Modulation of the mesospheric semiannual oscillation by the quasibiennial oscillation

Recent satellite and radar observations suggest that the semiannual oscillation (SAO) in the mesosphere is modulated by the stratospheric quasibiennial oscillation (QBO). The modulation is only apparent during the SAO easterly phase, which is considerably stronger when QBO winds are westerly than when they are easterly. We use an equatorial beta-plane model to demonstrate how such a modulation could come about through selective damping of the equatorial wave spectrum excited by deep convection. The waves affected most strongly are easterly inertia-gravity waves of phase speeds slower than ~ -40 m s^-1. This is close to the zonal wind speed during the easterly phase of the QBO (-30 to -35 m s^-1), so the waves suffer strong thermal damping or even absorption as they propagate through the stratosphere. Because these waves are important for driving the easterly phase of the mesopause SAO in the model, that phase is weaker when the stratospheric QBO winds are easterly. A similar modulation of the westerly phase of the SAO does not occur for two reasons: (1) QBO westerlies are only half as strong as QBO easterlies, and (2) much of the driving of the westerly phase of the SAO is accomplished by Kelvin waves of phase speed ~40-60 m s^-1. As a consequence, the QBO winds have negligible influence on the vertical propagation of waves with westerly phase velocity and hence on the westerly phase of the modeled SAO.