SWOT fast-sampling observations of topographically modulated coastal-eddy propagation in the Algerian Basin

Abstract. The detailed evolution of coastal mesoscale eddies propagating along continental margins is difficult to observe with conventional nadir altimetry due to the limited spatio-temporal sampling. Here we use observations from the one-day repeat fast-sampling phase of the high-spatial-resolution Surface Water and Ocean Topography (SWOT) mission to examine the propagation of a small anticyclonic coastal eddy along the Algerian Coast in the Western Mediterranean Sea in May–June 2023. SWOT sea level anomaly fields are compared with conventional altimetry, sea surface temperature, and chlorophyll-a observations to assess the eddy position, structure, and evolution. SWOT resolves a more realistic nearshore eddy structure, leading to an improved characterization of its eastward displacement compared to conventional gridded altimetry, particularly during the eddy interaction with capes and steep bathymetric transitions. Ellipse-based centroid tracking indicates localized apparent propagation-speed maxima of about 20 cm s^-1, whereas conventional gridded altimetry produces smoother and weaker velocities, generally below 10 cm s^-1. These speed maxima coincide with a narrowing continental shelf and sharper shelf-break slopes, suggesting kinematic variability organized by the local coastal topography. This case study demonstrates the value of SWOT fast-sampling observations for resolving coastal eddy deformation and event-scale kinematic variability, highlighting new opportunities for understanding the interaction between small mesoscale eddies and coastal topography.