  Turbulence in the upper ocean in the submesoscale range (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. By means of numerical simulations, we explore the
Lagrangian dispersion properties of turbulent flows in a quasi-geostrophic
model system allowing for both thermocline and mixed-layer instabilities. The
results indicate that, when mixed-layer instabilities are present, the
dispersion regime is local from the surface down to depths comparable with that
of the interface with the thermocline, while in their absence dispersion
quickly becomes nonlocal 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 surface ones.
