Insights from Idealized Modelling into the Quasi-Biennial Oscillation

Most climate models that simulate the Quasi-Biennial Oscillation (QBO) substantially underestimate tropical zonal wind amplitudes in the lower stratosphere, which limits their ability to represent global QBO teleconnections. To investigate the causes of this long-standing bias and to identify the key requirements for simulating a realistic QBO, we use an idealized atmospheric model with simplified physics based on the GFDL dry spectral dynamical core. The model incorporates empirically derived latent heating from observed tropical precipitation to represent the effects of tropical convection on the generation of resolved waves that drive the QBO.

We perform an extensive set of sensitivity experiments that systematically vary tropical heating, parameterized gravity wave drag, gravity wave drag strength and formulation, vertical resolution, and horizontal resolution. The results demonstrate that high vertical resolution (L80) is the most critical factor for reproducing realistic QBO amplitudes in the lower stratosphere. Parameterized gravity wave drag is also essential, as tropical heating alone is insufficient to sustain a robust QBO. In contrast, increasing horizontal resolution beyond moderate values provides little benefit, with simulations at T42 resolution already producing a reasonable QBO.