NONSTATIONARY GRAVITY-WAVE FORCING OF THE STRATOSPHERIC ZONAL MEAN WIND

The role of gravity wave forcing in the zonal mean circulation of the stratosphere is discussed. Starting from some very simple assumptions about the momentum flux spectrum of nonstationary (non-zero phase spee d) waves at forcing levels in the troposphere, a linear model is used to calculate wave propagation through climatological zonal mean winds at solstice seasons. As the wave amplitudes exceed their stable limits , a saturation criterion is imposed to account for nonlinear wave brea kdown effects, and the resulting vertical gradient in the wave momentu m flux is then used to estimate the mean flow forcing per unit mass. E vidence from global, assimilated data sets are used to constrain these forcing estimates. The results suggest the gravity-wave-driven force is accelerative (has the same sign as the mean wind) throughout most o f the stratosphere above 20 km. The sense of the gravity wave forcing in the stratosphere is thus opposite to that in the mesosphere, where gravity wave drag is widely believed to play a principal role in decel erating the mesospheric jets. The forcing estimates are further compar ed to existing gravity wave parameterizations for the same climatologi cal zonal mean conditions. Substantial disagreement is evident in the stratosphere, and we discuss the reasons for the disagreement. The res ults suggest limits on typical gravity wave amplitudes near source lev els in the troposphere at solstice seasons. The gravity wave forcing i n the stratosphere appears to have a substantial effect on lower strat ospheric temperatures during southern hemisphere summer and thus may b e relevant to climate.