Internal variability of the troposphere-stratosphere coupled system simulated in a simple global circulation model

Internal variability of a troposphere-stratosphere coupled system is invest igated in a series of numerical experiments with a simple global circulatio n model under a perpetual winter condition. In order to examine the relativ e importance of forced planetary waves in the interaction with the zonal me an zonal flow and baroclinic disturbances, amplitude of a sinusoidal surfac e topography of zonal wavenumber 1 or 2 is swept as an experimental paramet er; 1000-day integrations are performed for 110 combinations of the externa l parameters. Intraseasonal variability of the extratropical winter stratosphere in the p arameter sweep experiment of the zonal wavenumber-1 topography is classifie d into four regimes, dependent on the topographic amplitude: (I) a nearly r adiative equilibrium state; (II) small undulations of the polar vortex; (II I) intermittent breakdowns of the polar vortex, or occurrence of stratosphe ric sudden warming events; and (IV) a usually weak and warm state of the po lar vortex. The behavior of planetary waves in the stratosphere is characte rized by linear propagation in regime I, and by quasi-linear, or weakly non linear, wave-mean flow interaction in regime II. In regimes III and IV, on the other hand, it is in a highly nonlinear state, reflecting the occurrenc e of sudden warming events and the usually distorted polar vortex, respecti vely. The Northern Hemisphere winter stratosphere corresponds to regime III in the intraseasonal variations, while the southern counterpart is close t o regime II. A lag-correlation analysis shows that the dynamical linkage between the str atosphere and the troposphere is also dependent on the regimes. The vertica l linkage is primarily the upward control in regime I; planetary waves are generated by nonlinear interaction of baroclinic disturbances in the tropos phere and propagate into the stratosphere. In regime II, stratospheric vari ations are largely confined in the stratosphere, although planetary waves a re generated in the troposphere. The linkage in regime III, on the other ha nd, is inevitably two-way. Planetary wave variability has upward influence from the troposphere to the stratosphere as well as the zonal mean zonal wi nd shows preconditioning in the troposphere before stratospheric sudden war ming events. Downward propagation of signals to the upper troposphere are a lso seen in the zonal mean temperature in high latitudes; it is higher than normal at the final stage of the sequence of sudden warming events. The li nkage is also two-way in regime IV.