The role of gravity waves in the quasi‐biennial oscillation

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Abstract
The role of gravity wave momentum transport in the quasi‐biennial oscillation (QBO) is investigated using a two‐dimensional numerical model. In order to obtain an oscillation with realistic vertical structure and period, vertical momentum transport in addition to that of large‐scale, long‐period Kelvin and Rossby‐gravity waves is necessary. The total wave flux required for the QBO is sensitive to the rate of upwelling, due to the Brewer‐Dobson circulation, which can be estimated from the observed ascent of water vapor anomalies in the tropical lower stratosphere. Although mesoscale gravity waves contributeto mean flow acceleration, it is unlikely that the momentum flux in these waves is adequate forthe QBO, especially if their spectrum is shifted toward westerly phase speeds. Short‐period Kelvin and inertia‐gravity waves at planetary and intermediate scales also transport momentum. Numerical results suggest that the flux in all vertically propagating waves (planetary‐scale equatorial modes, intermediate inertia‐gravity waves, and mesoscale gravity waves), in combination, is sufficient to obtain a QBO with realistic Brewer‐Dobson upwelling if the total wave flux is 2–4 times 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 important in the upper and lowermost levels of the QBO and subtropics.

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