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 the zonal mean state of the mesosphere, in particular the magnitude and variability 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 instability and hence the formation of the 2-day wave. The 2-day wave, in turn, interacts nonlinearly with the diurnal tide, producing secondary waves and a reduction 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 explanation in terms of interactions between planetary waves and tides. ¿ 1999 American Geophysical Union