The compressional beta effect: a source of zonal winds in planets?

TL;DR

This paper proposes a compressional Rhines mechanism driven by planetary rotation and fluid compression/expansion as a source of deep zonal jets in giant planets, supported by numerical simulations and applicable to planets like Jupiter and Saturn.

Contribution

It introduces a novel compressional Rhines mechanism for jet formation, extending classical theories to deep planetary interiors with density stratification.

Findings

The mechanism predicts jet widths consistent with observations.

Numerical simulations confirm the robustness of jet formation via this mechanism.

It explains the formation of vorticity and buoyancy staircases in deep rotating systems.

Abstract

Giant planets like Jupiter and Saturn feature strong zonal wind patterns on their surfaces. Although several different mechanisms that may drive these jets have been proposed over the last decades, the origin of the zonal winds is still unclear. Here, we explore the possibility that the interplay of planetary rotation with the compression and expansion of the convecting fluid can drive multiple deep zonal jets by a compressional Rhines-type mechanism, as originally proposed by Ingersoll and Pollard (1982). In a certain limit, this deep mechanism is shown to be mathematically analogous to the classical Rhines mechanism possibly operating at cloud level. Jets are predicted to occur on a compressional Rhines length $l_R = (2 \Omega \langle H_\rho^{-1} \rangle v_{jet}^{-1} )^{-1/2}$, where $\Omega$ is the angular velocity, $\langle H_\rho^{-1} \rangle$ is the mean inverse density scale height and $v_{jet}$ is the typical jet velocity. Two-dimensional numerical simulations using the anelastic approximation reveal that this mechanism robustly generates jets of the predicted width, and that it typically dominates the dynamics in systems deeper than $O(l_R)$. Potential vorticity staircases are observed to form spontaneously and are typically accompanied by unstably stratified buoyancy staircases. The mechanism only operates at large rotation rates, exceeding those typically reached in three-dimensional simulations of deep convection in spherical shells. Applied to Jupiter and Saturn, the compressional Rhines scaling reasonably fits the available observations. Interestingly, even weak vertical density variations such as those in the Earth core can give rise to a large number of jets, leading to fundamentally different flow structures than predicted by the Boussinesq models typically used in this context.