Role of forced planetary waves in the troposphere-stratosphere dynamical coupling

The dynamical coupling between the troposphere and stratosphere is mainly through vertically propagating planetary waves which are generated in the troposphere by the interactions among the zonal mean zonal wind, planetary-scale forcings such as topography and heat-contrasts and baroclinic disturbances. The interactions are highly nonlinear, so that behavior of the planetary waves might be sensitive to slight changes of the forcing process.

We have investigated the vertical coupling in numerical experiments with a simple global circulation model under perpetual-winter or seasonally-varying conditions. In order to examine relative importance of forced planetary waves in the interactions, a series of parameter sweep experiments are performed by changing amplitude of a sinusoidal surface topography.

Our previous study revealed different types of stratospheric variability, such as occurrence of sudden warmings, depending on the topographic amplitude. The coupling is also dependent on the topographic amplitude, or the stratospheric variability. The coupling is primarily upward for no topographic forcing, while it is two-way for small and medium topographic amplitudes (around 500 m and 1000 m, respectively) in seasons when the stratosphere shows large variations (spring for small amplitudes and autumn to spring for medium ones, corresponding to the real southern and northern hemispheres, respectively). Typical two-way coupling consists of preconditioning and aftereffects of stratospheric sudden warming events, which extend in the stratosphere and also troposphere. Time variations in the troposphere and stratosphere are rather independent in the other seasons.

The vertical coupling process is analyzed from a viewpoint of the annular mode variability in the perpetual-winter experiments. The dominant mode of low frequency variability qualitatively changes with the topographic amplitude. An ideal annular mode is obtained in the run with no topography, which barotropically extends from the surface to the stratosphere. Even if the topography is included, the leading mode shows a barotropic annular structure in the troposphere, containing some zonal asymmetry of zonal wavenumber one. The tropospheric annular mode is basically independent of stratospheric variability in runs of small topographic amplitudes, while it is significantly correlated in those of medium ones, with a baroclinic structure above the tropopause.