Numerical Models of the Atmospheric Circulation on Jupiter and Saturn:Zonal Jets, Long-term Variability, and QBO-type Oscillations

Jupiter and Saturn exhibit numerous robust zonal (east-west) jet streams at the cloud level; moreover, both planets exhibit long-term stratospheric oscillations involving perturbations of zonal wind and temperature that propagate downward over time on timescales of ~4 years (Jupiter) and ~15 years (Saturn). These oscillations, dubbed the Quasi Quadrennial Oscillation (QQO) for Jupiter and the Semi-Annual Oscillation (SAO) on Saturn, are thought to be analogous to the Quasi-Biennial Oscillation (QBO) on Earth, which is driven by upward propagation of equatorial waves from the troposphere. Similar dynamics are likely relevant on a wide population of exoplanets and brown dwarfs. To investigate the formation and maintenance of zonal jets and QBO-like oscillations on giant planets, we here present global, three-dimensional, high-resolution numerical simulations of the flow in the stratified atmosphere--overlying the convective interior. The effect of interior convection is parameterized by inducing small-scale, randomly varying perturbations in the radiative-convective boundary at the base of the model. In the simulations, the convective perturbations generate atmospheric waves and turbulence that interact with the rotation to produce numerous zonal jets. A robust eastward equatorial jet—equatorial superrotation--emerges in some models, leading to jet profiles similar to that on Jupiter and Saturn. Moreover, the equatorial stratosphere exhibits stacked eastward and westward jets that migrate downward over time, exactly as occurs in the terrestrial QBO, Jovian QQO, and Saturnian SAO. The periods and amplitudes of the simulated oscillations bracket the range observed on Jupiter and Saturn. We performed diagnostics that characterize the wave population generated by the convective forcing, with a particularly strong role for Kelvin, Rossby, and mixed Rossby-gravity waves, and we show how the interaction of these waves with the mean flow causes the stratospheric oscillations. This is the first demonstration of a QBO-like phenomenon in 3D numerical simulations of a giant planet.