The role of equatorial waves in the semiannual oscillation of the middle atmosphere

A review of the observed features of the semiannual oscillation (SAO) of the middle atmosphere is presented, together with numerical simulations that reproduce much of the observed behavior. It is argued that latent heat release by deep convection excites equatorial Kelvin and inertia-gravity waves that play a major role in driving the SAO. The waves in question have scales ranging from planetary (k = 1 — 3) to intermediate (k ≃ 4 — 25), and periods less than 5 days. In the calculations presented here, planetary-scale Kelvin waves provide over half of the driving for the westerly phase of the stratospheric SAO, while the remainder is supplied by intermediate scale waves; in the mesosphere, the westerly phase is driven by a combination of planetary and intermediate-scale Kelvin waves, while inertia-gravity waves of diurnal period contribute most of the forcing for the easterly phase. The inertia-gravity waves responsible for forcing the mesospheric easterly phase are particularly affected by dissipation during the easterly phase of the quasibiennial oscillation (QBO); this may account for the apparent modulation of the mesospheric SAO by the QBO seen in satellite observations.