Abstract

Oceanic motions at scales larger than few tens of km are quasi-horizontal due to seawater
stratification and Earth’s rotation and are characterized by quasi two-dimensional turbulence. While large (mesoscale) coherent vortices contain most of the kinetic energy, at sufficiently small scales (in the submesoscale range) the flow is populated by smaller eddies and filamentary structures associated with intense gradients, which play an important role in both physical and biogeochemical budgets. Turbulent mixing in such oceanic flows can be characterized through the relative dispersion of pairs of drifters. The relation between the observed dispersion behaviors and the statistical properties of the underlying turbulent flow, however, can be subtle. Furthermore, the knowledge of the transport properties at depth, which is essential to understand the coupling between surface and interior dynamics, is still limited.

In this talk, I will address submesoscale turbulence from a Lagrangian pair-dispersion point of view, relying on both experimental data and numerical simulations. I will focus on the identification of different dispersion regimes, as well as on seasonality effects. The results will be compared with theoretical predictions from idealized turbulence models. Within this context I will examine systems with more, or less energetic submesoscales to explore the relation between the Lagrangian statistics and the Eulerian flow features. I will then extend the analysis by considering more realistic dynamics accounting for both thermocline and mixed-layer instabilities. In particular, I will focus on the possibility to relate the characteristics of the spreading process at the surface and at depth, which is relevant for inferring the dynamical features of deeper flows from the experimentally more accessible (e.g. by satellite altimetry) surface ones. I will show that in the absence of a mixed layer, the properties of the spreading process rapidly decorrelate from those at the surface, but the relation between the surface and subsurface dispersion appears to be largely controlled by vertical shear. In the presence of mixed-layer instabilities, instead, the statistical properties of dispersion at the surface are found to be a good proxy for those in the whole mixed layer.

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