On the simulation of the quasi-biennial oscillation in the community atmosphere model, version 5

The quasi-biennial oscillation (QBO) of the tropical zonal mean wind is a prominent feature of the tropical stratosphere. The easterly and westerly wind regimes alternate with a period of about 28 months. The QBO is believed to be forced by a combination of equatorial waves, in particular, Kelvin and mixed Rossby-gravity waves, as well as smaller-scale gravity waves. Although the QBO is well observed and basic forcing mechanism well understood, it has been a challenge to simulate in General Circulation Models (GCMs). In this paper we examine the role of vertical resolution and gravity wave parameterization on the simulation of the QBO in the Community Atmosphere Model, version 5. We show that in this model vertical resolution of 500 m and adequate gravity wave drag are needed to obtain a realistic QBO. At 500 m vertical resolution, CAM5 generates significantly more mixed Rossby-gravity and Kelvin waves as compared to CAM5 with 700 m or 1200 m vertical resolution. These waves then contribute to the forcing of the easterly and westerly phases of the QBO, respectively. In this work, we also briefly explore the effects of horizontal resolution on the QBO and conclude that the QBO can be adequately represented with horizontal resolution of ∼200 km as long as vertical resolution of the model is fine enough.