The Dynamics of Jovian Polar Cyclones

Polar vortices are observed in the atmospheres of most solar-system planets, arising as a single cyclone centred on or close to the pole. In contrast, Jupiter’s polar vortices have an unprecedented structure. As revealed by NASA’s Juno spacecraft, they consist of geometric patterns of cyclonic vortices surrounding a central cyclonic vortex at the pole. These crystalline structures were not predicted prior to being observed, and the mechanisms explaining their formation and evolution remain poorly understood. One possible mechanism is that moist convection produces small vortices in the polar regions, with the cyclones then migrating polewards via the ‘beta-drift’ mechanism and merging. Nevertheless, models including these processes do not spontaneously produce polygonal patterns like those on Jupiter. In contrast, this study investigates the stability of an initialized pattern of fully formed polar vortices subjected to these small-scale short-lived processes. This forced-dissipative system is modelled using the shallow-water equations describing a single layer of fluid on a polar gamma-plane. The initialized cyclones are subjected to a stochastic forcing with a short decorrelation time and the factors affecting their stability and time-evolution are studied. These include their degree of shielding (an anticyclonic ring around each cyclone), their depth and the properties of the forcing, in addition to the role of potential vorticity mixing.