The Dynamics of Jupiter’s Polar Cyclones

The poles of Jupiter are hidden from the view of Earth-orbiting and solar-plane satellites. In 2016, the arrival of the Juno spacecraft into a pole-to-pole orbit around Jupiter provided the first direct images of the Jovian poles, revealing a unique meteorological phenomenon. Each pole features a symmetrical arrangement of cyclones, with radii of ~5,000 km and wind speeds reaching 100 m/s. These formations comprise one polar cyclone at each pole and a surrounding ring of circumpolar cyclones, with eight cyclones in the north and five in the south. Today, more than 7 years after Juno's arrival, these two polar vortex-"crystals" have proven stable, maintaining their numbers and, on average, their latitudes and relative positions. 

The balance holding the cyclones in place is explained in the first part of this research. This balance is between a natural tendency of cyclones to propagate poleward, known as "β-drift," and a similar effect that stems from an interaction between multiple cyclones, causing a rejection between them. Evaluating these components using the observed properties of the cyclones exposes a band of equilibrium at latitude ~84°, matching their observed latitude, and accounts for the different number of cyclones between the north and south poles occupying that band. In addition, we demonstrate that this equilibrium cannot form on Saturn, which, although dynamically similar to Jupiter, only has a single polar cyclone at each pole. 

Analyzing the location of the cyclones throughout the Juno mission, 2 motion patterns become apparent. These patterns are presented and explained mechanistically in the second part of this research. The first is a coherent oscillatory motion of the cyclones around their mean positions, with an amplitude of ~400 km and periods of ~12 months. We find that perturbations from the balance responsible for the cyclones' mean positions can account for their radial accelerations leading to these circular motion patterns, as illustrated in the observations and in a toy model. The second motion pattern is a general westward drift of all cyclones, with southern cyclones drifting on average 7° per year and northern cyclones 3°. By analyzing the group of cyclones from a center-of-mass perspective and thus eliminating the mutual interactions between them, we find that their center of mass only feels the β-drift, which is expected to lead to a westward motion. This analysis, supported by measurements and modeling, accounts for the different drift rates between the poles.

Lastly, we explore the vertical structure of these polar cyclones. By analyzing the westward drift in a single-layer Shallow-Water model, we constrain the deformation radius of the cyclones to fit the observations, where we find that deeper cyclones result in a stronger β-drift. We use this estimation of the deformation radius to explore the vertical modes of Jupiter’s polar upper atmosphere by solving the eigenfunction problem between the 2D and 3D quasi-geostrophic models. This analysis explores and predicts the vertical structure of the polar cyclones and lays a framework for interpreting the forthcoming Microwave Radiometer (MWR) measurements expected for the north pole of Jupiter.

This work provides a unified perspective on the dynamics of Jupiter’s polar cyclones, revealing the physical principles that govern their stability, motion, and vertical structure. By linking observed cloud-level motions to deeper atmospheric dynamics, our findings offer a foundation for interpreting future MWR data from Juno. This research enhances our broader understanding of cyclonic behavior in giant planet atmospheres and sets the stage for further explorations of similar phenomena on other planetary bodies.