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. Each of these mechanisms have actually been found able to produce seasonal variability in tidal models. It is perhaps likely that all of them contribute to the seasonal variability in the atmosphere.

We have therefore undertaken a systematic study of several of these mechanisms in the hope that we can identify which of them actually occur in nature. This presentation will focus on the interaction of the tides with the mean flow and with gravity wave drag. It is found that the mean winds can change the relative phase of the tidal response to tropospheric and stratospheric heating sources. A seasonal reversal of phase in the stratospheric response results in an amplitude modulation in the total response that is twice the size of the amplitude of the stratospheric response alone. The mean winds also modulate the interaction of a spectrum of gravity waves with the tide.

Gravity wave drag reduces tidal amplitude if the phase difference between the diurnal variations in the drag and the tide is greater than 90 degrees. (This phase relation may depend on the gravity wave drag scheme used.) The mean wind structure during solstice allows gravity waves near the peak of the spectrum at slower phase speeds to reach higher into the mesosphere, which causes larger reduction in tidal amplitude. The associated increase in gravity wave breaking could also cause an increase in eddy diffusion and provide an additional mechanism for tidal amplitude reduction.