The Behavior of Mean Zonal Wind and Planetary-Scale Disturbances in the Troposphere and Stratosphere during the 1973 Sudden Warming

Abstract

The behavior of mean zonal wind and planetary-scale geopotential height disturbances during the 1973 sudden warming is investigated in terms of their dynamical interactions, with use of the data of Nimbus 5 Selective Chopper Radiometer and conventional radiosonde. From the survey of the stratospheric temperature field in the 1972/73 northern hemisphere winter, it is found that two mid-winter warmings occurred successively at an interval of about 16 days. Examination of previous studies based on satellite observations reveals that a major or minor warming occurs about two weeks before a spectacular major warming, and an emphasis is laid on the effect of the first warming on the second one.
A large easterly region of mean zonal wind appeared in the stratospheric high latitudes in the second warming (Warming 2) while not in the first warming (Warming 1). With Warming 2, initially the easterly accelerations occurred in middle latitudes while the westerly accelerations in high latitudes in the upper stratosphere and these accelerations owe its existence generally to the momentum flux convergence mainly by wavenumber 2 disturbances. Eventually there occurred the intense easterly accelerations both in middle and high latitudes which brought about the circulation reversal in the stratosphere, which were due to the predominance of the Coriolis torque acting on the equatorward mean meridional flow induced mainly by wavenumber 1 disturbances. During pre-warming period for Warming 2, wavenumber 1 geopotential height disturbances propagated from the lower troposphere upward to the middle stratosphere and attained very large amplitude. This amplification of wavenumber 1 seems to come from the response to a characteristic mean zonal wind profile which continued through about 10 days after Warming 1. The wind profile is such that a westerly maximum lies in the high latitudes 40km level and then a region of large positive β-effective (latitudinal gradient of zonal mean quasi-geostrophic potential vorticity) in the high latitude stratosphere is surrounded by negative β-effective regions. It is speculated that this wavenumber 1 amplification is the result of resonance of planetary Rossby wave discussed by Tung and Lnidzen (1979).