The role of gravity waves in maintaining the QBO and SAO at equatorial latitudes

Abstract

We present a numerical spectral model (NSM) that describes the equatorial oscillations in the zonal circulation, the Quasi-biennial Oscillations (QBO) and Semi-annual Oscillations (SAO). The oscillations are generated by the momentum deposition of small scale gravity waves (GW) described with the Doppler spread parameterization (DSP) of Hines. We discuss here the GW mechanism and some of the conditions that are favorable for generating the QBO and SAO. Our model reproduces the salient features observed and leads to the following conclusions: (1) In 2D, a QBO can be generated with a period of about 30 months, peak amplitude close to 20 m/s near 30 km, and downward propagation velocity of 1.2 km/months, close to the observed values. The QBO phase reverses near 70 km and extends with significant amplitude into the upper mesosphere. (2) SAO amplitudes between 5 and 30 m/s are generated that peak at about 55 km with eastward phase during equinox. A second peak with opposite phase is produced above 70 km. (3) The computed oscillations depend on the rate of radiative cooling and on the chosen eddy viscosity that is tied in the DSP to the GW source but is uncertain to some degree. (4) GW filtering by the QBO at lower altitudes can account for the large variability in the SAO that is observed. The SAO in turn also affects the QBO, and under certain conditions the seasonal (semiannual) variations in the solar forcing can act as a pacemaker to seed and synchronize the QBO. (5) In a 3D version of the model that computes also tides and planetary waves, the amplitudes of the SAO, and even the QBO, are significantly reduced compared to 2D. The tides and planetary waves apparently absorb some of the GW momentum that otherwise in 2D goes into the equatorial oscillations of the zonal circulation. (7) The QBO is also generated in 3D with Kelvin and Rossby gravity waves that deposit momentum through nonlinear advection and radiative cooling. However, the wave amplitudes need to be carefully tuned in order to generate such an oscillation, in contrast to the GW mechanism which is robust.