| 24 Jun 2026
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.
The manuscript presents a consistent description of a coastal eddy observed during the SWOT fast-sampling phase in the Algerian Basin. The physical interpretation is generally plausible, and the comparison between SWOT, conventional gridded altimetry, SST, and chlorophyll-a is visually convincing. However, in my opinion the manuscript does not provide sufficient scientific scope for publication in Ocean Science in its present form.
The main result of the paper is that SWOT resolves the structure and short-term evolution of a coastal eddy better than standard interpolated L4 altimetry maps. While this is an interesting illustration, it is also largely expected. The manuscript repeatedly compares SWOT observations with daily 1/16° gridded products derived from conventional altimetry. It is already well established within the altimetry community that such mapped products smooth mesoscale and submesoscale variability and have difficulty representing features evolving on time scales shorter than several weeks. Consequently, the fact that SWOT performs better in this comparison does not, by itself, constitute a sufficiently novel scientific result.
Furthermore, the capability of SWOT to observe mesoscale and even submesoscale eddies has already been demonstrated by a rapidly growing body of literature. In this context, the present study remains primarily descriptive. The analysis does not introduce a clearly innovative eddy-detection methodology, nor does it provide a quantitative assessment of the improvements brought by SWOT relative to conventional products. The comparison with SST and chlorophyll-a is useful as qualitative support, but it is not developed into an independent validation of eddy position, propagation, or deformation.
The manuscript does not appear to provide substantial new insight into the oceanography of the Algerian Basin. The study documents an interesting case, but it does not convincingly demonstrate a new dynamical mechanism nor quantify the role of topography in a way that significantly advances our understanding of coastal eddy propagation. In particular, the interpretation of the apparent velocity maxima as evidence of topographic modulation remains suggestive rather than demonstrated, especially since the estimated velocities are derived from ellipse-centroid displacement during phases of strong eddy deformation.
Finally, the manuscript reads as a useful and pedagogically effective case study. It clearly illustrates that SST and chlorophyll-a patterns can be consistent with sea-level contours and that SWOT provides a much sharper two-dimensional representation of the eddy than conventional mapped altimetry. However, I do not think this is sufficient for the standards of Ocean Science, which states that it publishes studies with "important implications for our understanding of the state and behaviour of the ocean". In its present form, the manuscript demonstrates an expected observational advantage of SWOT but does not yet translate this advantage into a sufficiently significant scientific contribution.
To strengthen a potential resubmission, the authors should move beyond a qualitative description and provide a more quantitative analysis. This could include objective metrics of the improvement obtained with SWOT, independent estimates of eddy displacement based on SST or chlorophyll-a observations, a sensitivity analysis of the ellipse-fitting methodology, or a clearer separation between eddy translation and deformation. A more rigorous assessment of the relationship between the observed propagation-speed maxima and bathymetric parameters would also be necessary if the claim of topographically modulated propagation is to be retained.
Another potentially valuable direction would be the inclusion of the SWOT-MIOST product, in which SWOT observations are incorporated into mapped altimetric fields. Such a comparison would be more informative than a direct SWOT-versus-L4 comparison and would allow the authors to evaluate whether SWOT-enhanced mapped products improve the representation of high-frequency coastal variability. Moreover, SWOT-MIOST products are available beyond the CAL/VAL phase and extend into the science phase of the mission, increasing both the relevance and the broader applicability of the study.
For these reasons, I do not recommend publication of the manuscript in its current form. However, because I still see potential in the study, I recommend that the manuscript be considered for major revision rather than rejection. The requested revisions are substantial and will require significant additions to the analysis. A substantially revised manuscript could become suitable for publication if the authors either provide a considerably stronger quantitative and dynamical analysis or reformulate the work more explicitly as a methodological or product-evaluation study rather than as a scientific advance in coastal eddy dynamics.
Below I add some line-by-line additional comments:
Lines 10–12 — The final sentence of the abstract reads rather generic and shows signs of AI-generated phrasing. This is true for many other sentences in the manuscript. If AI tools were used in the preparation of the manuscript, this should be explicitly stated according to the journal policy, for example in the acknowledgements or in the dedicated AI-use declaration if required by the journal.
Lines 39–42 — The sentence referring to the “β-effect, lower-layer circulation, shelf width and depth, and slope characteristics” is technically correct but not very accessible. Please explain this point in simpler physical terms, so that the reader can understand which mechanisms are expected to influence eddy propagation near a continental slope.
Lines 47–48 — The statement that SST and ocean-colour observations are sensitive to cloud presence is generally true, but it is not clear whether this is a major limitation for the specific region and period analysed in this study. Please clarify whether cloud contamination was actually a significant issue for the Algerian Basin case considered here.
Lines 52–55 — The limitations of conventional altimetry in coastal regions are cited using Vignudelli et al. (2011), but this reference is quite old and does not reflect the progress made in coastal altimetry over the last 15 years, including improvements in sensor technology, retracking, data processing, and geophysical corrections. Since the study analyses recent data, please either update this statement with more recent references or remove the sentence if it is not directly relevant to the present analysis.
Lines 59–61 — The claim that SWOT provides observations “much closer to the coast compared to conventional along-track altimetry” is misleading. Reprocessed high-rate conventional altimetry can provide valid measurements up to a few hundred metres from the coast in favourable cases. The real added value of SWOT in the coastal zone should instead be framed as its two-dimensional mapping capability with low noise over the swath. Please revise this statement accordingly.
Lines 60–62 — The phrase “potential persistent small-scale uncertainties” is unclear. Please specify what type of uncertainty is meant, whether it refers to geophysical corrections, wet-tropospheric effects, residual errors, or another source of error, and explain why it is expected to be persistent.
Lines 86–87 — The manuscript states that SST and ocean-colour data were downloaded from the Copernicus Marine Service. Please provide the corresponding product DOIs in this section, or explicitly indicate where the reader can find the DOI information later in the paper.
Lines 88–90 — The manuscript states that PyEddyTracker was used for conventional altimetry. Please clarify how this was implemented. Was the code used as distributed publicly? Was it modified by the authors? If a specific version or customised implementation was used, please indicate how the reader can access it.
Lines 94–103 — The ellipse-fitting procedure appears to be a central methodological component of the study, but it is not described in sufficient detail to allow reproducibility. Please provide a clearer step-by-step description of the method, including the filtering, contour selection, boundary reconstruction, ellipse fitting, and centroid-speed calculation. Ideally, this should include the mathematical formulation and/or a worked example showing how the procedure is applied to one SWOT scene.
Lines 106–114 — The comparison with SST and chlorophyll-a is useful as qualitative support, but it should not be described as a “validation” of the SLA-derived eddy position and shape. In particular, the fact that SWOT gives higher velocities or sharper structures does not automatically imply that they are more accurate. If the authors want to use SST as an independent reference for the eddy displacement, the eddy centroid or another comparable positional metric should also be estimated from SST. Otherwise, the wording should be softened to “qualitative consistency” or “supporting evidence” rather than validation.
Lines 148–154 — The interpretation of the SWOT-derived speed peaks needs to be treated with more caution. The higher propagation velocities obtained from the ellipse-centroid displacement are not necessarily more accurate than those from conventional altimetry, especially because the eddy is deforming during the same period. To support the statement that SWOT better captures propagation variability, the authors should compare the centroid displacement with an independent estimate, for example from SST, or explicitly state that these are apparent kinematic velocities affected by deformation.