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Lagrangian Surface Signatures Reveal Upper-Ocean Subduction Near Oceanic Density Fronts

Aravind, H. M., V. Verma, S. Sarkar, M.A. Freilich, A. Mahadevan, P.J. Haley Jr., P.F.J. Lermusiaux, and M.R. Allshouse, 2022. Lagrangian Surface Signatures Reveal Upper-Ocean Subduction Near Oceanic Density Fronts. Ocean Modeling, sub-judice.

Subduction, the downward transport of fluid and material over a finite time interval, plays a critical role in the exchange of freshwater, heat, nutrients, and other biogeochemical tracers between the ocean surface and the interior. Sustained subduction is believed to occur through coherent pathways that act as conduits for fluid transport that exist near oceanic density fronts. Only a small fraction of the ocean surface undergoes sustained subduction, however, making direct observation of the phenomenon extremely difficult since it requires precise identification of the conduits from surface signatures. Analysis of three submesoscale-resolving ocean models spanning length-scales ranging from kilometers to hundreds of kilometers reveal that Lagrangian alternatives to widely employed Eulerian targeting approaches are more efficient at identifying these surface signatures. Using both Eulerian and Lagrangian metrics, we identify “target zones” that maximize these metrics and may correlate to the surface signatures of subduction conduits. Our analysis demonstrates that the Lagrangian metrics are more likely to target sites of strongest subduction and identify the majority of the strongest subducting conduits in the domain. Ensemble analysis of forecasts from a realistic data-assimilative ocean modeling system demonstrates that the target zones identified by the proposed Lagrangian metrics locate persistent subduction regions even without knowledge of the true surface velocity, allowing them to be used to predict the locations of subduction conduits during field observations.