Coastal finescale feature and associated transport using new high resolution surface currents (SWOT, AIS): a Gulf of Lion case study.

Coastal regions are characterized by intense mesoscale and submesoscale dynamics that play a critical role in cross-shore exchange, tracer transport, and ecological connectivity. Resolving finescale processes near the coast remains challenging due to the limitations of conventional satellite altimetry and sparse \textit{in situ} observations. Here, we investigate the complementarity of emerging high-resolution observing systems-wide-swath Surface Water and Ocean Topography (SWOT) altimetry and surface currents reconstructed from Automatic Identification System (AIS) vessel trajectories-for characterizing coastal submesoscale structures in the Gulf of Lion, northwestern Mediterranean Sea. We combined SWOT altimetry, AIS-derived currents, and satellite-derived sea surface temperature and chlorophyll to provide independent tracer-based evidence of the dynamical coherence and transport imprint of finescale coastal structures. Offshore Toulon was an ideal study area, with simultaneous observations from moorings, HF radar, conventional altimetry (DUACS), SWOT one-day repeat data (Cal/Val), and AIS for mid-May to mid-July 2023. SWOT resolved sharp velocity gradients and compact submesoscale features down to $\mathcal{O}$(10,km) absent in standard altimetry products. AIS currents, though coarser, captured temporal evolution and extended monitoring beyond SWOT daily overpasses. Together, these datasets revealed a short-lived submesoscale anticyclonic structure at the coastal edge of the Northern Current, with clear sea surface temperature and chlorophyll signatures. Lagrangian diagnostics, including backward particle tracking and finite-time Lyapunov exponents, showed this structure acted as a transient transport barrier, organizing coastal-to-offshore exchanges. These results demonstrate the strong complementarity between SWOT and AIS observations and highlight the unique capability of wide-swath altimetry for resolving coastal submesoscale dynamics.