Hi, I’m Clément Vic, physical oceanographer at the Laboratoire d’Océanographie Physique et Spatiale (University of Brest, Ifremer, IRD, CNRS). I work on small scales in the ocean and flow-topography interactions. In this project, I’m interested in the life cycle of internal tides and their impact on the large-scale circulation and mixing of tracers. Internal tides are internal waves of tidal frequencies that are generated by tidal currents sloshing back and forth on steep seafloor slopes. These waves propagate in the ocean interior and dissipate their energy along the way through interactions with background flows and the seafloor topography. How, and how much energy is dissipated remains unclear in the area we’re interested in, although the waves are among the most energetic in the global ocean, hence the current survey !
We routinely deploy a Vertical Microstructure Profiler (VMP), an instrument that measures the velocity shear with a high frequency (1024 Hz), which allows us to infer the rate of dissipation of kinetic energy. The VMP is tethered to the ship, but requires to be free falling to make accurate measurements. Here comes the physical challenge: manually unwind the tether fast enough to get enough slack in the water not to perturb the free fall of the VMP ! Roll up your sleeves, warm up your shoulders, stretch out your neck and your back, play out some Shakira music, 3…2…1… top à la vachette ! (French for let’s go)
The above figure shows the potential density (colors) and rate of dissipation of kinetic energy (bar plots) along a 24-h station near a site of strong interaction of tides with the seafloor topography. High dissipation rates are highlighted in orange and red, and are spanning the whole water column. They are associated with internal wave breaking, which triggers vertical mixing and potential fluxes of, e.g., nutrients, through the upper part of the water column.
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Clément Vic on behalf of the science team aboard R/V Antea