Gravity wave interactions with the diurnal propagating tide

Observations of the diurnal propagating solar tide in the mesosphere and lo wer thermosphere (similar to 50-150 km) indicate substantial annual variati ons in tidal amplitudes. Observations and modeling efforts suggest that int eractions with internal gravity waves can result in significant modificatio ns of the diurnal tide. Herein a two-dimensional (2-D) linear steady-state tidal model is used in conjunction with hybrid Lindzen/Matsuno gravity wave parameterization to examine the role of gravity wave interactions in the s easonal variability of the diurnal tide in the MLT. The analysis indicates that annual variations of the background atmosphere result in significant a nnual variations in the eddy diffusion and the diurnal harmonic of the mome ntum flux divergence (wave stress) produced by breaking gravity waves. The seasonal variability in these two mechanisms can explain the observed semi- annual variability in the diurnal tide: amplitude minima at solstice and ma xima at equinox. The relative roles of wave stress and diffusion are examin ed and it is shown that both play important roles in tidal modification by damping its amplitude. The effects of varying inputs to the gravity wave pa rameterization are explored.