  Effects of subgrid-scale gravity waves (GWs) on the diurnal migrating tides
are investigated from the mesosphere to the upper thermosphere for September
equinox conditions, using a general circulation model coupled with the extended
spectral nonlinear GW parameterization of Yi\u{g}it et al (2008). Simulations
with GW effects cut-off above the turbopause and included in the entire
thermosphere have been conducted. GWs appreciably impact the mean circulation
and cool the thermosphere down by up to 12-18%. GWs significantly affect the
winds modulated by the diurnal migrating tide, in particular in the
low-latitude mesosphere and lower thermosphere and in the high-latitude
thermosphere. These effects depend on the mutual correlation of the diurnal
phases of the GW forcing and tides: GWs can either enhance or reduce the tidal
amplitude. In the low-latitude MLT, the correlation between the direction of
the deposited GW momentum and the tidal phase is positive due to propagation of
a broad spectrum of GW harmonics through the alternating winds. In the Northern
Hemisphere high-latitude thermosphere, GWs act against the tide due to an
anti-correlation of tidal wind and GW momentum, while in the Southern
high-latitudes they weakly enhance the tidal amplitude via a combination of a
partial correlation of phases and GW-induced changes of the circulation. The
variable nature of GW effects on the thermal tide can be captured in GCMs
provided that a GW parameterization (1) considers a broad spectrum of
harmonics, (2) properly describes their propagation, and (3) correctly accounts
for the physics of wave breaking/saturation.
