the eastern Tibetan Plateau.In the extremely active phase of ISO,the low frequency westerly wind over the eastern Tibetan Plateau is the strongest.