Deep model simulation of polar vortices in gas giant atmospheres

Abstract

The Cassini and Juno probes have revealed large coherent cyclonic vortices in the polar regions of Saturn and Jupiter, a dramatic contrast from the east-west banded jet structure seen at lower latitudes. Debate has centered on whether the jets are shallow, or extend to greater depths in the planetary envelope. Recent experiments and observations have demonstrated the relevance of deep convection models to a successful explanation of jet structure, and cyclonic coherent vortices away from the polar regions have been simulated recently including an additional stratified shallow layer. Here we present new convective models able to produce long-lived polar vortices. Using simulation parameters relevant for giant planet atmospheres we find flow regimes of geostrophic turbulence (GT) in agreement with rotating convection theory. The formation of large scale coherent structures occurs via three-dimensional upscale energy transfers. Our simulations generate polar characteristics qualitatively similar to those seen by Juno and Cassini: they match the structure of cyclonic vortices seen on Jupiter; or can account for the existence of a strong polar vortex extending downwards to lower latitudes with a marked spiral morphology, and the hexagonal pattern seen on Saturn. Our findings indicate that these vortices can be generated deep in the planetary interior. A transition differentiating these two polar flows regimes is described, interpreted in terms of force balances and compared with shallow atmospheric models characterising polar vortex dynamics in giant planets. In addition, heat transport properties are investigated, confirming recent scaling laws obtained with reduced models of GT.

Interaction of Saturn’s Hexagon With Convective Storms

“…The stability of the hexagon with respect to the nearby strong storms, together with its long lifetime and survival to the strong seasonal insolation changes at its latitude, points toward a meteorological formation with roots deep in Saturn's atmosphere, as proposed by some recent models (Garcia et al, 2020;Yadava & Bloxhama, 2020). This would be consistent with the deep extension of the cloud top winds as deduced from gravity field measurements (Kaspi et al, 2020), reinforcing the idea of its possible relationship with the bulk rotation of the planet (Sánchez-Lavega, 2005, 2014.…”

The number and location of Jupiter’s circumpolar cyclones explained by vorticity dynamics

“…For the poles of Jupiter, the polygonal structure of the CPCs was modeled using a single-layer shallow water (SW) model 14 to investigate their depth and structure and provide evidence for anticyclonic shielding around the CPCs, which was shown to be necessary to inhibit cyclone mergers. In another approach, it was shown, using deep 3D models, that Jovian cyclones could extend deep and may emanate from convection of heat 15,16,17 and drift poleward 18,19 . Such 3D vortex behavior was also studied in laboratory experiments 20,21 .…”

Oscillations and Stability of the Jupiter Polar Cyclones

“…The Rayleigh number, or Ra, is the ratio of two time scales, that for diffusive thermal transport and that for convective thermal transport at a given speed, and was estimated for Jupiter to be greater than 10 12 (Manneville & Olson, 1996). One of the most recent applications of rotating thermal convective models, presented in Garcia et al (2020), for example, achieved only Ra ∼ 10 5 . It showed that deep convection can occur on Jupiter or Saturn poles, but did not reproduce the details of the observed structures or their stability.…”