Geostrophic Regimes, Intermediate Solitary Vortices and Jovian Eddies

Abstract

We examine the relevance to Jupiter's atmosphere of the solitary vortices favored at scales intermediate to those of the quasi-geostrophic (QG) and planetary-geostrophic motions. Horizontal divergence plays a crucial role in the intermediate-geostrophic (IG) dynamics and leads to asymmetries in vortex behavior; in partcular, anticyclonic vortices are generally more stable than cyclonic vortices when the mean flow is weak or westerly. The IG vortices always propagate westward at close to the planetary long-wave speed, regardless of the mean zonal flow. Meridional shear influences only secondary aspects of vortex behavior. Although governed by a form of the Korteweg-deVries (KdV) equation, vortex encounters produce coalescence not soliton behavior. Jupiter's Great Red Spot and Large Ovals appear to be in, or close to, an IG balance while the Small Ovals lie in a QG balance. The stability of anticyclonic IG vortices may explain why most of Jupiter's super-eddies prefer anticyclonic spin. Solutions to the shallow water (SW) equations, using Jovian parameters, show that an IG vortex with the scale and environment of the Great Red Spot has great longevity and that such a vortex may originate in a weak barotropic instability of the zonal currents. Strong barotropic instability on the IG scale differs from its counterpart on the QG scale and produces multiple, steep, isolated vortices resembling the Large Ovals. Equations are derived for all forms of geostrophic balance (three basic classes, ten subsets) to investigate the uniqueness of the IG system. Numerical studies use the IG ?-plane equation to examine basic modal properties and the full SW equations to examine the Jovian eddies.