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

TL;DR

This study uses numerical simulations to analyze how mixed-layer instabilities influence Lagrangian dispersion in upper-ocean turbulence, revealing different dispersion regimes and their relation to energetic structures and vertical shear.

Contribution

It provides new insights into the impact of mixed-layer instabilities on vertical dispersion regimes and the connection between surface and deep ocean flow dynamics.

Findings

Presence of mixed-layer instabilities leads to local dispersion from surface to thermocline depth.

Absence of instabilities results in nonlocal dispersion with depth.

Vertical shear influences particle spreading and dispersion properties.

Abstract

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.