The Role of Forced Planetary Waves in the Annual Cycle of the Zonal Mean Circulation of the Middle Atmosphere

Abstract

The annual cycle of the zonally averaged circulation in the middle atmosphere (16–96 km) is simulated using; a severely truncated semi-spectral numerical model. The model includes only a single zonal harmonic wave component which interacts with the mean flow. The circulation is driven by diabatic heating and by a specified perturbation in the topography of the lower boundary, which is taken to be the 100 mb surface. Damping is included as Newtonian cooling and Rayleigh friction. A comparison of the annual cycle simulated by this model with the results of an analogous two-dimensional model indicates that planetary waves have relatively little influence on the zonal mean temperature profiles and on the solstice mean zonal winds at high latitudes. The primary effects of the forced waves are in decelerating the mean winds in low latitudes in the winter hemisphere to produce a region of weak westerlies, and in generating “final warmings” at the spring equinoxes. Computed mean zonal winds and wave amplitudes display significant hemispheric asymmetries. These differences are associated with the strong dependence of the eddy structure on the mean zonal wind profile. The “residual” mean meridional circulation, computed by subtracting the portion of the Eulerian mean meridional flow which is exactly balanced by the eddy heat fluxes, is nearly identical at the solstices to the Eulerian mean circulation computed in the two-dimensional model. Except in low latitudes, the net mean flow forcing by the eddy fluxes is quite small at the solstices in agreement with the predictions of wave-mean flow non-acceleration theorems. There are no major midwinter sudden warmings produced in the model. However, final warmings occur in the spring equinoctial seasons in both hemispheres. The rapid transient adjustments which occur during these final warmings make the equinoctial circulations in this model much different from the circulations computed for corresponding seasons in the two-dimensional model.