On the momentum budget of the quasi-biennial oscillation in the Whole Atmosphere Community Climate Model

The model has high vertical resolution, 500 m, above the boundary layer and through the lower and middle stratosphere, decreasing gradually to about 1.5 km near the stratopause. Parameterized mesoscale gravity waves and resolved equatorial waves contribute comparable easterly and westerly accelerations near the equator.

Westerly acceleration by resolved waves is due mainly to Kelvin waves of zonal wavenumber in the range k = 1-15 and is broadly distributed about the equator. Easterly acceleration near the equator is due mainly to Rossby-gravity (RG) waves with zonal wavenumbers in the range k = 4-12. These RG waves appear to be generated in situ during both the easterly and westerly phases of the QBO, wherever the meridional curvature of the equatorial westerly jet is large enough to produce reversals of the zonal-mean barotropic vorticity gradient, suggesting that they are excited by the instability of the jet.