Modulation of the mesospheric semiannual oscillation by the quasibiennial oscillation

Recent satellite and radar observations suggest that the semiannual oscilla tion (SAO) in the mesosphere is modulated by the stratospheric quasibiennia l oscillation (QBO). The modulation is only apparent during the SAO easterl y phase, which is considerably stronger when QBO winds are westerly than wh en they are easterly. We use an equatorial beta-plane model to demonstrate how such a modulation could come about through selective damping of the equ atorial wave spectrum excited by deep convection. The waves affected most s trongly are easterly inertia-gravity waves of phase speeds slower than simi lar to -40 m s(-1). This is close to the zonal wind speed during the easter ly phase of the QBO (-30 to -35 m s(-1)), so the waves suffer strong therma l damping or even absorption as they propagate through the stratosphere. Be cause these waves are important for driving the easterly phase of the mesop ause SAO in the model, that phase is weaker when the stratospheric QBO wind s are easterly. A similar modulation of the westerly phase of the SAO does not occur for two reasons: (I) QBO westerlies are only half as strong as QB O easterlies, and (2) much of the driving of the westerly phase of the SAO is accomplished by Kelvin waves of phase speed similar to 40-60 m s(-1). As a consequence, the QBO winds have negligible influence on the vertical pro pagation of waves with westerly phase velocity and hence on the westerly ph ase of the modeled SAG.