Review of Hydrography and circulation below Fimbulisen Ice Shelf, East

Antarctica, from 12 years of moored observations

By Lauber et al

The authors describe in detail data from three moorings that were deployed in 2009 in the ocean cavity beneath Fimbulisen. Each mooring consisted of two sets of instruments, one within 50 m or so of the ice base, and the other somewhat deeper in the water column. The (up to 12-year long) time series were supplemented with ship-based data from up stream of the study sites: CTD sections and two moorings.

The authors find that the cavity is primarily ventilated by Winter Water (WW), but Antarctic Surface Water (ASW) intrudes along the ice base at particular times of the year, and that Warm Deep Water (WDW) can enter the cavity via warm-cored eddies. Various generation mechanisms for the eddies are discussed, although no evidence for any in particular is available from the data.

The data confirm and, to an extent, extend conclusions from previous studies that use shorter versions of the time series. As the salinity data were available only up until the end of 2010, the most detailed analysis was undertaken for the data from that year. This involved partitioning the water masses observed at the moorings using an Optimum MultiParameter (OMP) analysis with dissolved oxygen (DO), temperature (T) and salinity (S) as the parameters, with four end members (WW, WDW, ASW and glacial meltwater (GMW)).

This was a very detailed read, and I'm sure I managed to lose sight of many of the links that the authors make. By the very nature of the dataset, some of the conclusions are a bit speculative, and here and there the evidence base is weak. But those statements are appropriately caveated in the text, and significant effort has been made to quantify uncertainty.

I am attaching a marked up PDF that contains all my comments except for one major one given below. Most of the comments are minor suggestions for re-wording, or requests for re-phrasing to improve clarity. All can be readily dealt with by simple changes to the text.

Major comment.

I'm very concerned with the selection of ASW as an end member for the OMP analysis. Clearly, if ASW enters the cavity, and the authors do a good job of making the case that it does, then the analysis must handle it in some way. But ASW properties change during the year and it makes some of the longer term trends in Fig 6, for example, highly suspect. As the winter comes on, we would expect ASW to be cooled to the freezing point quite rapidly. In any case, there will be a strong seasonal signal to its properties. Therefore, the trends we see in Fig 6 are almost certainly a result of changes in the end member properties.

This is partly handled by the large uncertainty bounds that the Monte Carlo analysis places on the time series of concentrations, but the authors still refer to seasonal variation in the different water mass concentrations. Placing large uncertainty on the ASW end member properties doesn't solve the problem, as the properties move in a well-defined way within the box in properties space - essentially, the distribution is not Gaussian.

Perhaps the authors can explain their thinking on this point. This can be resolved by carrying out experiments to see the extent to which a likely seasonal profile in ASW properties would change end member concentrations. That would then indicate whether more caveats need to be declared.

Additional remark.

Identifying the participation of WDW and ASC in basal melting is useful, but much more useful would be an attempt to quantify the contributions of WW, WDW and ASW. Or at least place bounds on the contributions. It's likely that a model would be required to extrapolate from the mooring data, but it might be possible to make progress shy of a model.  

Review of Hydrography and circulation below Fimbulisen Ice Shelf, East Antarctica, from 12 years of moored observations, by Lauber et al.

I don’t need to be anonymous … Laurie Padman

Overall rating: I rated this "accepted subject to minor revisions" as I think the science is excellent and, despite all my comments, the presentation is good, my comments should all be fairly easily addressed, and the editor can assess whether the responses are sufficient. However, it is almost "major revisions", and I'm willing to re-review it.  

This manuscript describes an impressive record of data, often longer than a decade, from three moorings that were deployed in 2009 in the ocean cavity beneath Fimbulisen. Analyses of these records are supported by other data sets from outside the cavity.

The authors use these data, with Optimum MultiParameter (OMP) analyses, to identify source water masses that enter the cavity, and to explore their seasonality. They conclude that Winter Water (WW) in the most important inflow, but that there are seasonally varying contributions from Antarctic Surface Water (ASW) and Warm Deep Water (WDW). The latter appears to be caused by warm-core eddies.

I am including a marked-up PDF with all my comments, both minor and major. There are a lot of these!  However, the paper was well written and easy to read, and I think most comments are fairly “minor” and can be decided on quickly. Below, however, I will repeat some of the more major comments.

MAJOR COMMENTS (repeating Anon Ref. #1)

I agree with all major comments by Anonymous referee #1, and these are repeated here for emphasis.

“I'm very concerned with the selection of ASW as an end member for the OMP analysis. Clearly, if ASW enters the cavity, and the authors do a good job of making the case that it does, then the analysis must handle it in some way. But ASW properties change during the year and it makes some of the longer term trends in Fig 6, for example, highly suspect. As the winter comes on, we would expect ASW to be cooled to the freezing point quite rapidly. In any case, there will be a strong seasonal signal to its properties. Therefore, the trends we see in Fig 6 are almost certainly a result of changes in the end member properties.

