In recent years, GCMs have had some success in generating quasi-biennial oscillations (QBOs), usually with parameterized gravity wave drag. These simulated QBOs are, in general, model-dependent; their characteristics depend on the particular parameterization scheme used, as well as on the amount of gravity wave drag relative to the drag from resolved equatorial planetary waves. An understanding of how the QBO is controlled by the different types of waves is important for a realistic representation of the QBO by GCMs.

In this study, we re-examine our basic understanding of the QBO using a simple one-dimensional equatorial model and a two-dimensional balance model that includes a seasonal cycle and the upwelling of the tropical Hadley circulation. The forcing due to gravity waves and equatorial planetary waves is represented using several different parameterization schemes. Attention is focused on the contrast between the schemes that are based on the theory of wave breaking and saturation and those that depend on thermal damping of the waves, by examining such issues as the mechanism of shear layer descent and the role of diffusion in the switching mechanism at the lowest levels. Constraints on the various parameters and on the gravity wave source spectrum are discussed. The ultimate goal of the project is to provide constraints that may be applied in GCMs.