Reply to comments from Seelye Martin

First, I’d like thank Dr. Martin for his positive and helpful comments on this paper. The bold elements below note explicit statements about how I will address his suggestions in the revision.

I’d also like to thank Dr. Martin for pointing out the lack of my referencing in the text of Parkinson (1990). I had intended to make the points raised by Dr. Martin – hence the presence of the reference in the reference list – but the writing of the paper was spread out over several months so by the time I completed it I had forgotten that this had not been added. I will add suitable comments in the Introduction and the Discussion sections looking back to the Parkinson reference to extensive sea-ice in the Weddell Sea.

With respect to the embargoed supplementary data, my cover letter to the journal contained information on how the reviewers could register with the UK Polar Data Centre to access this data. I refer the reviewers to the journal editorial staff if they wish to access the data at this time. I will have the UKPDC remove the wider embargo once the paper is accepted. This approach is normal practice according to the UKPDC. Please note that Bigg (2024) is already in the reference list.

With respect to the iceberg part of the dataset I will  amend the text at l. 139-140 as suggested by Dr. Martin. I split the record into three categories, as noted by Dr. Martin. The category 3 entries (iceberg fields) are used in the lower panel of Figure 6 (I will add this note to the Figure 6 legend). In Figure 5, I decided for clarity in an already busy figure to only note the presence of icebergs. This is the summary iceberg figure, and the detail of iceberg density is better left to Figure 6, for comparison with the modern observations. I stand by this division, which I think makes the paper clearer.

Specific Concerns

My purpose here was to compare the records of the two Antarctic circum-navigations of the early historical period, rather than to concentrate on one part of the Southern Ocean. The earlier paper by Love and Bigg (2023), referenced in this paper, covered the Weddell Sea in the 1820-1845 period, and included sea-ice data from Weddell’s expedition. Fig. 1 in Love and Bigg shows that Weddell didn’t reach as far east as the sea-ice peak region recorded by Cook and Bellingshausen. Dr. Martin is right that more could be said about the 1820s record in the Discussion and I will add an additional couple of sentences in the revision on what Love and Bigg found of relevance here.

I agree with Dr. Martin that Wales’ journal of Cook’s expedition needs to be checked for the iceberg data. It is worth noting however that while there are a few occasions where Forster recorded icebergs and Cook’s journal did not, on the whole the two records were very similar so I suspect that Wales’ journal is unlikely to add too much.

1. 32: I think it is an interesting context that a British expedition was seeking the North Pole at the same time as Cook’s expedition south, but that they seem not to be linked. While of tangential importance to the paper overall I believe it is an important contextual observation regarding contemporary polar expeditions and do not wish to remove it.
2. 86-88: My point here was that the other nineteenth century expeditions were more geographically focused and not circum-Antarctic in concept. Therefore they do not fit within the context of this paper. I stand by this statement. One has to define limits to any specific work or it could lose focus.
3. 69: apologies. This should be section 2.1. I will correct this in the revision.

General question: I did try to consider color blindness in my colour choice. I think I have largely succeeded, but in some cases there were a lot of lines to consider.

1. 115: thank you for the positive comment on the chronometer accuracy.
2. 143: I agree this text should be changed as suggested in the revision.
3. 208: The Comiso and Worby result doesn’t have a significant impact on the analysis (note the large error bar). I note this in the second phrase of the sentence.

Figure 1: The fish-hook south of Africa is an illusion. The outbound green line starts only a few days before ice was observed, and so is well south of Africa at the top if the fish-hook, while the inbound green line goes all the way to Cape Town. I will add a note to the Figure legend explaining this.

Figure 2: I agree this Figure could be confusing. I will put all the colour (Cook is red Xs, Bellingshausen is blue crosses) and shape information into the Figure legend in the revision.

Figure 3: The purpose of this figure is to demonstrate that there may be open water or polynyas south of the northern sea ice limit, particularly, but not exclusively, in the Weddell Sea. I tried to find an example where this was most obvious, and related to one of the observation days from both Cook and Bellingshausen’s journals. The text does imply this, but is not too clear, so I suggest I add some explanatory text to the Figure legend in the revision.

