Subsurface floats in the Filchner Trough provide first direct under-ice tracks of eddies and circulation on shelf

Sallée, Jean-Baptiste; Vignes, Lucie; Minière, Audrey; Steiger, Nadine; Pauthenet, Etienne; Lourenco, Antonio; Speer, Kevin; Lazarevich, Peter; Nicholls, Keith

doi:https://doi.org/10.5194/egusphere-2023-2952

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https://doi.org/10.5194/egusphere-2023-2952

Preprints

11 Dec 2023

| 11 Dec 2023

Subsurface floats in the Filchner Trough provide first direct under-ice tracks of eddies and circulation on shelf

Jean-Baptiste Sallée, Lucie Vignes, Audrey Minière, Nadine Steiger, Etienne Pauthenet, Antonio Lourenco, Kevin Speer, Peter Lazarevich, and Keith Nicholls

Abstract. Bottom water formation in the Weddell Sea and mass loss from the Filchner-Ronne Ice Shelf are tightly linked by the supply of Warm Deep Water to the continental shelf. Heavy sea ice cover and icebergs restrict ship access and upper ocean measurements by moorings, compelling us to try new sampling methods. We present results from the first dedicated under-sea-ice float experiment tracking circulation on the continental shelf between the Brunt Ice Shelf and Filchner Ice Shelf. Seven Apex profiling floats were deployed in 2017 at three different locations, targeting the sources of modified Warm Deep Water (mWDW) inflow and Ice Shelf Water (ISW) circulation in the Filchner Trough. The floats capture a warm mWDW regime with southward inflow over the eastern continental shelf and a cold ISW regime with a recirculation of ISW in the Filchner Trough throughout the four years of observations. The mWDW flowing onto the continental shelf follows two pathways: the eastern flank of the Filchner Trough and via a Small Trough on the shallow shelf farther east. In the present circulation regime, this warm water is blocked from reaching the ice shelf cavity due to the presence of the thick ISW layer inside the Filchner Trough. The floats' trajectories and hydrography reveal the dynamically active front, flow reversal, and eddy generation between these two water masses along the eastern flank of the Filchner Trough.

Received: 07 Dec 2023 – Discussion started: 11 Dec 2023

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18 Oct 2024

Subsurface floats in the Filchner Trough provide the first direct under-ice tracks of the circulation on shelf

Jean-Baptiste Sallée, Lucie Vignes, Audrey Minière, Nadine Steiger, Etienne Pauthenet, Antonio Lourenco, Kevin Speer, Peter Lazarevich, and Keith W. Nicholls

Ocean Sci., 20, 1267–1280, https://doi.org/10.5194/os-20-1267-2024,https://doi.org/10.5194/os-20-1267-2024, 2024

Short summary

Jean-Baptiste Sallée, Lucie Vignes, Audrey Minière, Nadine Steiger, Etienne Pauthenet, Antonio Lourenco, Kevin Speer, Peter Lazarevich, and Keith Nicholls

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2952', Anonymous Referee #1, 17 Jan 2024

See attached.

Citation: https://doi.org/10.5194/egusphere-2023-2952-RC1
- AC2: 'Reply on RC1', Jean baptiste SALLEE, 03 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2952/egusphere-2023-2952-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-2952-AC2
RC2:
'Comment on egusphere-2023-2952', Anonymous Referee #2, 02 Feb 2024

Subsurface floats in the Filchner Trough provide first direct under-ice tracks of eddies and circulation on shelf
by Jean-Baptiste Sallee, Lucie Vignes, Audrey Miniere, Nadine Steiger, Etienne Pauthenet, Antonio Lourenco, Kevin Speer, Peter Lazarevich, and Keith W. Nicholls

--- Summary ---
Sallee et al present a fascinating study of trajectories and watermass transport implications from 7 RAFOS-enabled profiling floats on the eastern Weddell Sea continental shelf of Antarctica (Filchner Trough region). The floats operated in a mode similar to floats in the global Argo array but with a 5-day profiling interval (daily profiling in the summer), drifting at 250m or 400m between profiles and receiving RAFOS positions up to 4 times daily, with total float lifetime ranging from 1 to 5 years. Acquiring this dataset is in itself an impressive accomplishment.

