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Introduction

Until recently middle atmosphere general circulation models (GCMs) have been unable to simulate long-period zonal mean wind oscillations in the tropics like the quasi-biennial oscillation (QBO). The inability of first principles models to generate the QBO can be attributed in part to Brewer-Dobson upwelling in the equatorial lower stratosphere which acts to suppress the oscillation. This was demonstrated by Dunkerton [1997] who showed that in the presence of mean upwelling the total wave momentum flux which is needed to maintain the QBO is several times larger than that estimated from observed planetary-scale waves and that the additional flux most likely comes from small-scale gravity waves which are not resolved by most GCMs.

In this presentation the Doppler spread gravity wave parameterization (DSP) of Hines [1997] is used to generate equatorial oscillations. The discussion consists of two parts. In Part 1 a mechanistic model is used to demonstrate the ideas of Dunkerton [1997]. Here the DSP is used in the context of a one-dimensional model like that used by Holton and Lindzen [1972]. In Part 2 results from the Canadian Middle Atmosphere Model (CMAM) are presented. Here the relative roles of parameterized and resolved wave drag, the tendency for the period of the oscillation to lock itself to the seasonal cycle, as well as the impact of the finite-difference formulation of the parameterization scheme, are discussed.