the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Feb 2025
Merging of a mesoscale eddy into the Lofoten Vortex in the Norwegian Sea captured by an ocean glider and SWOT observations
Abstract. The Lofoten Vortex (LV) is an intense, apparently permanent anticyclone in the Lofoten Basin of the Norwegian Sea. It is characterised by a 1200 m thick core of Atlantic Water, with a radius of 15–20 km, in nearly solid-body rotation reaching speeds up to 0.8 m s-1. Potential vorticity in the core is nearly two orders of magnitude lower than the surroundings, creating a barrier to lateral mixing. It has previously been postulated that anticyclonic eddies in the Lofoten Basin, shed from the eastern branch of the Norwegian Atlantic Current along the Lofoten Escarpment, merge into the LV, contributing to maintaining its large heat and salt content and energetics, but such merging events have proven difficult to observe directly due to their transient and unpredictable nature. In April 2023, an eddy merger event was successfully observed using a combination of in-situ data from an autonomous ocean glider and absolute dynamic topography (and derived velocities) from the fast sampling calibration phase of the Surface Water Ocean Topography (SWOT) satellite altimeter. During the observed merging process an incoming eddy gradually approaches the LV, then elongates as the two begin to corotate and then merge, with a corresponding spin up of vorticity and eddy kinetic energy and possible ejection of low potential vorticity water from the merged LV core. The incoming eddy had a smaller radius and higher Rossby number than the LV. It has a similar density range as the LV and therefore a double-core vertical structure did not form after the merger. During the observed period, merging eddies were the dominant process affecting the evolution of the LV, clearly outweighing vertical 1D processes due to atmospheric forcing and lateral mixing between the LV core and the outer rim. Through influx of buoyant waters, spin-up of eddy kinetic energy and increasingly anticyclonic vorticity, eddy mergers contribute to the longevity of the LV.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Ocean Science.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2025-433', Anonymous Referee #1, 15 Mar 2025
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I have read “Merging of a mesoscale eddy into the Lofoten Vortex in the Norwegian Sea captured by an ocean glider and SWOT observations”, by Damerell et al. The manuscript details the successful observation of an eddy merging event within the Lofoten Vortex (LV) in April 2023, using data from an ocean glider and the SWOT satellite. The merger resulted in increased vorticity and eddy kinetic energy. This study provides in-situ evidence supporting the hypothesis that anticyclonic eddies from the Norwegian Atlantic Current contribute to the LV’s heat, salt content, and energy. The findings suggest that eddy mergers play a dominant role in the LV’s evolution and persistence, surpassing the influence of atmospheric forcing and lateral mixing.
Scientific Significance:
The manuscript presents new in-situ and satellite data that provide valuable insights into the mechanisms sustaining the Lofoten Vortex (LV). The findings support previous hypotheses that eddy mergers play a crucial role in maintaining the LV by offering compelling new evidence of such an event. The conclusions are well-supported by the data.
Scientific Quality:
The study employs a valid scientific approach with appropriate methods. The manuscript references relevant literature and provides substantial background information. The processing of glider and SWOT data appears thorough, and dynamical considerations, including cyclogeostrophic balance, are properly accounted for. The analysis effectively supports the conclusion that an eddy merger occurred during the study period.
However, the manuscript lacks an error analysis, which would significantly strengthen the study. Addressing potential sources of error—such as those in the glider data, cyclogeostrophic velocities, potential vorticity, and radial gradients—would enhance the manuscript’s rigor. Including an error analysis would also provide valuable guidance for future studies integrating glider data with SWOT observations.
Presentation Quality:
The manuscript's presentation could be improved. While the introduction provides excellent background information, its structure is disorganized, with topics presented in a scattered manner. The lack of clear connections between key points weakens the manuscript’s motivation. A more coherent and structured introduction would greatly improve readability and strengthen the overall impact of the study. I strongly encourage the authors to refine this section to enhance clarity and cohesion.
The figures in general are ok, with good text size and readability, but I found some things hard to understand:
Fig. 3 – This takes up a lot of real estate for useful but not essential information.
Fig 4 – Can the authors please discuss what smoothing methods they use? Why not just show raw or binned data here? I think the discrete colorbars smear the data resolution and this muddle the results.
Fig 5 – the green line is difficult to see. So are the red markers. One suggestion would be to change the color of the green line, and possibly make marker sizes larger, or make them the same shape but different colors.
Fig 6. I didn’t find the figure useful and was difficult to read. I found that references to this figure in the manuscript were easier to see in Fig. 5 – I suggest removing this or putting it in the supplementary section.
Eq 3- Do you use this? Or so you use the cyclogeostrophic vorticity from swot? Can you clarify this please?
Eq. 5 – I don’t think this needs its own equation.
Citation: https://doi.org/10.5194/egusphere-2025-433-RC1 -
RC2: 'Comment on egusphere-2025-433', Anonymous Referee #2, 18 Mar 2025
reply
This paper presents in situ observations of a vortex merger in the Lofoten Basin, based on a combination of glider and altimetric data. The contribution of this study to scientific progress is substantial and twofold. First, it provides a 3D in situ observation of an asymmetric vortex merger in the ocean with unprecedented resolution. Second, it offers new insights into the mechanisms that sustain the quasi-permanence of the Lofoten Vortex, confirming that vortex mergers play a crucial role, as previously suggested.
I recommend this article for publication in Ocean Sciences, with only minor revisions to improve the clarity of the presentation.
General comments:
-The manuscript does an excellent job of reviewing and presenting current knowledge on Lofoten Vortex dynamics. However, it lacks even a brief introduction to the broader context of vortex mergers. Since the 1980s, oceanographers have extensively studied vortex mergers (e.g., Polvani et al. 1989, https://doi.org/10.1017/S0022112089002016). This study is highly significant for that research community, as it provides a rare 3D in situ observation of a phenomenon usually examined through idealized simulations.
I recommend adding a short paragraph in the introduction to introduce this field of study, as well as incorporating a discussion of the present observations in the context of vortex merger research. The authors could refer to the literature cited in Carton et al. 2016.-The manuscript's clarity could be improved by using shorter, more concise sentences throughout the text. Also please check the synthax and the vocabular you use for the vortex merger; examples, L301 "more strongly anticyclonic" is poorly said, or L304 "dissipating [...] while reinforcing[...]", no it's an exchange of mass.
-Could the authors consider adding subsesction in the result section? It would greatly improve the clarity of the different messages of the manuscript
-In my opinion, if Figs. 5,6,7,8 were placed in appendix, the manuscript would gain clarity. Then, putting in regards Figs 4 and 9 would be easier, and enough to describe the merger.
Specific comments:
L7 (and all the manuscript) : in-situ/in situ -> \textit{in situ}. Also italized all latin expressions (e.g., i.e.,...)
Fig. 1: it could be useful to add a schematic of LV and small eddies shed from the NwASC in Fig. 1
Authors could use the newly released v2.0 of SWOT data for the study
A T/S diagram for each realisation could be added in Fig. 4, where the water masses "inside" LV and Eddy B are identified.
The authors are not clear (or maybe I missed it) in exlpaining why the azimuthal velocity of the LV is way larger in realisation 3 than realisation 1
Citation: https://doi.org/10.5194/egusphere-2025-433-RC2
Data sets
Physical oceanography data from Seaglider missions in the Lofoten Basin, Norwegian Sea, January - November 2023 Gillian M. Damerell, Ilker Fer, Ailin Brakstad, and Fiona Elliott https://doi.org/10.21335/NMDC-440347600
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