MODE SELECTION, WAVE BREAKING AND PARAMETRIC SENSITIVITY IN THE QUASI-BIENNIAL OSCILLATION

An analytical model is developed for the propagation of equatorially t rapped waves on an equatorial B-plane in a uniform zonal flow in the p resence of Rayleigh friction and Newtonian cooling. By comparing solut ions obtained from this model under conditions appropriate to the trop osphere and stratosphere, it is shown that wave modes which are most L ikely to contribute to the excitation of zonal flow oscillations occur in a 'spectral window', for which modes propagate freely in the verti cal in the troposphere but are strongly damped in the stratosphere. Th is spectral window leads to a mode-selection criterion which accounts for the dominant modes observed in association with the main stratosph eric quasi-biennial oscillation (QBO), and other aspects of equatorial -wave propagation in the tropical stratosphere and mesosphere. Solutio ns from this model are also used to verify a new set of numerical mode ls of equatorial-wave propagation (using the 'transformed Eulerian-mea n' formulation) in the presence of zonal flows with general lateral an d vertical shear and arbitrarily specified stratification. The latter numerical models are shown to simulate a realistic stratospheric QBO s uccessfully, under conditions similar to those observed. The models ar e then used to investigate the probable role of large-scale wave-break ing processes in modifying the form of the QBO, via a convective param etrization scheme, which is shown to act in certain respects like Rayl eigh friction. Finally, a sensitivity study is carried out to investig ate 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.