--- General Comments ---
The manuscript's presentation of the scientific context, the float experiment and results, and the discussion of implications is well done and informative. Figures are generally clear. In particular, the floats are simultaneously able to illustrate flow pathways and the evolution and variability of layer temperature and thickness along those pathways. Overall, not a lot of changes are needed before publication, although I do have a number of suggestions and questions which the authors may choose to address.
Although the discussion of pathways and transport mechanisms is interesting and informative, there has not been an attempt to make quantitative estimates of volume or heat transport or diffusive watermass transformations. Clearly any attempt to do this would be subject to some guesses at unmeasured quantities (such as flow width or duration), but even very rough values would be helpful for comparing these observations to numerical models or shipboard or moored measurements.
In one especially interesting and curious feature of Fig.8 (the comparison of a float's measured trajectory and temperature with the progressive-vector trajectory and temperature from a mooring), the temperature at the float made a sudden change immediately before reaching the mooring. Is there any suggestion of how this might have happened (either through diffusive heating or non-Lagrangian movement of the float)? The change brought the float measurement up to the mooring's temperature just before the closest approach, but the warm temperature had been present at the mooring considerably earlier. Later, the warm patch seems to leave the mooring but the float continues to follow it.
The authors make the intriguing suggestion that ISW could block the heat flux of mWDW toward the FRIS. Is this energetically possible? (Meaning, what is the source of energy maintaining the potential energy barrier of the dense water and accompanying geostrophic front?) Or is this really a statement about a different cause? For example, that the atmospheric cooling that produces the ISW (say close to the ice face) removes all heat before it can reach the cavity? Or simply that the amount of on-shelf mWDW transport and heat flux here are small relative to the West Antarctic shelf?

--- Specific Comments ---
The bathymetry in Fig 1 is difficult to make out against the temperature shading. Would a light color work better? Also some bathymetric contour labels would be helpful. The bathymetry is presented in a more readable form in other panels, but there is no definitive label of any isobath and the colorbar is too finely graded to help. How about making one isobath (e.g. 300m or 400m) thicker than the others to provide at least one clear reference (along with the fixed 100m contour interval).
Also on the subject of bathymetry, I'm curious whether the floats drifting at 400m ever got stuck on the bottom during their drift phase. From counting the number of 100m-interval contours on the shelf east of Filchner Trough (e.g., in Fig.3), most of the shelf appears to be shallower than 300m. However, the depth-time plots in Fig.4 show water mostly between 400m and 500m on the shelf. Does the float-inferred bathymetry actually match the mapped contours? Or is the shallowest contour shown not the 100m one?
I suspect that a T-S diagram would help clarify some of the discussion of watermasses and layers. And coloring by or otherwise indicating variations in latitude or water depth might be a good way to illustrate the location, density range, and T-S characteristics of the ISW and mWDW watermasses and the front between the two.
In addition, it would be useful to mark some candidate isopycnals (e.g., important ones for mWDW and/or ISW) on the depth-time plots (figs 4,5,7).
Ending the Fig.4 trajectories on the shelf but continuing the timeseries off the shelf is confusing (and it is difficult to tell whether this is a choice that has been made deliberately or due to a lack of GPS data). Is there at least an approximate trajectory or direction of the floats that left the shelf (12682 and 12703) that could be indicated? They must have reached the surface at some point to send the data back.

--- Minor Comments/Typos ---
l.9 typo: Pobservations
l.202-210. The description of the southern float trajectories is interesting in it's comparison to the behavior of the eastern floats but should also re-iterate the fact that these floats parked at a different depth (250m) than the others (400m).
Fig.3 caption lists different colors for the floats than the legend in the figure. Which is correct? (I'm guessing it's the legend.)
Fig.4 mentions float 12672 but probably means 12682
"Hovmoller diagram" is usually not what a timeseries of vertical profiles is called (at least in oceanography). Not sure whether there's a definitive definition, though. "Depth vs. time" or "profile timeseries" seems more common for the types of plots shown in Figs 4, 5, and 7. Usually Hovmoller is reserved for horizontal distance vs. time (and in particular as a way to emphasize wave propagation that might be seen in an animation of 2D structures changing in time).
Fig.6 shows temperature on the 27.75 isopycnal (mWDW). Can this isopycnal be added to Figs 4+5?
Fig.7 shows ISW layer thickness. Mention the definition of this layer as water colder than -1.9C in the caption.

Citation: https://doi.org/10.5194/egusphere-2023-2952-RC2
- AC1: 'Reply on RC2', Jean baptiste SALLEE, 03 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2952/egusphere-2023-2952-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-2952-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2952', Anonymous Referee #1, 17 Jan 2024

See attached.

