A Moist Convective Mechanism for Polar Cyclone Formation on the Giant Planets

The giant planets each have distinct polar behavior. Saturn has the most striking features observed to date, with a deep, hot and rapid cyclone situated directly over each pole, and a rapid jet marking the cyclone boundary at 3 degrees from the pole. We propose and test a moist convective hypothesis for polar cyclone formation. Using purely baroclinic forcing in a 2.5 layer shallow water model, motivated by moist convection observed on Jupiter and Saturn, a robust tendency to form an equivalent barotropic polar cyclone is identified. Simulations span several orders of magnitude of energy density, ranging from weak jet-like flows to strong cyclones that experience instabilities. We find that a range of behavior, including what is observed on the giant planets as well as in previous simulation studies, can be expressed by varying 2 nondimensional control parameters. They are a second baroclinic deformation radius scaled by the planetary radius, and a total energy parameter that scales with the kinetic+potential energy density of the system at statistical equilibrium. In the context of a shallow, idealized model, the difference between Jupiter's and Saturn's polar flow regimes may be explained by their different planetary and deformation radii.