Turbulent zonal jets interacting with isolated topography: an experimental study

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 gas giants, the subsurface oceans of icy moons and the liquid metallic cores of telluric planets. 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 laboratory experiments to study the interaction between self-sustained turbulent zonal jets and an isolated topography. We use the Coreaboloid device at UCLA (Lonner et al., 2022, doi:10.1029/2022JE007356) to robustly produce turbulent zonal jets. The setup is a 75cm-diameter water tank rotating at speeds up to 72 revolutions per minute. The deflection of the free surface due to the fast rotation provides a strong topographic β-effect. 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. To simulate a localised topography, we attach acrylic disks of different radii and heights on the bottom plate. We visualise the flow using 1) a thermal infrared camera to image the temperature field at the free surface 2) particle image velocimetry (PIV) on a horizontal laser plane and 3) ultrasonic doppler velocimetry (UDV) along three chords. We find that stationary Rossby waves develop downstream of the topography in prograde jets and influence the amplitude, number, and position of the zonal jets. The observed zonal wavelength of stationary lee Rossby waves agrees with theoretical predictions for plane Rossby waves, provided that the feedback of the zonal flow amplitude and curvature is taken into account. Remarkably, the topography leaves a visible imprint on the flow even for heights as small as h=3 mm, corresponding to just 1.2% of the total fluid depth H. For larger topography (h/H=5.9% to 17.5%), upstream blocking is observed, and a cyclonic circulation forms above the topography.