Citation: https://doi.org/10.5194/egusphere-2023-2952-RC1
- AC2: 'Reply on RC1', Jean baptiste SALLEE, 03 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2952/egusphere-2023-2952-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-2952-AC2
RC2:
'Comment on egusphere-2023-2952', Anonymous Referee #2, 02 Feb 2024

Subsurface floats in the Filchner Trough provide first direct under-ice tracks of eddies and circulation on shelf
by Jean-Baptiste Sallee, Lucie Vignes, Audrey Miniere, Nadine Steiger, Etienne Pauthenet, Antonio Lourenco, Kevin Speer, Peter Lazarevich, and Keith W. Nicholls

--- Summary ---
Sallee et al present a fascinating study of trajectories and watermass transport implications from 7 RAFOS-enabled profiling floats on the eastern Weddell Sea continental shelf of Antarctica (Filchner Trough region). The floats operated in a mode similar to floats in the global Argo array but with a 5-day profiling interval (daily profiling in the summer), drifting at 250m or 400m between profiles and receiving RAFOS positions up to 4 times daily, with total float lifetime ranging from 1 to 5 years. Acquiring this dataset is in itself an impressive accomplishment.

--- General Comments ---
The manuscript's presentation of the scientific context, the float experiment and results, and the discussion of implications is well done and informative. Figures are generally clear. In particular, the floats are simultaneously able to illustrate flow pathways and the evolution and variability of layer temperature and thickness along those pathways. Overall, not a lot of changes are needed before publication, although I do have a number of suggestions and questions which the authors may choose to address.
Although the discussion of pathways and transport mechanisms is interesting and informative, there has not been an attempt to make quantitative estimates of volume or heat transport or diffusive watermass transformations. Clearly any attempt to do this would be subject to some guesses at unmeasured quantities (such as flow width or duration), but even very rough values would be helpful for comparing these observations to numerical models or shipboard or moored measurements.
In one especially interesting and curious feature of Fig.8 (the comparison of a float's measured trajectory and temperature with the progressive-vector trajectory and temperature from a mooring), the temperature at the float made a sudden change immediately before reaching the mooring. Is there any suggestion of how this might have happened (either through diffusive heating or non-Lagrangian movement of the float)? The change brought the float measurement up to the mooring's temperature just before the closest approach, but the warm temperature had been present at the mooring considerably earlier. Later, the warm patch seems to leave the mooring but the float continues to follow it.
The authors make the intriguing suggestion that ISW could block the heat flux of mWDW toward the FRIS. Is this energetically possible? (Meaning, what is the source of energy maintaining the potential energy barrier of the dense water and accompanying geostrophic front?) Or is this really a statement about a different cause? For example, that the atmospheric cooling that produces the ISW (say close to the ice face) removes all heat before it can reach the cavity? Or simply that the amount of on-shelf mWDW transport and heat flux here are small relative to the West Antarctic shelf?

--- Specific Comments ---
The bathymetry in Fig 1 is difficult to make out against the temperature shading. Would a light color work better? Also some bathymetric contour labels would be helpful. The bathymetry is presented in a more readable form in other panels, but there is no definitive label of any isobath and the colorbar is too finely graded to help. How about making one isobath (e.g. 300m or 400m) thicker than the others to provide at least one clear reference (along with the fixed 100m contour interval).
Also on the subject of bathymetry, I'm curious whether the floats drifting at 400m ever got stuck on the bottom during their drift phase. From counting the number of 100m-interval contours on the shelf east of Filchner Trough (e.g., in Fig.3), most of the shelf appears to be shallower than 300m. However, the depth-time plots in Fig.4 show water mostly between 400m and 500m on the shelf. Does the float-inferred bathymetry actually match the mapped contours? Or is the shallowest contour shown not the 100m one?
I suspect that a T-S diagram would help clarify some of the discussion of watermasses and layers. And coloring by or otherwise indicating variations in latitude or water depth might be a good way to illustrate the location, density range, and T-S characteristics of the ISW and mWDW watermasses and the front between the two.
In addition, it would be useful to mark some candidate isopycnals (e.g., important ones for mWDW and/or ISW) on the depth-time plots (figs 4,5,7).
Ending the Fig.4 trajectories on the shelf but continuing the timeseries off the shelf is confusing (and it is difficult to tell whether this is a choice that has been made deliberately or due to a lack of GPS data). Is there at least an approximate trajectory or direction of the floats that left the shelf (12682 and 12703) that could be indicated? They must have reached the surface at some point to send the data back.

