the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Apr 2025
Surface evolution and wind effects during a cyclonic eddy splitting event in the Balearic Sea
Abstract. During the period of 23–28 February 2022, a cyclonic eddy in the Balearic Sea was observed to split into two smaller eddies. Serendipitously, a wealth of data was collected of the event, including satellite chlorophyll maps, Lagrangian drifters at several depths, hydrographic sections intersecting the splitting eddy, and wind speed and direction. Sufficiently many drifters were in the area to estimate kinematic properties (divergence, vorticity, and strain rate) from clusters along the edge of the eddy before and during the elongation period that led to the splitting. The vertical velocity w can be computed from colocated divergence values from surface drifters (CARTHE and CODE, within the top meter) and near-surface drifters (SVP, at 15 m depth). Together, the observations delineate the process of eddy elongation, leading to vorticity and strain-rate intensification on February 25, followed by the collapse of the ridge in the center of the eddy and the emergence of smaller eddies on February 26, terminating with the splitting into submesoscale cyclones on February 28. The consecutive daily hydrography and drifter observations supplement the remotely sensed descriptive view of the eddy splitting process. In particular, they confirm dominant internal dynamics, consistent with isopycnal doming, but also point to a role played by the winds, which shifted from predominantly southwesterly to predominantly northeasterly and strengthened significantly before weakening again in the area of interest during the eddy splitting period. Nonlinear Ekman pumping WEknl is estimated from the wind data and drifter-derived vorticities to capture the contribution of the wind effects to the patterns of up- and downwelling accompanying the eddy splitting. The WEknl patterns are consistent with the drifter-based w and divergence estimates. Moreover, the nonlinear Ekman pumping is found to be of the same order of magnitude as (though generally less than) w, suggesting that the wind likely plays a role in the observed surface processes.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-1475', Hong Li, 08 Jun 2025
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RC2: 'Comment on egusphere-2025-1475', Anonymous Referee #2, 16 Aug 2025
General comments
The manuscript presents a detailed observational analysis of a rare and scientifically relevant phenomenon: the splitting of a cyclonic eddy in the Balearic Sea, documented during the CALYPSO 2022 experiment. The combination of drifter clusters, hydrography, wind measurements, and satellite observations provides a dataset that allows the authors to link eddy internal dynamics with surface wind forcing and nonlinear Ekman pumping. This manuscript contributes to the understanding of mesoscale–submesoscale interactions and the role of winds in modulating eddy-driven vertical exchanges.
Overall, the analysis is rigorous, the methods are clearly described, and the paper is well written. However, some aspects of the interpretation could be better balanced between observational evidence and theoretical expectations. In particular, the discussion of wind effects sometimes appears speculative given the sparse spatial coverage, and could benefit from a more cautious phrasing. Additionally, the manuscript would gain from a clearer separation of what is firmly supported by the observations versus what is inferred or hypothesized.
With these refinements, the paper has the potential to be an excellent contribution. I therefore recommend publication after moderate revisions.
Specific comments
1. Novelty and positioning:
While the introduction situates the work within the literature, the manuscript relies heavily on Middleton et al. (2025, Science Advances), which has already analyzed the causal mechanisms of the same eddy breakup. To strengthen the impact, the authors should articulate more clearly what is genuinely new in this study compared to Middleton et al. (2025). For example, the use of surface drifters to resolve fine-scale divergence and vertical velocity, and the explicit quantification of wind-driven nonlinear Ekman pumping, could be emphasized as distinct contributions. Without this clarification, the present work risks being perceived as a secondary companion to the Science Advances article rather than an independent advance.2. Main objective:
In lines 50–58, the manuscript outlines the approach taken in this study. However, it would improve clarity if the authors explicitly formulated the study objectives (e.g., as a concise list). This would help readers clearly distinguish the novel aims of the paper and align them with the results and conclusions.
3. Previous history:
The authors inform that “The eddy could be identified in satellite chlorophyll concentration maps as early as 17 February 2022 and was targeted for intense sampling starting 23 February”. Could they provide an animation with the history of the eddy from the beginning with remote sensing pictures as Supplementary material?4. Eddy shape and Rossby number. Line 84
Could the authors justify how the eddy shape induces instability? Could they provide the Rossby number for this eddy?
