  Brown dwarfs and directly imaged giant planets exhibit significant evidence
for active atmospheric circulation, which induces a large-scale patchiness in
the cloud structure that evolves significantly over time, as evidenced by
infrared light curves and Doppler maps. These observations raise critical
questions about the fundamental nature of the circulation, its time
variability, and the overall relationship to the circulation on Jupiter and
Saturn. Jupiter and Saturn themselves 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. To investigate these issues, we here present global,
three-dimensional, high-resolution numerical simulations of the flow in the
stratified atmosphere--overlying the convective interior--of brown dwarfs and
Jupiter-like planets. 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. 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. This is the first demonstration of a QBO-like
phenomenon in 3D numerical simulations of a giant planet.
