Marco A. Giorgetta and Elisa Manzini

Max Planck Institute for Meteorology, Hamburg, Germany

Abstract:

The quasi-biennial oscillation (QBO) in zonal wind in the equatorial stratosphere results from wave mean-flow interaction. This mechanism transforms high frequency variability of tropical vertically propagating waves excited by troposheric tropical meteorology to a quasi-biennial oscillation of the zonal wind in the stratosphere. The QBO exerts filtering on the wave spectrum propagating to higher layers and therefore may contribute to the interannual variability of wave mean-flow interaction in higher regions. The QBO is a robust phenomenon, in the sense that no QBO cycle broke down during the available observation record. Nevertheless modelling the QBO in GCMs is still a challenge and there is only a small number of GCMs that have shown an equatorial oscillation, some of them having rather quasi- annual than quasi-biennial periods. Here we show that the QBO is successfully simulated in a general circulation model of the newest generation. Key factors are a sufficient spatial resolution, a realistic simulation of tropical convection, and the consideration of the effects of gravity waves. From this simulation it is inferred that a broad spectrum of atmospheric waves is necessary to generate the QBO in the model. Sensitivity tests address the role of the parameterized gravity wave drag and the sensitivity of the simulated QBO to modifications of the parameterization for deep convection.