Vorticity-gradient forces and a center-of-mass approach explain the mean and oscillatory motion of Jupiter's polar cyclones.

Abstract

The polar cyclones on Jupiter have been observed regularly since their discovery by the Juno mission in 2016. While the symmetrically spaced 9 and 6 cyclones at Jupiter's north and south pole (respectively) have largely maintained their locations, 5 years of Juno's observations showed oscillatory perturbations in their positions. In addition, an overall westward drift was measured for the cyclones at both poles. In this study, a mechanism for these motions is presented. This mechanism is driven by the known "beta-drift" effect, a poleward-westward acceleration experienced by cyclones under beta (the meridional gradient in planetary vertical vorticity). When considering the relative vorticity of other cyclones, in addition to beta, to evaluate beta-drift on each cyclone, the polar group of cyclones forms a dynamical system analogous to a system of springs. Using the Juno observations, we show that such a representation agrees well with the data describing the location and acceleration of the cyclones with time. In addition, a toy model, driven by such prescribed beta-drift forces, is able to reproduce motions similar to the observations. To explain the mean westward motion exhibited by the circumpolar cyclones in the north and south poles (4° and 7. 5° degrees longitude per year, respectively), we propose a center-of-mass approach. Using simulations, we show that the motion of cyclones in a group can be primarily divided into a contribution from beta and a contribution from the interactions between cyclones. When considering the group as a whole, their center of mass is only subject to beta, manifesting in a polar orbit of the group, which precesses westward. This precession is proposed as the mechanism for the westward drift of the individual cyclones. We conclude by showing observational evidence for this interpretation.