CALCULATIONS OF THE STRATOSPHERIC QUASI-BIENNIAL OSCILLATION FOR TIME-VARYING WAVE FORCING

One-dimensional calculations are carried out for the time evolution of the equatorial lower stratospheric mean zonal wind forced by time-var ying equatorial Kelvin and mixed Rossby-gravity waves. If the time var iation of the wave momentum forcing is given by a steady forcing plus a sinusoidal modulation, a tendency toward phase locking between the p eriod of the wave forcing's modulation and the period of the resulting mean wind oscillation is found in some cases, depending on the period and magnitude of the wave forcing as well as the phase difference bet ween variations of the easterly and westerly momentum fluxes. Regime d iagrams are shown to make these dependences clearer. If the wave forci ngs are irregularly modulated, the resulting time variation of the win d oscillation shows no resemblance to the imposed time variation of th e wave forcing. These simple calculations are used to indicate that fo r nonlinear phenomena, such as the quasi-biennial oscillation (QBO), o ne cannot conclude that a lack of correlation between two data records means that these are physically unrelated. When the equatorial wave m omentum fluxes are modulated according to the eastern Pacific sea surf ace temperatures, the simulated time variation of the QBO period somet imes (depending on the phase relation between the easterly and westerl y time-varying fluxes) shows a great resemblance to the observations. This suggests that easterly and westerly momentum fluxes into the equa torial lower stratosphere are related to SST variations.