--- Minor Comments/Typos ---
l.9 typo: Pobservations
l.202-210. The description of the southern float trajectories is interesting in it's comparison to the behavior of the eastern floats but should also re-iterate the fact that these floats parked at a different depth (250m) than the others (400m).
Fig.3 caption lists different colors for the floats than the legend in the figure. Which is correct? (I'm guessing it's the legend.)
Fig.4 mentions float 12672 but probably means 12682
"Hovmoller diagram" is usually not what a timeseries of vertical profiles is called (at least in oceanography). Not sure whether there's a definitive definition, though. "Depth vs. time" or "profile timeseries" seems more common for the types of plots shown in Figs 4, 5, and 7. Usually Hovmoller is reserved for horizontal distance vs. time (and in particular as a way to emphasize wave propagation that might be seen in an animation of 2D structures changing in time).
Fig.6 shows temperature on the 27.75 isopycnal (mWDW). Can this isopycnal be added to Figs 4+5?
Fig.7 shows ISW layer thickness. Mention the definition of this layer as water colder than -1.9C in the caption.

Citation: https://doi.org/10.5194/egusphere-2023-2952-RC2
- AC1: 'Reply on RC2', Jean baptiste SALLEE, 03 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2952/egusphere-2023-2952-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-2952-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Jean baptiste SALLEE on behalf of the Authors (03 Apr 2024) Author's response Manuscript

EF by Lorena Grabowski (05 Apr 2024) Author's tracked changes

ED: Referee Nomination & Report Request started (09 Apr 2024) by Ilker Fer

RR by Anonymous Referee #1 (28 Apr 2024)

Suggestions for revision or reasons for rejection

I'm quite satisfied with the author's reponses to my review. I'm happy to recommend publication after addressing the minor typographic and clarifying issues listed below. The figures in particular are beautiful. I also really appreciate that the authors clearly separate their description of the observations from their interpretations of the meaning of those observations. Overall I think this is tightly argued and well presented piece of work on an important region.

10, omit "via"
54, "time series"
55, "flow onto the continental shelf and subsequently southward" would be better
59, I think that the authors are saying that the western pathway is estimated to contribute about 1/2 of the heat that is delivered to the FRIS; in this case, the "at the shelf break" could be omitted for clarity, or some other slight rewording
59, please provide a reference for the asserting beginning "CTD observations..."
67, I believe what is meant here is "Thus far, mWDW has only been observed at the ice fron thear 79 S once, in 2013". It's a little unclear what the "only" is intended to mean.
75, "sea ice"
77, "water masses"
80, Argo floats would not be suitable for a shelf study because of their typically deep profile depth, so it would be better to simply say "Autonomous instruments"
89, "infer" or "triangulate" would be better than "provide"
92, I think what the authors are meaning to say is that the approach taken in the present study is similar to that taken in a recent study in a different region, but that is not coming through in the current wording.
100, since you've aleady introduced the term Small Trough, perhaps use it here?
101, "but never directly measured" is better
109, "sea ice"
136, "transmitted a frequency-modulated signal" (no with) is better
140 and afterward, since TOA is used to refer to multiple times of arrive, I believe it should be written as TOAs is almost all cases (line 148 and 149 are exceptions)
150, "unrealistic records were removed", "time series"
151, please specify the bandwidth of the LOESS filter for reproducibility
157, "postition was estimated" or "inferred", not "computed"
164, it is not clear if "smoothed trajectories obtained from ... " is referring to the process just decribed. If it would be better to just say it like that. "raw TOA" is confusing because it sounds like you are now referring to a different processing
165, "time series"
175, "hydrographic"
177, Can you reword this without saying layers twice?
183, "sea-ice-covered" I believe
184, six hours is not the frequency; it is the sampling interval.
186, "open ocean"; "transmitting" is better than "emitting"
193, "the Small Trough"
217, "and a recirculation"
222, "regime of recirculation mWDW on the eastern shallow continental shelf" is better
225 & 226, "water mass"
232, "appears to be more efficient... compared to" is better
239, The clause after the colon is about transit time is awkward and would be better as an independent sentence that clarifies the meaning of "about two months"
242, "time series"
245, "mWDW range"
248, "that this region is the site of" or "that in this region there is" are better, since it is not the region that poses.
250, "that is dominated by"
256, "over the eastern flank of the Filcher trough" can be omitted.
258, ", however, "
259, ", hence turbulence mixing, "
275, "with a timescale" is probably what is meant hear, as "within" would mean two months or less
278, "available" is better than "existing"
286, semicolon should be "and". "in which" would be better than "where" as it would clarify that you are explaining the meaning of the mode change.
292, "were" is better than "got"
299, "water masses"
300, "sea-ice-covered"
308, "directly observed" is better than "demonstrated"
314, "Limitations... do not"
323, "out of the trough" can be omitted
329, perhaps emphasize at the end of this sentence that there would then be a associated impact on the heat fluxes to the base of the ice shelf.
332, "recirculating" is better than "convoluted"; the latter is more often used to denote logically muddled than physically wiggly
334, It is not clear what "irregular positioning" means since "positioning can mean the act of inferring a position"; perhaps "available positions" is what is meant.
345, ", however,". "motion along"
346, "hence turbulent mixing, "
349, "first Lagrangian observations over the Filchner–Ronne ice shlef were acquired" is I think what is meant.
351, "sea ice". The phase after the colon is awkward and would be better phrases as "an eastern passage..., and a western passage..."
359, "time scale". "up to four years" is all that can be said based on the data, not "up to at least".
373, "float data" not "floats data"
375, "Float" is not capitalized.
388, journal name capiltalization
390, missing journal name
394, journal name capiltalization
403, missing journal name
433, article title capitalization
448, Southern Ocean
466, journal name capiltalization
477, missing journal name, tech report details, or link
486, article title capitalization
497, missing journal name

