Wind measurements with the UARS spacecraft show that between 90 and 100 km altitude the fundamental diurnal tide is modulated semi-annually with peak amplitudes of almost 100 m/s near equinox, more than a factor of two larger than during solstice. Applying Hines' Doppler-Spread Parameterization (DSP) for small scale gravity waves (GW), we present results from a Numerical Spectral Model (NSM) to describe the seasonal variations in the mean zonal circulation interacting self consistently with the diurnal tide. With the DSP, the GW momentum source increases the amplitude of the tide at altitudes above 70 km, in contrast to the Rayleigh friction parameterization that causes damping. Since the tide absorbs GW momentum, it reduces the wave-generated semi-annual oscillations (SAO) in the mean zonal winds at equatorial latitudes. Outside the tropics, GW momentum is absorbed by the mean zonal winds that peak near the summer and winter solstices. As a result, the GW reach the upper mesosphere with more momentum during equinox periods and thus cause a seasonal modulation of the tide that is predominately semi-annual. ¿ 1998 American Geophysical Union