A Multiwave Model of the Quasi-biennial Oscillation

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Abstract
A simple one-dimensional model of the quasi-biennial oscillation is discussed. Our model is essentially a generalization of the Holton–Lindzen models. We consider a large number of vertically propagating internal waves interacting simultaneously with the mean flow. Effects of both wave damping and critical level absorption are included, but wave–wave interaction is neglected. The effects of momentum advection due to the Hadley circulation are also parameterized. This model is used to study how the mean flow in the equatorial lower stratosphere would respond to forcing by a tropospheric wave spectrum with a significant amount of momentum flux at slow horizontal phase speeds. We find that a “continuous” wave spectrum forces mean flow oscillations in a manner quite similar to a “discrete” two-wave spectrum. But the factors that control the period and amplitude of the oscillations, in the case of a continuous spectrum, seem to be quite different. Our results also suggest that mean rising motion in the tropics may play an important role in determining the vertical structure of the QBO near the tropopause.

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