STRATOSPHERIC VACILLATION CYCLES.

Abstract

A quasi-geostrophic beta -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. It is found 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. It is concluded that oscillations in stratospheric long waves do not necessarily reflect oscillating tropospheric forcing but may occur even in the presence of steady forcing.