A Diagnostic Numerical Model of the Quasi-Biennial Oscillation

Abstract
The long-period variations in zonally symmetric circulations of the tropical stratosphere are investigated. The zonally averaged momentum, continuity, and heat energy equations, subject to the assumptions of hydrostatic balance and a geostrophically balanced zonal wind, are used to formulate a numerical model which is integrated in time to study the evolution of the flow. Terms representing diabatic heating (thermal forcing) and divergence of eddy momentum flux (dynamical forcing) appear as input parameters whose distributions in space and time are specified in various ways to obtain diagnostic information on the dynamics of the observed quasi-biennial wind oscillation. Experiments with thermal forcing through time varying diabatic heating indicate that an enormous variation in the radiative heating is required to produce the observed amplitude of the zonal wind oscillation, and that a thermally driven model is incapable of reproducing the observed characteristics of the downward propagating wind regimes. Dynamical forcing with reasonable amplitudes produced zonal wind changes of comparable magnitude to those observed in the atmosphere. However, experiments with a time varying momentum source in the region above 25 km were unable to simulate the downward propagation of the atmospheric wind regimes. It is concluded that the time variations of momentum fluxes which force the zonal wind oscillation exhibit a phase dependence on height.