Zonal jets in the laboratory: a transition towards zonostrophic turbulence?

Abstract:

Earth-based telescope observations and records from various spacecrafts, including the ongoing Juno mission, have revealed the complex multi-scale dynamics at play on Jupiter. Among others, the origin, structure and evolution of gas giants’ zonal jets are still poorly understood, especially in terms of coupling with their deep molecular interior. Here, we use an experimental approach to address the questions of zonal jets formation and long-term evolution. A strong topographic β-effect is obtained inside a fast rotating water-filled tank thanks to the paraboloidal shape of the fluid free surface due the centrifugally-induced pressure. The bottom of the tank is sculpted so that this β-effect is spatially uniform. A small-scale forcing is performed by circulating water at the base of the tank, through 128 inlets and outlets, allowing a control of its spatial distribution and intensity. Time-resolving PIV measurements reveal two regimes where zonal jets with strong instantaneous signature are present. In the first regime, obtained at low forcing amplitude, the jets are weak, locally forced and steady. In the second one, obtained at large forcing amplitude, the system evolves towards broader and stronger jets, decoupled from the forcing scale. This regime is closer to the so-called zonostrophic turbulence relevant to gas giants' applications. An analytical modeling based on the quasi-geostrophic approximation reveals that the observed transition and associated bistability result from the resonance between the directly forced Rossby waves and the background zonal flow.