Influence of parameterized small-scale gravity waves on the migrating diurnal tide in Earth's thermosphere

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 Yigit 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 anticorrelation 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. Plain Language Summary Atmospheric waves generated by meteorological processes in the lower atmosphere influence the state and evolution of the atmosphere at higher altitudes. Using a three-dimensional model of the atmosphere, we study the effects of small-scale gravity waves on the large-scale tides. In addition to the study of the interaction of these waves in the mesosphere and lower thermosphere region of the atmosphere, we explore gravity wave effects even at much higher altitudes in the thermosphere (up to about 300 km). Our results show that gravity waves strengthen the tide in the lower thermosphere. In the upper thermosphere, gravity waves can either enhance or damp the tides. The underlying mechanisms are investigated.