THE ROLE OF GRAVITY-WAVES IN THE QUASI-BIENNIAL OSCILLATION

The role of gravity wave momentum transport in the quasi-biennial osci llation (QBO) is investigated using a two-dimensional numerical model. In order to obtain an oscillation with realistic vertical structure a nd period, vertical momentum transport in addition to that of large-sc ale, long-period Kelvin and Rossby-gravity waves is necessary. The tot al wave flux required for the QBO is sensitive to the rate of upwellin g, due to the Brewer-Dobson circulation, which can be estimated from t he observed ascent of water vapor anomalies in the tropical lower stra tosphere. Although mesoscale gravity waves contribute to mean flow acc eleration, it is unlikely that the momentum flux in these waves is ade quate for the QBO, especially if their spectrum is shifted toward west erly phase speeds. Short-period Kelvin and inertia-gravity waves at pl anetary and intermediate scales also transport momentum. Numerical res ults suggest that the flux in all vertically propagating waves (planet ary-scale equatorial modes, intermediate inertia-gravity waves, and me soscale gravity waves), in combination, is sufficient to obtain a QBO with realistic Brewer-Dobson upwelling if the total wave flux is 2-4 t imes as large as that of the observed large-scale, long-period Kelvin and Rossby-gravity waves. Lateral propagation of Rossby waves from the winter hemisphere is unnecessary in this case, although it may be imp ortant in the upper and lowermost levels of the QBO and subtropics.