Sensitivity of mesospheric mean flow, planetary waves, and tides to strength of gravity wave drag

A global circulation model which extends from the surface to 125 km is used to study how the strength of gravity wave drag affects the dynamics of the mesosphere. The strength of gravity wave drag has a strong influence on th e zonal mean state of the mesosphere, in particular the magnitude and varia bility of the summer mesopause temperature and the shear on the top of the mesospheric jets. This change in the zonal mean state strongly affects the susceptibility of the mesosphere to baroclinic and/or barotropic instabilit y and hence the formation of the 2-day wave. The 2-day wave, in turn, inter acts nonlinearly with the diurnal tide, producing secondary waves and a red uction in amplitude of the diurnal tide. Previous studies with quasi-linear mechanistic tidal models have captured some semiannual variation in tidal amplitude through direct interactions between tides and gravity waves. Our fully nonlinear global circulation model results support an alternative exp lanation in terms of interactions between planetary waves and tides.