Wave-Mean Flow Interaction and Stratospheric Sudden Warming in an Isentropic Model

Abstract

A multilayer isentropic model, with constant potential temperature in each model layer, has been developed to investigate the linear and nonlinear characteristics of motions in the stratosphere. The transition from linear to nonlinear behavior in the content of wave-mean flow interaction is studied. The planetary wave in the model is excited by wavelike forcings at the lower boundary. It propagates upward into the middle atmosphere. Ale planetary wave breaking and the mean zonal flow modification in the model are closely associated with critical layer of the quasi-stationary planetary wave. However, the region with strong mean flow deceleration and severe potential vorticity (PV) contour deformation is broad. As the forcing amplitude increases, this region shifts poleward and the maximum center of mean flow deceleration extends upward. In cases with large forcing amplitudes, the polar vortex is pushed away from the pole and easterly winds are found in the polar region. The responses of the model to varying forcing amplitude at the lower boundary suggest that the preconditioned mean zonal flow is not essential to the occurrence of stratospheric sudden warminglike events. The mean zonal flow can be self-preconditioned from a state that supports equatorward propagation to a state that supports poleward propagation of waves. The linear theory can be used to describe the model behavior as long as the meridional gradient of zonal mean PV is maintained. The nonlinearity becomes important once the PV gradient is destroyed in cases with large forcing amplitude.