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Atmospheric Vacillations in a General Circulation Model. II: Tropospheric-Stratospheric Coupling and Stratospheric Variability

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  • 1 Australian Numerical Meteorology Research Centre, P.O. Box 5089AA, Melbourne, Australia 3001
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Abstract

The transient behavior of the stratosphere and its relationship to fluctuations in the dynamical coupling between troposphere and stratosphere has been investigated. The analysis is based on a 124-day sequence of atmospheric statistics generated in an 18-level hemispheric general circulation model set up for annual mean conditions. The model results are assumed to be representative of the winter half-year because of a bias in the model toward winter conditions.

Examination of the various forcing terms demonstrates that the dynamical coupling is dominated by the w′ϕ′ flux. This flux maximizes at subpolar latitudes and experiences a quasi-periodic fluctuation which is controlled by the tropospheric vacillation cycle, thus indicating a plausible connection between this vacillation cycle and stratospheric warnings. Both long- and short-term stratosphere variations are largely controlled by the fluctuations of the w′ϕ′ flux. The convergence of this flux is shown to force corresponding variations in the eddy kinetic energy and the eddy and zonal available potential energies in the model stratosphere. Results are discussed for a number of different levels and latitudes. Fluctuations in the poleward flux of sensible heat and possibly relative angular momentum owing to large-scale eddies can also be related to variations in the w′ϕ′ flux. The induced responses in the mean zonal wind and zonal mean temperature distributions relate well to observations. These findings suggest that not only are intra-annual stratospheric variations governed by fluctuations in the tropospheric vacillation cycle, but that inter-annual variations are also attributable to the same cause. The synoptic response of the model stratospheric fields is illustrated for wavenumbers 2 and 5, and shown to vary considerably during the course of a vacillation cycle.

Abstract

The transient behavior of the stratosphere and its relationship to fluctuations in the dynamical coupling between troposphere and stratosphere has been investigated. The analysis is based on a 124-day sequence of atmospheric statistics generated in an 18-level hemispheric general circulation model set up for annual mean conditions. The model results are assumed to be representative of the winter half-year because of a bias in the model toward winter conditions.

Examination of the various forcing terms demonstrates that the dynamical coupling is dominated by the w′ϕ′ flux. This flux maximizes at subpolar latitudes and experiences a quasi-periodic fluctuation which is controlled by the tropospheric vacillation cycle, thus indicating a plausible connection between this vacillation cycle and stratospheric warnings. Both long- and short-term stratosphere variations are largely controlled by the fluctuations of the w′ϕ′ flux. The convergence of this flux is shown to force corresponding variations in the eddy kinetic energy and the eddy and zonal available potential energies in the model stratosphere. Results are discussed for a number of different levels and latitudes. Fluctuations in the poleward flux of sensible heat and possibly relative angular momentum owing to large-scale eddies can also be related to variations in the w′ϕ′ flux. The induced responses in the mean zonal wind and zonal mean temperature distributions relate well to observations. These findings suggest that not only are intra-annual stratospheric variations governed by fluctuations in the tropospheric vacillation cycle, but that inter-annual variations are also attributable to the same cause. The synoptic response of the model stratospheric fields is illustrated for wavenumbers 2 and 5, and shown to vary considerably during the course of a vacillation cycle.

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