This is partly handled by the large uncertainty bounds that the Monte Carlo analysis places on the time series of concentrations, but the authors still refer to seasonal variation in the different water mass concentrations. Placing large uncertainty on the ASW end member properties doesn't solve the problem, as the properties move in a well-defined way within the box in properties space - essentially, the distribution is not Gaussian.

Perhaps the authors can explain their thinking on this point. This can be resolved by carrying out experiments to see the extent to which a likely seasonal profile in ASW properties would change end member concentrations. That would then indicate whether more caveats need to be declared.

Additional remark.

Identifying the participation of WDW and ASC in basal melting is useful, but much more useful would be an attempt to quantify the contributions of WW, WDW and ASW. Or at least place bounds on the contributions. It's likely that a model would be required to extrapolate from the mooring data, but it might be possible to make progress shy of a model.”

MY GENERAL COMMENTS

1) Most of the "supplemental" figures are just as important to the flow of the paper as the "main" figures. I would prefer that they just get cited and appear in main text in the right place.

2) There is a lot of Introductory material in Results (section 4); e.g., the discussions about other data sets that explain what is already known about mWDW/WDW upwelling across sills, and the annual cycle of mWDW/WDW temperatures on the continental shelf. IMO, a better way to write the paper would be so tell us everything that is important, that is already known, setting up the rest of the paper to demonstrate the consequences of those external processes on what happens in the cavity, and surprises you find.

The problem the present format causes is that it is hard to tell what was previously known from prior studies, with what the new manuscript adds.  However, like I said, the paper is well-written, so unless you see an easy way to make this structural change, I wouldn't recommend it.

3) {really minor} I prefer citing as “BY {citation}” rather than “IN {citation}” wherever possible, to create the authors more for their past work.

4) I don’t really like the term “velocity shear” for what is really a “magnitude of the velocity difference”. Since the last expression is long, you could introduce a symbol and use that everywhere.

5) it is important that text and figures are consistent. Three examples: (a) you talk about “percent” of source water masses from OMP analyses, but figures are labeled with “Fraction”; (2) Fig. 7 has “Speed difference” on the y-axes, but the text always refers to “velocity shear”; (c) places in the annual cycle are always referred to by month name, but most time series of annual cycles use {1, 2, … 12}.

6) I mentioned three Ross Sea papers that are relevant to your comments on ASW: Porter et al. (2019), Tinto et al. (2019) and Stewart et al. (2019). You don’t need to cite these, but they might be worth looking at.

7) I’m not sure of the plan for the three Appendices: Will they be published in the main paper, or will there be Supplementary Online Material?  I think the SOM approach is cleaner; however, it’s a journal and editor issue. However, the flow of the paper sometimes relies on the reader jumping to an “Appendix” figure, and it’d be easier if anything that needed to be read sequentially was in Main Text figures. It’s okay to hide away things that aren’t needed to understand the science (provided the reader trusts the authors!), but important science content should not be in non-main-text figures.

MY MAJOR COMMENTS

1) You need to be *very* clear, at all times, whether you are referring to the presence of a water mass, or the presence of a contribution from a water mass. e.g., no sub-ice-shelf moorings show WDW, so it is wrong to talk about WDW being present. But the OMP analysis finds a fraction of WDW, and so maybe mWDW intrudes *after* production involving WDW (and WW) offshore. This might take a while: every sentence involving a water mass name needs to be checked.

2) The introductory “map” figure needs to be improved. There are too many features that are discussed which are not included in Fig. 1a. In addition, I have recommended adding a new first section to Data and Methods, “2.1 geometry”, that describes (briefly) where sub-ice-shelf bathymetry and ice draft come from, and shows bathymetry, ice draft, and water column thickness separately.

3) You should probably define the T, S and DO ranges for mWDW. Often, I think you are referring to presence of mWDW but you describe it as WDW (which it is too cold for). mWDW is useful to define since it is a “water mass” that is found in the cavity, even though its heat content is all from the WDW source water mass.

4) Sometimes you claim that the expected inflow path is following water column thickness (wct), sometimes following bathymetry. You note that this is seasonal (depending on baroclinicity), but any statement about expected flow path should, therefore, specify what season you're talking about. Or, possibly, in this region the bathymetry and wct are closely aligned, so it doesn’t matter?  But, especially if that's the case, an extra figure devoted to geometry (bathy, draft and wct) is needed.

5) As an example of mixing water masses with fractions of source water types, on line 287 you state “and cold/oxygen-rich WW, which is the most abundant water mass at all sub-ice-shelf instruments.” But … this is not true, right?  For all upper sub-ice-shelf instruments, ISW dominates. WW is probably the dominant "source water" mass (as ISW is mostly a lot of WW and a little GMW), but it isn’t the dominant water mass.