Atmospheric circulation on rapidly rotating giant planets: from brown dwarfs to Jupiter and Saturn
Prof. Adam P. Showman
University of Arizona and Peking University
Atmospheric circulation on rapidly rotating giant planets: frombrown dwarfs to Jupiter and SaturnBrown dwarfs are objects of several to ~80 Jupiter masses that formlike stars but have insufficient mass to undergo thermonuclearreactions. They therefore cool off gradually with time, and resemblemassive, rapidly rotating versions of Jupiter and Saturn, but withmuch higher temperatures and internal heat fluxes that are orders ofmagnitude greater. Brown dwarfs exhibit significant evidence foractive atmospheric circulation, which induces a large-scale patchinessin the cloud structure that evolves significantly over time, asevidenced by infrared light curves and observed surface maps. Theseobservations raise critical questions about the fundamental nature ofthe circulation, its time variability, and the overall relationship tothe circulation on Jupiter and Saturn. Jupiter and Saturn themselvesexhibit numerous robust zonal (east-west) jet streams at the cloudlevel; moreover, both planets exhibit long-term stratosphericoscillations involving perturbations of zonal wind and temperaturethat propagate downward over time on timescales of ~4 years (Jupiter)and ~15 years (Saturn). These oscillations, dubbed the QuasiQuadrennial Oscillation (QQO) for Jupiter and the Semi-AnnualOscillation (SAO) on Saturn, are thought to be analogous to theQuasi-Biennial Oscillation (QBO) on Earth, which is driven by upwardpropagation of equatorial waves from the troposphere. To investigatethe atmospheric circulation on this broad class of objects, I herepresent global, three-dimensional, high-resolution numericalsimulations of the flow in the stratified atmosphere---overlying theconvective interior---of brown dwarfs and Jupiter-like planets. Inthe simulations, convective perturbations at the base of thestratified atmosphere generate atmospheric waves and turbulence thatinteract with the rotation to produce numerous zonal jets andeddies. Moreover, the equatorial stratosphere exhibits stackedeastward and westward jets that migrate downward over time, exactly asoccurs in the terrestrial QBO, Jovian QQO, and Saturnian SAO. This isthe first demonstration of a QBO-like phenomenon in 3D numericalsimulations of a giant planet. I will discuss the dynamics of thesephenomena, the relationship to dynamical mechanisms familiar fromEarth, and implications for explaining the observations. If time isavailable, I will also present some models aimed at understanding theinteraction of cloud feedbacks and atmospheric dynamics, which islikely to play a critical role in shaping the large-scale atmosphericcirculation and global-scale cloud patchiness of giant planetsgenerally.