The diurnal tide in the mesosphere and lower thermosphere is observed to have large seasonal and interannual variations in amplitude and structure. Many theories have been proposed to explain this phenomenon, including seasonal variations in heating, gravity wave drag, gravity wave induced diffusion, planetary wave interaction, and interaction with the seasonally varying mean flow. It has been found that each of these mechanisms is actually able to produce seasonal variability in tidal models. It is still unclear which mechanism is primarily responsible for the seasonal variability in the atmosphere.

This paper will present results of experiments in which we explore the interaction between gravity waves and tides as modulated by the mean winds. These experiments employ a time dependent linearized primitive equation model that incorporates either the Alexander-Dunkerton or Hines gravity wave schemes. Both of these schemes parameterize the wave-drag on the mean flow that results when the gravity waves saturate and break, but are based on completely different saturation criteria. It is well known that the summer and winter jet reversals in the mesosphere are accomplished by the net gravity wave drag resulting from selective filtering of a spectrum of gravity waves by the jets themselves. The tidal winds also filter the gravity waves, causing a diurnal modulation of the wave drag that acts upon the tide, changing its amplitude and structure. A peak in the amplitude of the diurnal component of gravity wave drag coincides with the location of the jet reversal in the winter mesosphere. This seasonal dependence of the wave drag acting on the tide provides a mechanism by which tidal amplitude can be significantly reduced during solstice, as observed. It will be shown how this interaction between the tide, mean flow and gravity waves depends upon to the mean wind structure, the GW parameterization used, and choice of the GW source spectrum.