Jupiter’s polar cyclones hang together

When NASA’s Juno spacecraft reached Jupiter in 2016, it discovered cyclonic vortices thousands of kilometers wide at the gas giant’s poles. What made those cyclones unique compared with others in planetary atmospheres was their configurations: They formed stable, polygonal patterns. The north pole has nine vortices, shown in the photo, and the south pole has six. Except for one temporary interloper at the north pole, the arrangements have remained stable since Juno‘s arrival. That stability is surprising because planetary rotation pushes cyclones toward the poles, so storms would be expected to accumulate there and grow in number, or merge, as they do on Saturn. But Jupiter’s cyclone configurations remain stable.

The first hint at an explanation came from measurements of the storm sizes. Cloud radii ranged from 2000 km to 3500 km, but the storms reached their maximum wind speeds only at about 1000 km from their centers. Beyond that, wind speeds dropped off faster than expected for an isolated cyclone. Researchers suspected that a region of anticyclonic vorticity—air swirling in the opposite direction—might surround and stabilize each storm. Now simulations from Cheng Li , a postdoc at the University of California, Berkeley, and collaborators at Caltech show that under certain conditions, such anticyclonic layers can shield vortices from their neighbors and stabilize polygonal configurations.

The researchers modeled Jupiter’s atmosphere as a single fluid layer on a rotating sphere. Some of the vortices’ characteristics—including wind speeds, radii, and positions—were constrained by Juno‘s observations. Other parameters, particularly the depth of the vortex-containing fluid layer and amount of vorticity shielding, were varied over a range of physically reasonable values. When shielding wasn’t strong enough, the simulated vortices coalesced (video above); when it was too strong, they flew apart. Between those extremes were stable polygonal configurations (video below).

Although the simulations show how shielding could produce Jupiter’s polar cyclone pattern, they leave many questions unanswered. It’s still unclear whether the vortices formed in their current locations or accumulated at the poles, how their shielding develops, and why their number doesn’t grow. (C. Li et al., Proc. Natl. Acad. Sci. USA 117, 24082, 2020 .)