Séminaire
Turbulence in the upper ocean at submesoscales (scales smaller than the deformation radius) plays an important role for the heat exchange with the atmosphere and for oceanic biogeochemistry. Its dynamics should strongly depend on the seasonal cycle and the associated mixed-layer instabilities. The latter are particularly relevant in winter and are responsible for the formation of energetic small scales that extend over the whole depth of the mixed layer. The knowledge of the transport properties of oceanic flows at depth, which is essential to understand the coupling between surface and interior dynamics, however, is still limited, due to the complexity of performing measurements below the surface.
In this talk, I will present a numerical work focused on Lagrangian pair dispersion in turbulent flows from a quasi-geostrophic model allowing for both thermocline and mixed-layer instabilities. We find that, in the presence of mixed-layer instabilities, the dispersion regime is local (meaning governed by eddies of the same size as the particle separation distance) from the surface down to depths comparable with that of the interface with the thermocline, while in their absence dispersion rapidly becomes nonlocal (i.e. dominated by the transport by the largest eddies) versus depth. We then identify the origin of such behavior in the existence of fine-scale energetic structures due to mixed-layer instabilities. We further discuss the effect of vertical shear on the Lagrangian particle spreading and address the correlation between the dispersion properties at the surface and at depth, which is relevant to assess the possibility of inferring the dynamical features of deeper flows from the more accessible (e.g. by satellite altimetry) surface ones.
Connexion : https://www.gotomeet.me/SeminairesGeosciencesENS