A Linear Stability Analysis of the Stratospheric and Mesospheric Zonal Mean State in Winter and Summer

Abstract

The linear normal modes of large-scale atmospheric motions are computed in a spherical quasi-geostrophic model extending from the surface up to 80 km to investigate the stability of zonal motions in the stratosphere and mesosphere. The basic states are taken from the Northern Hemisphere winter and summer solstitial conditions. The winter basic state is able to generate growing planetary waves 1 and 2, which extend into the stratosphere and the mesosphere at high latitudes. They are identified as Green modes which propagate very slowly. The structure of the Green mode compares favorably with the observed quasi-stationary planetary waves. Waves 4?6, generated in the troposphere, can also penetrate into the lower stratosphere but an trapped them and resemble Charney modes. Energetics analysis indicates that the ratio of the local energy conversion versus energy flux from below is about 1:2 for the Omen modes waves 1 and 2 in the stratosphere while the mean flow and the planetary waves in the mesosphere are all supported by wave energy flux from below. In summer, the growing normal mode solutions are significant only either in the troposphere or in the mesosphere, with little amplitude in the stratosphere. The mesosphere supports westward propagating waves 1?4, especially wavenumber 3, which may be identified as a 2-day wave observed in the summer mesosphere. In addition, a comparison is made of the stabilities of the stratospheric basic states before and after the 1976/1977 sudden warming event. Before the sudden warming, waves 1 and 2 were almost stationary. They are identified as the deep Green modes with their maximum geopotential amplitude in the upper stratosphere. After the sudden warming, the planetary waves 1, 2 and 3 were all confined to the troposphere, resembling the external Charney mode, and propagated with a much larger phase speed. Ale associated energetics in the stratosphere underwent a reversal from a typical baroclinic energy conversion (before the sudden warming) to the opposite conversion (after the sudden warming). These characteristics of transient planetary waves and the associated enegetics agree fairly well with the observations.