Figure 4: I welcome the helpful comments regarding this pivotal figure. In the revision I will extend the y-axis to 50S and consider a polar projection, although this might be difficult to do. I will certainly alter the size of the symbols so that the historical and modern observations are the same size. Currently, the historical ones are the larger ones. I will experiment with the colours (the current colours were an attempt to implement the color blindness guidelines of the journal template …). I think adding the Parkinson ice tongue would complicate an already problematic figure – I will note this in the Discussion, as noted above. I will also clarify the text at l. 216 to make the assertion clearer.

Figure 5: I will put all the information about the figure in the legend in the revision. This is a summary iceberg figure – I use the extreme iceberg field data in the lower panel of Figure 6 and suggest this is a clearer way of allowing the reader to see the difference.

Figure 6: I can change the panels to be Figure 6a and b, as suggested, both in the Figure and references in the text, in the revision. I will add a note to the legend to explain that the bottom panel is indeed showing the category ‘3’ iceberg data.

Figure 7: Again, I will put all the figure colour and symbol information in the legend, rather than mix between legends on and off figure, in the revision. The iceberg data is the same as in Figure 5, but it is true that some ship observations go further ast than the Tournadre data of Figure 6 suggests. I think this is because the ship data goes further back in time and will add such an explanatory note to the text discussion in the revision. I do currently note in the Conclusion that marine ice (ie sea-ice and icebergs) suggest a colder Weddell Sea in the past than today, but will make it more explicit in the revision that it is both icebergs and sea-ice that show this eastwards trend.

Reply to comments from Irina Rogozhina

First, I’d like thank Dr. Rogozhina for her positive and helpful comments on this paper, and very much thank her for stepping in to provide a review when the original second reviewer became unavailable.

Major Concerns

The Introduction section was difficult to write as it needs to combine background with some discussion of the two expeditions’ timeframes and comparisons. The suggestion of adding a Table to make the timings of the different ship voyages clearer is an excellent one. The combination of such a Table with more explicit reference to Figure 1 in the Introduction would make the setting of the scene better and shorter.

I thank Dr. Rogozhina for pointing out that the modern data methods are not given sufficient background. I agree with her, on re-reading the paper. I think there is some risk of duplicating material if too much is added to the Introduction, while Sections 2.2 and 2.3 are improved by adding more detail. I propose to largely explain the background and the currently used datasets to modern sea-ice and iceberg data distributions through an enhancement of Sections 2.2 and 2.3, although I can add a sentence or two more in the Introduction to set the scene.

I agree that much of the first paragraph of section 3 belongs in the Introduction. Martin et al. (2022) is mentioned there, but I propose to move much of this section 3 paragraph to expand the Introduction mention of the Martin et al. work.

The apparent difference in uncertainties, or current variability, between the Cook and Bellingshausen data is almost certainly linked to the day of the year when a given ship was in a particular longitude. Cook spent less time in southerly latitudes, and this was mostly in the early summer months of December and January, while Bellingshausen was mostly in such latitudes in January and February, when there is less sea-ice typically. I can add an explanation in the text associated with the figure. I am loath to make the figure even more complicated – the first review has already suggested some improvement – but could add a Figure 4b where the day of the summer (ie day relative to 1 December) is plotted for each historical observation to make this clear.

Minor Concerns

1. 53: You are correct – I should have said “temperate climate” and will do so in the revision.
2. 79: Again, you are correct. I will add a “were” in the revision.

Figs. 1-3 and 5-7: Thank you pointing out the offset of some of the longitude markers. This must be something to do with the matlab code and I will correct it in the revision.

Figure 2: The tif file for Figure 2 is the same physical size as for Figure 3. I think the small size in the pdf is an artefact of trying to fit the figure in the page in WORD. I will provide the tif file directly in the revision.

Figure 6: I will make it clear in the revised section 2.3 how the iceberg probabilities are calculated by Tournadre et al. I can expand the discussion of Figure 6 in the text to make clearer the correspondence between past observations and modern “iceberg alleys” such as the Weddell iceberg tongue.

1. 269: Thanks very much for pointing this out. I will move the comma to after the Budge et al. reference in a revision.