5. Ekman pumping:
The manuscript introduces the relative wind stress formulation (lines 130), which is appropriate when surface currents are of comparable magnitude to the wind. However, it would be helpful if the authors quantified whether this was indeed the case during the studied event (e.g., comparing typical current speeds from drifters with wind speeds). Otherwise, the reader cannot assess whether the nonlinear Ekman pumping calculation captures a first-order effect or only a minor correction.
6. Vertical velocity estimation:
The derivation of vertical velocities from colocated divergence estimates (Eq. 11, Sect. 4.2) is central to the analysis. While the method is standard, the final estimates may be highly sensitive to errors in the horizontal divergence (e.g., due to drifter separation or noise). It would strengthen the manuscript if the authors quantified or at least discussed the associated uncertainties, so that readers can better assess the robustness of the reported magnitudes.
7. Figures:
Figures 5–9 are informative but quite dense. In some cases (e.g., panels b–d), it is difficult to visually link divergence, strain, and vorticity fields. A suggestion would be to add schematic overlays or arrows indicating the main features discussed in the text. Enlarging key panels could also improve readability. And zoom in the maps to present the information with larger detail might also help.8. Traceability of results:
The derivation of vertical velocities from divergence estimates (Eq. 11) is central to the analysis. While the method is standard, it would help readers if the uncertainties associated with this estimate (due to drifter separation, divergence noise, etc.) were quantified or at least discussed more explicitly.9. Biogeochemical implications:
The introduction briefly mentions the role of cyclones in nutrient pumping and carbon export. The conclusions could be strengthened by revisiting these implications in light of the observed upwelling/downwelling magnitudes and their potential impact on tracer transport.10. Link to M24:
There is a direct link to a manuscript that wasn’t unpublished at the time of the submission. I have found it published now. In any case, the continuous link to that manuscript is maybe excessive. For instance, the “summary and discussion” section is totally focused on a comparison with that manuscript and almost no other is mentioned (only Dauhajre and McWilliams, 2018). The manuscript would benefit from a major rewriting of this final section to provide a wider comparison with other manuscripts; moreover, that section might also compile the main limitations of the methodology applied and also state some final conclusions linked to the objectives of the manuscript.
Minor comments
- Page 3, l. 55: “drifters with drogues centered at 0.4 m and 0.75 m for the surface and at 15 m depth for the near-surface” → rephrase to avoid possible ambiguity (“surface drifters at 0.4–0.75 m, near-surface at 15 m”).
- Page 6, ll. 173 to 178 and elsewhere: CaC instead CARTHE and CODE
- How many drifters were deployed?
- Sections 5.1.2, 5.1.3, 5.1.4. Letters in the text referring to the panels in figures 7 and 8 are mostly wrong.
- Sections 5.1.2 to 5.1.5 provide a comparison with the M24 manuscript. As that would be expected for a discussion section, I suggest the authors to move those comments to the final Discussion section.
- Minor typos:
- l. 163: “aignificant” → “significant”.
- l. 259: “exteme” → “extreme”.
- l. 421: “velocitity” → “velocity”.
Citation: https://doi.org/10.5194/egusphere-2025-1475-RC2
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Review of the manuscript ‘Surface evolution and wind effects during a cyclonic eddy splitting event in the Balearic Sea’ by Donnet1 et al.
I find the manuscript well written and logically clear. The dynamics of the impact of the wind on the eddy split are also clear. I suggest publishing the manuscript with some minor revisions. My detailed comments are below.
References
Stern, M. E.: Interaction of a uniform wind stress with a geostrophic vortex, Deep Sea Res., 12, 355–367, https://doi.org/10.1016/0011-610 7471(65)90007-0, 1965.
Li, H., Xu, F., & Wang, G. (2022). Global mapping of mesoscale eddy vertical tilt. Journal of Geophysical Research: Oceans, 127, e2022JC019131. https://doi.org/10.1029/2022JC019131