SEASONAL-VARIATIONS OF THE DIURNAL TIDE INDUCED BY GRAVITY-WAVE FILTERING

Wind measurements with the UARS spacecraft show that between 90 and 10 0 km altitude the fundamental diurnal tide is modulated semi-annually with peak amplitudes of almost 100 m/s near equinox, more than a facto r of two larger than during solstice. Applying Hines' Doppler-Spread P arameterization (DSP) for small scale gravity waves (GW), we present r esults 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 th e Rayleigh friction parameterization that causes damping. Since the ti de absorbs GW momentum, it reduces the wave-generated semi-annual osci llations (SAO) in the mean zonal winds at equatorial latitudes. Outsid e the tropics, GW momentum is absorbed by the mean zonal winds that pe ak near the summer and winter solstices. As a result, the GW reach the upper mesosphere with more momentum during equinox periods and thus c ause a seasonal modulation of the tide that is predominately semi-annu al.