Journal
PHYSICS OF FLUIDS
Volume 33, Issue 3, Pages -Publisher
AIP Publishing
DOI: 10.1063/5.0041036
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Funding
- French CNES TOSCA program
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Turbulence in the upper ocean, especially in the submesoscale range, is crucial for heat exchanges with the atmosphere and oceanic biogeochemistry. The dynamics are influenced by the seasonal cycle and mixed-layer instabilities, particularly in winter. The presence of fine-scale energetic structures due to mixed-layer instabilities affects the dispersion properties of turbulent flows.
Turbulence in the upper ocean in the submesoscale range (scales smaller than the deformation radius) plays an important role for the heat exchanges 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 with 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.
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