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Zonostrophic turbulence on gas giants: insights on turbulent transport and mixing from laboratory experiments

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ADIW03 - Climate Applications of Layering

The colourful bands of Jupiter are sustained by intense east-west winds called zonal jets, which extend well below Jupiter’s weather layer into its mantle of liquid hydrogen. These jets constitute a fascinating natural example of how a rapidly-rotating turbulent flow self-organises at large scale. Despite decades of observations and modelling, understanding the long-term, nonlinear equilibration of zonal jets and the feedback with the underlying turbulence is still a challenge, notably because of the extreme “zonostrophic” regime of turbulence on gas giants.  In this talk, I will discuss the challenges that arise to study such extreme regime, and describe insights from recent laboratory experiments built in Marseille where instantaneous turbulent zonal jets spontaneously emerge from the small-scale forcing, equilibrate at large scale, and can contain up to 70% of the total kinetic energy of the flow once in a quasi-steady state. I will show that the spectral properties of the experimental flows are consistent with the theoretical predictions in the zonostrophic turbulence regime. This constitutes the first fully-experimental validation of the zonostrophic theory in a completely three-dimensional framework.  Next, I will quantify the local potential vorticity mixing by measuring the equivalent of a Thorpe scale, and confirm that it can be used to estimate the upscale energy transfer rate of the flow, which otherwise needs to be estimated from a much more demanding spectral analysis. Finally, I will analyse Lagrangian trajectories to discuss the turbulent transport properties in terms of effective diffusivity and investigate the effect of the zonal flow on the isotropy and homogeneity of the turbulent transport. Our estimates of meridional diffusivities are consistent with predictions from mixing length and zonostrophic theories, and confirm the so-called suppression effect of zonal flows on eddy-diffusivity, which may hence be important to take into account in models parametrizing small-scale processes. These results are cross-validated by complementary 2D quasi-geostrophic numerical simulations. Contributors – Benjamin Favier (IRPHE, CNRS ), Michael Le Bars (IRPHE, CNRS ), Simon Cabanes (IPGP), Jonathan Aurnou (UCLA)

This talk is part of the Isaac Newton Institute Seminar Series series.

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