Turbulent zonal winds in laboratory experiments

Zonal jets are coherent east-west winds or currents observed – or expected to emerge – in many planetary fluid layers, from the Earth’s oceans and atmosphere, to the atmospheres of Jupiter and Saturn, the subsurface oceans of icy moons and the liquid metallic cores of telluric planets. Understanding the long-term, nonlinear equilibration of zonal jets and their interaction with underlying turbulence and waves remains a major challenge in planetary fluid dynamics. A further complication is that, in many of these systems, zonal jets interact with a solid boundary with topography: the bathymetry in Earth’s oceans is known to influence the dynamics of the Antarctic Circumpolar Current, flows in liquid cores interact with the topography at the Core-Mantle boundary, and icy moon oceans are in direct contact with a global ice crust of spatially varying thickness.

In this talk, I will present two experimental setups able to reproduce the spontaneous emergence of turbulent zonal jets from rotating turbulence. Both experiments take advantage of the deflection of the free surface due to the centrifugal force to obtain a topographic beta-effect, necessary for the formation of zonal jets. In the first experiment (IRPHE, Marseille), the forcing is mechanical with source and sinks of water generating small scale turbulence at the bottom of the tank. In the second experiment, we use the Coreaboloid device (Spinlab, UCLA). The flow is forced by thermal convection, driven by starting the experiment with hot water, and cooling the inner cylinder with a block of ice. I will explain the emergence of zonal jets in terms of Rossby wave-mean flow interactions, including the nonlinear saturation of the flow and the coalescence of zonal jets. I will also discuss preliminary results on the interaction between turbulent zonal jets and localised topography, including the formation of stationary Rossby waves downstream of the topography, that feed back on the amplitude, number and position of zonal jets.