Table 1, Please indicate whether the time given is UTC, local time, etc.

Figure 1, "thence" needs to be unpacked into more words to explain the deeper contours. "Lambert" should be capitalized.

Figure 2, what does "maximum" mean here? I believe you mean that for all position fixes associated two or more TOA records, the distance from the float to the farthest sound source is reported, but that's coming through at the moment.

Figure 3, a minor point, the cross are not very visible; small black dots or filled circles might be better. No hyphen after "S" in "T-S diagram". "estimated as the maximum profile depth" would be better than "inferred from the profile depth." Is the colorbar axis in the right panel the same as that used in the left panel? I'm seeing a lot more dark blue on the right then I would expect to see based on the left, suggesting perhaps the colorbar axes not the same. In this and future maps, heavy gray lines are float tracks while light gray lines are isobaths corresponding to changes in the shading color. You might suppress the latter as they are not really needed; otherwise, in this and following maps, please clarify that it is the heavy gray lines that are float tracks.

Figure 4, the meaning of "for" in "for the ISW" and "for the mWDW" is not clear. Something like, e.g. "marking a typical value for" would be better. Second to last sentence, I believe "colored circles" should be "colored triangles".

Figure 8, in the last sentence, the construction "black (mooring) and green (float) circles is confusing; it would be better to first tell us about the black circles and then about the green circles.

Hide

RR by Anonymous Referee #2 (16 May 2024)

ED: Publish subject to minor revisions (review by editor) (16 May 2024) by Ilker Fer

AR by Jean baptiste SALLEE on behalf of the Authors (28 May 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (04 Jun 2024) by Ilker Fer

AR by Jean baptiste SALLEE on behalf of the Authors (10 Jun 2024)

Journal article(s) based on this preprint

18 Oct 2024

Subsurface floats in the Filchner Trough provide the first direct under-ice tracks of the circulation on shelf

Jean-Baptiste Sallée, Lucie Vignes, Audrey Minière, Nadine Steiger, Etienne Pauthenet, Antonio Lourenco, Kevin Speer, Peter Lazarevich, and Keith W. Nicholls

Ocean Sci., 20, 1267–1280, https://doi.org/10.5194/os-20-1267-2024,https://doi.org/10.5194/os-20-1267-2024, 2024

Short summary

Jean-Baptiste Sallée, Lucie Vignes, Audrey Minière, Nadine Steiger, Etienne Pauthenet, Antonio Lourenco, Kevin Speer, Peter Lazarevich, and Keith Nicholls

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In the Weddell Sea, one of the regional seas fringing Antarctica, we investigated how warm deep currents and cold waters containing freshwater released from the Antarctic are connected. We used autonomous observation devices never been used in this region previously that allow us to track the movement and characteristics of water-masses under the sea ice. Our findings show a dynamic interaction between warm-masses, providing key insights to understand climate-related changes in the region.


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