TIDAL GRAVITY-WAVE INTERACTIONS AND THEIR INFLUENCE ON THE LARGE-SCALE DYNAMICS OF THE MIDDLE ATMOSPHERE - MODEL RESULTS

Results from a modified version of the quasi-geostrophic model of McLandress and McFarlane [1993] in combination with the tidal model of Forbes [1982a, b] are used to delineate the role of tides in modulating gravity wave momentum fluxes and the ensuing effects on the large-scale circulation of the middle atmosphere for solstice conditions. The influence of the tidal wind fields on the location and strength of the gravity wave drag is strongest in the mesosphere and lower thermosphere where the tidal winds are of the same order as the zonal mean component. Due to the nonlinear dependence on the local wind speed, the daily mean gravity wave drag in the model can differ significantly depending on whether or not tidal variations are included. At middle and low latitudes these differences are largest and occur as a result of the filtering effects of the shorter vertical wavelength diurnal tide. At more polar latitudes where the longer wavelength semidiurnal tide dominates, a slight increase in drag occurs. The interaction between the tidal wind fields and the localized orographic gravity wave momentum flux has important consequences for the in situ generation of planetary scale disturbances. In particular, the amplification of stationary planetary wavenumber 1 is enhanced by the presence of tides. In addition, forcing of nonmigrating tidal modes is predicted to occur as a result of the localized drag.