SWOT and the Mediterranean Sea fine-scale dynamics: observability and improvement in coastal processes detection.

Louise Rousselet1, Camille Cardot1, Andrea Doglioli2, Anne Petrenko2, Anthony Bosse2, Jean-Baptiste Roustan3,4, Alexandre Barboni1, Pascale Bouruet-Aubertot5, Francesco d'Ovidio5
1Univ Toulouse, CNES, CNRS, IRD, LEGOS, Toulouse, France
2Aix Marseille Univ, Université de Toulon, CNRS, IRD, MIO, Marseille, France
3Laboratoire d’Océanographie Physique et Spatiale (LOPS), IUEM, CNRS, Ifremer, IRD, University of Brest, 29280 Plouzané, France
4Direction Générale de l’Armement, Ministère des Armées, Paris, France
5Sorbonne University, CNRS, IRD, MNHN, Oceanography and Climate Laboratory: Experiments and Numerical Approaches (LOCEAN-IPSL), 75005, Paris, France

A comprehensive understanding of fine-scale variability and its role on the global ocean remains a key objective that has been limited by the lack of observations at these scales. However, the new SWOT mission launch, together with the international SWOT-AdAC experiment, provide an unprecedented opportunity to capture fine-scale processes both with in situ and satellite-derived measurements. The SWOT fast sampling phase offers a temporal repetitivity of one day allowing for the identification and tracking of specific fine-scale events. The use of multi-instruments high frequency in situ data acquired in the Mediterranean Sea during two months (Apr-May 2023) under SWOT swaths (C-SWOT/WEMSWOT, https://doi.org/10.17600/18002077, and BioSWOT-Med, https://doi.org/10.17600/18002392 campaigns) provides sufficient observations to significantly evaluate SWOT capabilities to capture fine-scale dynamics. SWOT observations show similar variations as the in situ reconstructed surface geostrophic currents, revealing fine-scale dynamics down to 20 km. Specific comparisons highlights the significant (negligible) contribution of internal waves (cyclogeostrophy) in SWOT-captured signal. To illustrate SWOT capacities to detect fine-scale coastal processes, undetectable by conventional gridded altimetry product (DUACS), we specifically focus on a short-lived anticyclonic structure developing at the coastal edge of the Northern Current. The structure clearly signs in SWOT-derived surface currents, sea surface temperature and chlorophyll fields. Lagrangian diagnostics, including backward particle tracking and finite-time Lyapunov exponents, show that this structure acts as a transient transport barrier and particle trap, organizing coastal-to-offshore exchanges.These observations highlight the unique capability of wide-swath altimetry for resolving and interpreting, not only open ocean, but also coastal fine scale dynamics.