Laboratory experiments of downslope oceanic currents at the CORIOLIS ROTATING PLATFORM

Predicting Earth’s climate is one of today’s greatest challenges and it depends on our understanding of the ocean dynamics over a large variety of interacting scales in time and space. Oceanic currents are driven at the surface by wind stress and in the deep interior by buoyancy differences induced by variations in temperature and salinity (thermo-haline convection). The resulting flow depends on energy dissipation processes, occurring after a cascade of instabilities and poorly understood turbulent processes. The main goal of this master project is to study the transition from a downslope oceanic current to a quasi-horizontal current under the effect of the Coriolis force through laboratory experiments at the CORIOLIS ROTATING PLATFORM (LEGI).

When buoyancy driven flows encounter topography, a downslope dense current, also called gravity current, is created. Gravity currents play an important role for the transport of water masses in the oceans. Their exchange involve boundary layers, high shears and density gradients, instabilities and generation of sub-mesoscale vortices at [km] scale. The resulting mixing influences the final stabilisation depth of the water mass, which controls the whole convection process. These flows however, occur at such small scales and are so rapid that a full resolution of their dynamics is out of reach for numerical models. Consequently, they need to be correctly represented in general ocean circulation models.
The main goal of this master project is to study the transition from a downslope oceanic current to a quasi-horizontal current under the effect of the Coriolis force. This process is a form of geostrophic adjustment leading to a horizontal current where the downslope gravity force is balanced by the upslope Coriolis force. A density front appears at the edge of this current, leading to specific instabilities and dissipation processes, which control the slow downward drift of the current and the final equilibrium location and density of the transported water mass.
Laboratory experiments will be performed at the Coriolis Rotating Platform, a world wide unique facility for its size of 13m in diameter, which allows to reproduce ocean currents in dynamical similarity for the main driving forces of buoyancy and Earth’s rotation. The experiments will be realized using saline solutions to generate GCs injected at several points equispaced on the Coriolis platform’s circonference at the top of an inclined boundary, shaped as an inversed cone, with circular or elliptical cross section. The ambient reservoir will be discrete-layered to allow the current to detach from the boundary. The velocity and concentration fields will be measured using simultaneous PIV/PLIF techniques. The data will be processed with MATLAB/Phyton in order to determine the influence of the experimental parameters on the development of the shear layer properties, current detachment and boundary layer ejection into the ocean interior and on entrainment at the sheared interface.