Dynamics of Planetary-Scale Baroclinic Waves during Southern Hemisphere Winter

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  • 1 Space Science Division, NASA-Ames Research Center, Moffett Field, California
  • | 2 Mycol, Inc. Sunnyvale, California
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Abstract

An investigation is conducted of the characteristics of linear and finite amplitude baroclinic waves during Southern Hemisphere winter. Emphasis is placed on the dynamics of planetary scale modes, which propagate into the stratosphere. Linear stability analysis of the June–August 1979, time mean geostrophic zonally averaged wind and temperature fields shows that both Charney and Green baroclinic modes exist at planetary scales, with the Green modes having smaller than, but comparable growth rates to, those of the Charney modes. Barotropic unstable modes of the June–August mean geostrophic zonal wind had growth rates too small to make them dynamically significant. Fully nonlinear computations showed that wave-wave coupling among intermediate scale baroclinic modes was important for determining the evolution of planetary scale modes. Computed maximum amplitudes of planetary modes are comparable to peak amplitudes observed for eastward traveling modes in Southern Hemisphere winter. Lack of coherence between stratosphere and troposphere for eastward traveling modes with zonal wavenumbers 1 and 2 appears to be due, based on the computations, to nonlinear effects in the stratosphere. EP fluxes are generally upwards and equatorwards as observed. Zonal accelerations implied by the EP flux divergence in the stratosphere show a dipole pattern typical of that observed, with westerly acceleration on the poleward side of the jet, and easterly acceleration on the equatorward side. The stratospheric jet tends to move downwards and polewards as a result, also in accord with observation. Transport of isentropic potential vorticity on isentropic surfaces in the stratosphere did not indicate irreversible mixing across or outside the vortex by planetary baroclinic waves.

Abstract

An investigation is conducted of the characteristics of linear and finite amplitude baroclinic waves during Southern Hemisphere winter. Emphasis is placed on the dynamics of planetary scale modes, which propagate into the stratosphere. Linear stability analysis of the June–August 1979, time mean geostrophic zonally averaged wind and temperature fields shows that both Charney and Green baroclinic modes exist at planetary scales, with the Green modes having smaller than, but comparable growth rates to, those of the Charney modes. Barotropic unstable modes of the June–August mean geostrophic zonal wind had growth rates too small to make them dynamically significant. Fully nonlinear computations showed that wave-wave coupling among intermediate scale baroclinic modes was important for determining the evolution of planetary scale modes. Computed maximum amplitudes of planetary modes are comparable to peak amplitudes observed for eastward traveling modes in Southern Hemisphere winter. Lack of coherence between stratosphere and troposphere for eastward traveling modes with zonal wavenumbers 1 and 2 appears to be due, based on the computations, to nonlinear effects in the stratosphere. EP fluxes are generally upwards and equatorwards as observed. Zonal accelerations implied by the EP flux divergence in the stratosphere show a dipole pattern typical of that observed, with westerly acceleration on the poleward side of the jet, and easterly acceleration on the equatorward side. The stratospheric jet tends to move downwards and polewards as a result, also in accord with observation. Transport of isentropic potential vorticity on isentropic surfaces in the stratosphere did not indicate irreversible mixing across or outside the vortex by planetary baroclinic waves.

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