in a Stratospheric Model

Abstract

The structure of stationary planetary waves in the winter stratosphere is computed by. means of a steady-state hemispheric quasi-geostrophic model with a zonal basic state and lower boundary forcing obtained from climatology. The nonlinear wave solution is found to resemble rather closely the linear one, despite the large wave amplitudes which distort considerably the westerly zonal flow. Zonal wavenumber one is the most affected by the wave-wave interactions, experiencing an increase in amplitude and a decrease in westward phase tilt in the northerly regions of the middle stratosphere. Comparison of the solutions to the corresponding climatological wave structure indicates that the inclusion of the nonlinear terms leads to an improvement of the structure of wavenumber one.
An examination of the 5-year January climatological basic state reveals a distinct linear relationship between the zonal streamfunction and the tonal potential vorticity in middle and northerly latitudes. Consequently, the wave-wave interactions are to a first approximation a result of the presence of the model dissipation. Weak dissipation in this region implies only weak interactions, which explains the quasi-linear structure of the solutions.