北半球冬季高空西风急流环路“闭合-断开”的年际振荡及其影响机制

Northern Hemisphere High Jet Circuit “On-off” Oscillation in the Boreal Winter

本文利用1979—2018年的200hPa纬向风月数据,从北半球高空急流轴的纬度-频率分布出发,定义了高空西风急流环路(High Jet Circuit,HJC)和HJC活动区。研究结果表明,冬季北半球HJC为环绕地球中纬度一圈的窄而强的风速带,在大西洋上空不闭合,类似于环路中的“开关”。其年际变化表现为HJC“闭合-断开”振荡,当振荡为闭合(断开)位相时,大西洋上空HJC“开关”的左(右)两支急流分别南(北)移和减弱(增强),同时HJC在欧亚大陆略南(北)移,在太平洋增强(减弱)。HJC的“闭合-断开”年际振荡受厄尔尼诺型海温以及高纬欧亚大陆和格陵兰岛东西偶极子型热力异常强迫的影响。厄尔尼诺(拉尼娜)型海温异常通过激发太平洋-北美型大气遥相关异常环流及影响天气尺度瞬变涡,使HJC在太平洋上空增强(减弱),HJC“开关”的左支急流南(北)移,使HJC“开关”趋于闭合(断开)。格陵兰岛和欧亚大陆北部的正-负(负-正)异常加热分布与北大西洋涛动(NAO)负(正)位相时北极地区的异常反气旋(气旋)环流造成的冷暖平流异常有关,而NAO负(正)位相的中纬度异常环流通过影响西风急流南压(北抬),使HJC“开关”趋于闭合(断开)。这两种机制的共同影响形成了北半球HJC“闭合-断开”的年际振荡。

Based on the latitude-frequency distribution of the high jet axis in the boreal winter,the high jet circuit(HJC) and the activity region of the HJC are defined by using the 200 hPa zonal wind monthly data from 1979 to 2018.The results show that the HJC is a narrow and strong wind speed band around the middle latitude of the earth,which is usually not closed over the Atlantic(30°W—50°W),and the ARHJC range here is large,which is similar to the “switch” in a closed circuit.Its interannual variability is characterized by the HJC“ on-off” oscillation,and the Atlantic part is the key “switch” of it.When the oscillation is in the on(off) phase,the left and right branches of “switch” over the Atlantic move southward(northward) and weakened(strengthened),respectively,making the “switch” tend to be closed(off).Meanwhile,the HJC moves southward(northward) over Eurasia and strengthened(weakened) over the Pacific.The HJC “on-off” oscillation is influenced by the El Nino(La Nina) SST anomalies and the thermodynamic dipole anomalies over high latitude Eurasia and Greenland.El Nino(La Nina) SST anomaly stimulates Pacific-North America Teleconnection anomalous circulation and affect the synoptic scale transient eddy,which makes the HJC strengthened(weakened) over the Pacific,and the left branch of the HJC “switch” moves southward(northward),so that the “switch” tends to closed(off).The positive-negative(negative-positive) anomalous heating distribution over Greenland and northern Eurasia is related to the anomalous cold and warm advection caused by the anomalous anticyclonic(cyclonic) circulation over the Arctic during the negative(positive) phase of North Atlantic Oscillation(NAO),while the anomalous circulation in the middle latitude during the negative(positive) phase of NAO tends to close(off) the “switch” of HJC by influencing the westerly jet to move southward(northward).The joint influence of these two mechanisms leads to the HJC “on-off” oscillation.