WAVE MEAN FLOW INTERACTION AND STRATOSPHERIC SUDDEN WARMING IN AN ISENTROPIC MODEL

A multilayer isentropic model, with constant potential temperature in each model layer, has been developed to investigate the linear and non linear characteristics of motions in the stratosphere. The transition from linear to nonlinear behavior in the content of wave-mean flow int eraction is studied. The planetary wave in the model is excited by wav elike forcings at the lower boundary. lt propagates upward into the mi ddle atmosphere. The planetary wave breaking and the mean zonal flow m odification 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 def ormation is broad. As the forcing amplitude increases, this region shi fts poleward and the maximum center of mean flow deceleration extends upward. In cases with large forcing amplitudes, the polar vortex is pu shed away from the pole and easterly winds are found in the polar regi on. The responses of the model to varying forcing amplitude at the low er boundary suggest that the preconditioned mean zonal flow is not ess ential to the occurrence of stratospheric sudden warminglike events. T he mean zonal flow can be self-preconditioned from a state that suppor ts equatorward propagation to a state that supports poleward propagati on of waves. The linear theory can be used to describe the model behav ior 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.