Mode selection, wave breaking and parametric sensitivity in the quasi-biennial oscillation

An analytical model is developed for the propagation of equatorially trapped waves on an equatorial B-plane in a uniform zonal flow in the presence of Rayleigh friction and Newtonian cooling. By comparing solutions obtained from this model under conditions appropriate to the troposphere and stratosphere, it is shown that wave modes which are most Likely to contribute to the excitation of zonal flow oscillations occur in a 'spectral window', for which modes propagate freely in the vertical in the troposphere but are strongly damped in the stratosphere. This spectral window leads to a mode-selection criterion which accounts for the dominant modes observed in association with the main stratospheric quasi-biennial oscillation (QBO), and other aspects of equatorial-wave propagation in the tropical stratosphere and mesosphere. Solutions from this model are also used to verify a new set of numerical models of equatorial-wave propagation (using the 'transformed Eulerian-mean' formulation) in the presence of zonal flows with general lateral and vertical shear and arbitrarily specified stratification. The latter numerical models are shown to simulate a realistic stratospheric QBO successfully, under conditions similar to those observed. The models are then used to investigate the probable role of large-scale wave-breaking processes in modifying the form of the QBO, via a convective parametrization scheme, which is shown to act in certain respects like Rayleigh friction. Finally, a sensitivity study is carried out to investigate the effect of varying parameters - such as the vertical diffusion coefficient, buoyancy frequency and forcing by tropospheric waves - on the form, strength and period of the QBO.