Submesoscale Impacts on Tracer Transport in the Gulf of Mexico

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Liu, Guangpeng
Bracco, Annalisa
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Mesoscale and submesoscale dynamics and their role on the transport and mixing of ocean biogeochemical tracers have been investigated widely in recent literatures. In general, large circulations and mesoscale eddies control the absolute dispersion of tracers horizontally, while submesoscale motions affect horizontal mixing and, with their strong vertical velocities, control upwelling or subduction processes in the boundary layers, at the ocean surface and near its bottom. However, the seasonality of mesoscale and submesoscale circulations varies greatly over time and space, and observational studies quantifying their biophysical consequences remain scarce. In this thesis, several numerical experiments are conducted either to explore the relative importance of submesoscale contributions within and across the surface and bottom boundary layers, or to interpret in-situ measurements of particle-like tracers at the ocean bottom, focusing on the Gulf of Mexico. In the upper ocean, intense submesoscale-associated vertical motions are found inside and around the Loop Current and the Loop Current eddies. Maps of vertical diffusivity calculated in mesoscale resolving and submesoscale permitting simulations indicate that diffusivities are an order of magnitude larger in the submesoscale permitting case. Those energetic surface submesoscale structures aggregate particles in regions where downward velocities are predominant and accelerate significantly their descent into the ocean interior. While mesoscale circulations determine size and shape of the catchment area of sinking particles, submesoscale circulations contribute to their convergence and vertical accelerations. Near the bottom, submesoscale circulations are also relevant for connectivity studies. They enhance diapycnal mixing, as done at the surface, but they also trap particles -or coral larvae - in their interior, transporting them in isolation from the surrounding. As a result, depth differences on scales of tens to at most few hundreds of meters, are enough to limit the vertical connectivity among sites few tens of kilometers apart.
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