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Stratospheric Vacillation Cycles

James R. HoltonDepartment of Atmospheric Sciences, University of Washington, Seattle 98195

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Clifford MassDepartment of Atmospheric Sciences, University of Washington, Seattle 98195

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Abstract

A quasi-geostrophic β-plane channel model is used to study wave-mean flow interaction in the stratosphere. The zonal mean circulation in the model is driven by differential radiative heating (parameterized in terms of a “Newtonian cooling”) and by horizontal eddy heat fluxes due to vertically propagating planetary waves excited by steady forcing at the lower boundary. We find that for sufficiently low-amplitude wave forcing the response is a steady stratospheric circulation very close to radiative equilibrium conditions. However, when the wave forcing is raised beyond a critical amplitude (typically of order 150 m) the response is no longer steady; rather, the mean zonal flow and eddy components oscillate quasi-periodically. We conclude that oscillations in stratospheric long waves do not necessarily reflect oscillating tropospheric forcing but may occur even in the presence of steady forcing.

Abstract

A quasi-geostrophic β-plane channel model is used to study wave-mean flow interaction in the stratosphere. The zonal mean circulation in the model is driven by differential radiative heating (parameterized in terms of a “Newtonian cooling”) and by horizontal eddy heat fluxes due to vertically propagating planetary waves excited by steady forcing at the lower boundary. We find that for sufficiently low-amplitude wave forcing the response is a steady stratospheric circulation very close to radiative equilibrium conditions. However, when the wave forcing is raised beyond a critical amplitude (typically of order 150 m) the response is no longer steady; rather, the mean zonal flow and eddy components oscillate quasi-periodically. We conclude that oscillations in stratospheric long waves do not necessarily reflect oscillating tropospheric forcing but may occur even in the presence of steady forcing.

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