On the Existence and Non-Existence of QBO-like Oscillations in Dynamical Cores of General Circulation Models

The quasi-biennial oscillation (QBO) is one of the most dominant phenomenon in the equatorial middle atmosphere. Model simulations of the QBO has proved difficult despite sufficiently high vertical grid resolutions. Most often, gravity wave drag parameterizations, convection schemes and diffusion coefficients are tuned to obtain quasi-realistic QBO oscillations. Therefore, it is an interesting observation that QBO-like oscillations can exist in idealized dynamical core experiments. These model simulations only utilize simple Rayleigh friction and a Newtonian temperature relaxation that replace the complex physical parameterization package.

The paper discusses QBO-like oscillations have been simulated with NCAR's spectral semi-Lagrangian dynamical core that is part of the Community Atmosphere Model CAM3.1. The dynamical core supports QBO-like oscillation with a period of about 4.5 years. Despite this long period the overall characteristics of the QBO-like are simulated quite realistically. The paper discusses an in-depth analysis of the driving wave mechanisms and addresses the QBO's sensitivity to parameters in the dynamical core. Special attention is paid to wave-mean flow interactions.