Baroclinic turbulence

Collaborators: R. Ferrari, CLIMA collaboration, J. Meunier, B. Miquel

The mean state of the atmosphere and ocean is set through a balance between external forcing (radiation, winds, heat and freshwater fluxes) and the emergent turbulence, which transfers energy to dissipative structures. The forcing gives rise to jets in the atmosphere and currents in the ocean, which spontaneously develop turbulent eddies through the baroclinic instability. A critical step in the development of a theory of climate is to properly include the eddy-induced turbulent transport of properties like heat, moisture, and carbon. Through a combination of numerical and analytical work, we have developed a scaling theory that predicts the scaling behavior of the eddy diffusivity associated with baroclinic turbulence. The theory can be used as a quantitative parameterization in the case of meridionally dependent forcing, in the fully turbulent regime. Our current research aims at gradually including the physical ingredients of a realistic patch of ocean in the idealized theory.

Radiatively driven convection

Collaborators: G. Hadjerci, S. Aumaître, B. Miquel, V. Bouillaut, S. Lepot

Turbulent convection is ubiquitous in geophysical and astrophysical contexts: It drives winds in the atmosphere and currents in the ocean, it generates magnetic fields inside planets and stars, and it triggers supernova explosions inside collapsing stellar cores. In many such natural flows, convection is driven by the absorption of incoming radiation (light or neutrinos). We designed an experiment to reproduce such radiatively driven convection in the laboratory. Our radiatively driven convection setup achieves the diffusivity-free or “ultimate” regime of turbulent convection, which is believed to be the one relevant to many natural flows. By adding global rotation to the laboratory experiment, we performed the first experimental observation of the diffusivity-free or "ultimate" regime of rapidly rotating convection, also known as the Geostrophic Turbulence scaling regime.

Wave mean-flow interactions

Collaborators: C. Higgins, W.R. Young, S. Aumaître, T. Humbert, K. Seshasayanan.

In the Ocean: Swell are long surface waves propagating on the surface of oceans. Swell is generated in stormy regions of the globe and can travel for approximately one week before breaking on distant beaches. We have shown that this propagation is strongly affected by surface currents. This explains the inference of swell sources on land by Munk et al. (1959): because of refraction by surface currents, the swell did not propagate on great-circle routes, creating a mirage for the California-based observers.

In the laboratory: We reproduced such wave refraction by a vortex in the laboratory. We focused on the nonlinear regime of wave refraction: when strong enough surface waves are refracted by a vortex, the vortex gets distorted by the strong radiation pressure forces. This distortion in turn weakens the wave refraction: the laws of refraction then strongly depend on the wave intensity. When the vortex shown in the top-left panel interacts with the wave field in the top-right panel, we observe wave refraction (bottom-right) together with wave-induced vortex recoil (bottom-left).

Rotating and magnetohydrodynamic turbulence

Collaborators: E. Monsalve, P.-P. Cortet, F. Moisy.

Theory: When turbulent flows are subject to either rapid global rotation or a strong external magnetic field, they tend to become two-dimensional. How far does this two-dimensionalization proceed? And what if the flow is driven in the form of waves only? When the flow is driven by vertically invariant forcing, I proved that the turbulence becomes exactly two-dimensional for strong enough magnetic field or rotation, with dramatic consequences for the energy dissipation rate of the flow. By contrast, when the forcing drives wave modes only, we quantitatively observed the inertial-wave cascade predicted by Wave Turbulence Theory.

Past work: The dynamo effect

Collaborators: K. Seshasayanan, S. Fauve, F. Pétrélis, the Von Karman Sodium collaboration

I was a member of the Von Karman Sodium collaboration. This experiment generates magnetic field through dynamo action in a turbulent flow of liquid sodium. My main contribution was to establish a connexion between randomly reversing magnetic fields, such as the geodynamo, and hemispherically localized magnetic fields, such as the remanent magnetic field of Mars. We then confirmed this prediction experimentally, with the first observation of localized dynamo magnetic fields.