| Summary: | Recent studies demonstrate that energetic sub-mesoscale fronts (10–50 km width) extend in the ocean interior, driving large vertical velocities and associated fluxes. However, diagnosing the dynamics of these deep-reaching fronts from in situ observations remains challenging because of the lack of information on the 3-D structure of the horizontal velocity. Here, a realistic numerical simulation in the Antarctic Circumpolar Current (ACC) is used to study the dynamics of submesocale fronts in relation to velocity gradients, responsible for the formation of these fronts. Results highlight that the stirring properties of the flow at depth, which are related to the velocity gradients, can be inferred from finite-size Lyapunov exponent (FSLE) at the surface. Satellite altimetry observations of FSLE and velocity gradients are then used in combination with recent in situ observations collected by an elephant seal in the ACC to reconstruct frontal dynamics and their associated vertical velocities down to 500 m. The approach proposed here is well suited for the analysis of sub-mesoscale-resolving datasets and the design of future sub-mesoscale field campaigns. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Received: 10 August 2020; Accepted: 25 August 2020; Published: 28 August 2020. High-end computing resources for the numerical simulation were provided by the NASA Advanced Supercomputing (NAS) Division at the Ames Research Center. Thanks to Christopher Henze at NASA Ames Hyperwall and MITgcm developers and NAS scientists that made available the model outputs. This work was supported by the CNES-TOSCA project Elephant seals as Oceanographic Samplers of sub-mesoscale features led by C. Guinet with support of the French Polar Institute (Programs 109 and 1201). Thanks to Fabien Roquet and Baptiste Picard that made available the ...
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