ASIA-PACIFIC WINTER LFO STRUCTURE AND LOWFREQUENCY VORTEX ACTIVITY FEATURES*

Based on the winter 1984/1985 ECMWF grid point data subjected to the 30-60 day band-pass filtering and composite analysis,a study is undertaken of the LFO(low-frequency oscillation) structure in the eastern Asian westerly jet entrance and exit regions and the Asia-Pacific low-frequency vortex activity characteristics.Results show that zonal wind oscillations on both sides of the jet core are in anti-phase,in close relation to the E-W displacement of the core.Ranging in NW-SE direction is a low-frequency vortex train(LFVT) emanating from Ural via central Asia to East Asia.A low-frequency vortex of Ural origin,when reaching around 50°N,80°E,is split into two parts,one travelling eastward and the other southward,and finally they arc connected cyclonically or anticyclonically at low latitudes,forming a vigorous low frequency cyclone or anticyclone in the eastern part of China mainland,completing a full cycle of the LFVT.Further,observed in the central Pacific are a meridional LFVT and a cyclone/anticyclone couplet looping in a counterclockwise sense,giving rise to the LFVT phase shift over this region.