The Oscillatory Motion of Jupiter's Polar Cyclones Results From Vorticity Dynamics

Abstract

The poles of Jupiter were observed in detail for the first time by the Juno spacecraft in 2016 (Bolton et al., 2017). In contrast with the polar regions of Saturn, which are inhabited by a single polar cyclone (PC) (Baines et al., 2009;Sánchez-Lavega et al., 2006), Jupiter's PCs are surrounded by a ring of stable circumpolar cyclones (CPCs) (Adriani et al., 2018;Orton et al., 2017). There are eight CPCs at the north pole and five at the south (Figure 1), each with a diameter of roughly 5,000 km and velocities reaching 100 ms −1 (Adriani et al., 2020;Grassi et al., 2018). Such cyclones can be generated by moist convection (O'Neill et al., 2015(O'Neill et al., , 2016, where 2D inverse energy cascade in the turbulent polar regions brings the kinetic energy from the convective scale up to the horizontal scale of the cyclones (Moriconi et al., 2020;. These regions are bounded by prograde jets at around latitudes 65°N∖S (Rogers et al., 2017(Rogers et al., , 2022, which may act as a separating barrier. In contrast with the Great Red Spot, which is centered around latitude 20°S and has a shallow depth (less than 500 km, Parisi et al. ( 2021)) relative to the deep surrounding jets (∼3,000 km, Kaspi et al. ( 2018)), the polar cyclones, subject to the Taylor-Proudman theorem (Vallis, 2017) with a vertical axis parallel to the planetary rotation axis and a smaller Rossby number, potentially extend deeper, suggesting a 2D dynamical regime.The beta-drift is a force that results from a dipole of vorticity (usually termed "beta-gyres," Sutyrin and Flierl (1994)) that is induced by an interaction between the tangential velocity of a cyclone and a gradient of potential vorticity (PV) that is present in the background of the cyclone (Chan, 2005;Gavriel & Kaspi, 2021;Rossby, 1948). This force creates a poleward drift on cyclones and an equatorward drift on anticyclones when only the planetary vorticity gradient (β) is considered (Chan, 2005;Franklin et al., 1996;Merlis & Held, 2019). Beta-drift is a known contributor to the poleward motion of Earth's tropical cyclones (Zhao et al., 2009), and was shown in shallow-water (SW) models to result in cyclones merging into a PC in settings characterizing gas-giants such as Jupiter and Saturn (

Vortex dynamics in rotating Rayleigh–Bénard convection

“…Another crucial issue awaiting theoretical descriptions is the hydrodynamic interactions among closely located convective vortices that give rise to the long-range collective motion of the vortices. What are the implications of the vortex dynamics presented in this study for the correlated vortex motion observed in giant gaseous planets and other large-scale geophysical and astrophysical flows (Li et al 2020;Gavriel & Kaspi 2021, 2022Mura et al 2021)? We expect future progress in answering these questions.…”

“…What are the implications of the vortex dynamics presented in this study for the correlated vortex motion observed in giant gaseous planets and other large-scale geophysical and astrophysical flows (Li et al. 2020; Gavriel & Kaspi 2021, 2022; Mura et al. 2021)?…”

The Westward Drift of Jupiter's Polar Cyclones Explained by a Center‐of‐Mass Approach

“…Since the discovery of Jupiter's polar cyclones, the role of vorticity gradient forces (or generalized β ‐drift), which takes into account both β and the relative vorticities of all neighboring cyclones, presented a consistent picture. These forces were used to explain the mean latitude of the cyclones and their number (Gavriel & Kaspi, 2021), the oscillatory motion patterns of the cyclones (Gavriel & Kaspi, 2022), and now their mean westward drifts as well. This series of studies, revealing different aspects of these forces, implies that these polar cyclones behave, to leading order, like discrete objects, linearly forced by a “spring‐like” system driving them all in the poleward‐westward direction while pushing them one from another.…”

“…(2022); see Figure S3 in Supporting Information for the equivalent north‐pole figure, based on the existing observations). It can be seen (Figure 4a) that the CM's trajectory is indeed very clean, and possesses mostly the 12‐month oscillations exhibited by the individual cyclones (Figures 4d and 4e, Gavriel and Kaspi 2022). This suggests that this 12‐month mode of motion is mostly synchronized between the cyclones, and is therefore largely due to the motion of the CM (i.e., the first mode of motion where all cyclones oscillate together as a group).…”

“…One motion trend is a mostly circular motion with a period of ∼12 months and a radius of ∼400 km (Gavriel & Kaspi, 2022). This motion was shown to follow a strong correlation between the instantaneous acceleration of each cyclone and the estimated total vorticity gradient under which it moves, suggesting that this motion is driven by the generalized β ‐drift mechanism (Gavriel & Kaspi, 2022). The second motion trend is an overall westward tendency of each cyclone (Blue arrows in Figures 1e and 1f), averaging to about 3° longitude per year at the north pole and 7° at the south (Mura et al., 2022).…”