Direct driving of simulated planetary jets by upscale energy transfer

Abstract

The precise mechanism that forms jets and large-scale vortices on the giant planets is unknown. An inverse cascade has been suggested. Alternatively, energy may be directly injected by small-scale convection. 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. We analyze the nonlinear scale-to-scale transfer of kinetic energy in such simulations as a function of the azimuthal wave number, m. 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 from small-scale convective flows. Initial correlations are caused by the effect of uniform background rotation and shell geometry on the flows. 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 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 as ${\ell }^{-5}$, the same as for the zonostrophic regime.

15 pages, 14 figures, accepted for publication in A&A

References in corpus (13)

• The NumPy array: a structure for efficient numerical computation
• Inverse cascade and symmetry breaking in rapidly-rotating Boussinesq convection
• Saturn's deep atmospheric flows revealed by the Cassini Grand Finale gravity measurements
• Astrophysical Fluid Dynamics via Direct Statistical Simulation
• Zonal flow scaling in rapidly-rotating compressible convection
• Universality of the Small-Scale Dynamo Mechanism
• Deep rotating convection generates the polar hexagon on Saturn
• Evidence for multiple Ferrel-like cells on Jupiter
• Jets and large-scale vortices in rotating Rayleigh-Bénard convection
• The number and location of Jupiter's circumpolar cyclones explained by vorticity dynamics
• Zonal jets experiments in the gas giants' zonostrophic regime
• Linking Zonal Winds and Gravity II: explaining the equatorially antisymmetric gravity moments of Jupiter
• Contrasting turbulence in stably stratified flows and thermal convection