Stable, baroclinic eddies on Jupiter and Saturn: A laboratory analog and some observational tests

Abstract

Laboratory experiments (and corresponding numerical simulations) on free thermal convection in a rotating fluid, subject to horizontal differential heating and cooling, are examined in the light of recent observations of the longest-lived eddies on Jupiter and Saturn (including Jupiter's Great Red Spot and White Ovals). Both laboratory and atmospheric systems are shown to be capable of satisfying scaling requirements for mutual dynamical similarity, within the uncertainties of the Jovian observations. By employing a suitable distribution of heat sources and sinks in the laboratory, a pattern of zonally averaged flow analogous to a laterally sheared belt or zone on Jupiter can be obtained at upper levels. Baroclinic eddies may develop in such a flow, whose properties are remarkably similar in structure and appearance to the long-lived features on the major planets. Difficulties in determining from global energy budget studies the essential processes maintaining and dissipating stable eddies are discussed with reference to the laboratory and atmospheric systems. Such difficulties do not arise when considering the potential vorticity budget for the flow. Inviscid, unforced “free mode” (i.e., nonadvecting) solution to a suitable potential vorticity equation are first-order components of most recent models of the Jovian eddies (including the baroclinic eddy analog described herein). Small departures from such a “free mode” in a realistic flow arise as a result of the dynamically crucial processes maintaining and dissipating it, with baroclinic eddies in the laboratory corresponding largely to a residual balance between thermal forcing (via internal heating) and viscous dissipation. On Jupiter, diabatically forced and transient eddy-driven flows are shown to differ primarily in the implied role of transient eddies in transporting potential vorticity q across closed geostrophic streamlines in the time mean. The feasibility of using observations of naturally occurring chemical tracers to infer aspects of the transport of q on Jupiter are discussed with a view to testing models of the long-lived eddies using data from the Galileo mission.