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Turbulence in the upper ocean in the submesoscale range (scales smaller thanthe deformation radius) plays an important role for the heat exchange with theatmosphere and for oceanic biogeochemistry. Its dynamics should strongly dependon the seasonal cycle and the associated mixed-layer instabilities. The latterare particularly relevant in winter and are responsible for the formation ofenergetic small scales that extend over the whole depth of the mixed layer. Theknowledge of the transport properties of oceanic flows at depth, which isessential to understand the coupling between surface and interior dynamics,however, is still limited. By means of numerical simulations, we explore theLagrangian dispersion properties of turbulent flows in a quasi-geostrophicmodel system allowing for both thermocline and mixed-layer instabilities. Theresults indicate that, when mixed-layer instabilities are present, thedispersion regime is local from the surface down to depths comparable with thatof the interface with the thermocline, while in their absence dispersionquickly becomes nonlocal versus depth. We then identify the origin of suchbehavior in the existence of fine-scale energetic structures due to mixed-layerinstabilities. We further discuss the effect of vertical shear on theLagrangian particle spreading and address the correlation between thedispersion properties at the surface and at depth, which is relevant to assessthe possibility of inferring the dynamical features of deeper flows from themore accessible surface ones.

Lagrangian pair dispersion in upper-ocean turbulence in the presence of mixed-layer instabilities