Direct driving of simulated planetary jets by upscale energy transfer

Abstract

Context. The precise mechanism that forms jets and large-scale vortices on the giant planets is unknown. An inverse cascade has been suggested by several studies. Alternatively, energy may be directly injected by small-scale convection. Aims. Our aim is to clarify whether an inverse cascade feeds zonal jets and large-scale eddies in a system of rapidly rotating, deep, geostrophic spherical-shell convection. Methods. We analyze the nonlinear scale-to-scale transfer of kinetic energy in such simulations as a function of the azimuthal wave number, m. Results. We find that the main driving of the jets is associated with upscale transfer directly from the small convective scales to the jets. This transfer is very nonlocal in spectral space, bypassing large-scale structures. The jet formation is thus not driven by an inverse cascade. Instead, it is due to a direct driving by Reynolds stresses, statistical correlations of velocity components of the small-scale convective flows. Initial correlations are caused by the effect of uniform background rotation and shell geometry on the flows and provide a seed for the jets. While the jet growth suppresses convection, it increases the correlation of the convective flows, which further amplifies the jet growth until it is balanced by viscous dissipation. To a much smaller extent, energy is transferred upscale to large-scale vortices directly from the convective scales, mostly outside the tangent cylinder. There, large-scale vortices are not driven by an inverse cascade either. Inside the tangent cylinder, the transfer to large-scale vortices is even weaker, but more local in spectral space, leaving open the possibility of an inverse cascade as a driver of large-scale vortices. In addition, large-scale vortices receive kinetic energy from the jets via forward transfer. We therefore suggest a jet instability as an alternative formation mechanism of largescale vortices. Finally, we find that the jet kinetic energy scales approximatively as −5 , the same as for the so-called zonostrophic regime.

Turbulent Drag at the Ice‐Ocean Interface of Europa in Simulations of Rotating Convection: Implications for Nonsynchronous Rotation of the Ice Shell

“…If our simulations predict both mean and turbulent zonal speeds that are too large, then a more realistic torque may be smaller than that calculated using only the numerical results in Figure 4. For superrotation, the jets here are driven by the turbulent (eddy) flux of angular momentum away from the rotation axis, which must be balanced by the viscous/drag fluxes of angular momentum (Böning et al., 2023; Kaspi et al., 2009). This essentially means that, roughly speaking, $$\langle \overline{KE}\rangle $$ is a function of $$\langle \overline{K{E}^{\prime }}\rangle $$, the fluid's viscosity, ν , and the drag coefficient at the boundaries, c D .…”

Asymptotic scaling relations for rotating spherical convection with strong zonal flows

“…Rotating convection plays an important dynamical role in stars and planets, where it is believed to be one of the primary drivers of global scale magnetic fields (Busse 1975;Glatzmaier & Roberts 1995;Kageyama & Sato 1995;Stanley & Glatzmaier 2010;Jones 2011;Aurnou et al 2015), and possibly gives rise to coherent large-scale flows such as zonal jets and vortices, as observed on the giant planets (Heimpel et al 2022;Siegelman et al 2022;Böning et al 2023). Understanding the physics of turbulence driven by rotating convection remains challenging due to the vast range of spatiotemporal scales.…”

“…2022; Böning et al. 2023). Understanding the physics of turbulence driven by rotating convection remains challenging due to the vast range of spatiotemporal scales.…”

Shadowgraph measurements of rotating convective planetary core-style flows

“…It is likely that convective energy is injected into planetary fluid layers at this scale (Calkins, 2018;Schwaiger et al, 2019;Calkins et al, 2021). Further, these local scale convective flows likely drive the formation of large-scale phenomena such as jets and vortices (Stellmach et al, 2014;Lonner et al, 2022;Böning et al, 2023), and generate local-scale induction that can cascade upscale to -2-manuscript submitted to Journal of Geophysical Research: Planets form planetary and stellar-scale magnetic fields (Roberts & King, 2013;Aubert et al, 2017;Tobias, 2021;Yan & Calkins, 2022).…”