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Rigid Sets and Coherent Sets in Realistic Ocean Flows

Feppon, F. and P.F.J. Lermusiaux, 2022. Rigid Sets and Coherent Sets in Realistic Ocean Flows. Nonlinear Processes in Geophysics, sub-judice. doi:10.5194/npg-2022-1

This paper focuses on the extractions of Lagrangian Coherent Sets from realistic velocity fields obtained from ocean data and simulations, each of which can be highly resolved and non volume-preserving. We introduce two novel methods for computing two formulations of such sets. First, we propose a new “diffeomorphism-based” criterion to extract “rigid sets”, defined as sets over which the flow map acts approximately as a rigid transformation. Second, we develop a matrix-free methodology that provides a simple and efficient framework to compute “coherent sets” with operator methods. Both new methods and their resulting rigid sets and coherent sets are illustrated and compared using three numerically simulated flow examples, including a high-resolution realistic, submesoscale to large-scale dynamic ocean current field in the Palau Island region of the western Pacific Ocean.