An interannual variation of the propagating diurnal tide in the mesosphere is simulated using a middle atmosphere general circulation model (GCM) and interpreted using a linear mechanistic tidal model. The GCM results show that the semiannual amplitude variation of the tide, which is characterized by strong tides near equinox and weak tides near solstice, exhibits a strong interannual variation in the presence of a long-period mean wind oscillation in the tropics. Changes in the latitudinal gradients of the mean winds in the subtropical mesosphere are shown to be primarily responsible for this behavior. It is suggested that this shear mechanism may explain the possible link between the quasi-biennial oscillation in the zonal-mean zonal winds in the stratosphere and the amplitude of the diurnal tide in the lower thermosphere that is seen in observations.