This paper adds the information (from 129I and 236U) for two Fram Strait sections in 2020 and 2021 to earlier (2016) radionuclide data, in order to better understand changes in mixing and Arctic circulation time scales (for the Atlantic water component). It also compares results from binary and TDD model approaches to interpret the results (including in the surface layer). The difference found between the 2020 and 2021 results is potentially interesting, fitting with the expectation that a positive polar index would trigger more confined Beaufort circulation (this more toward cyclonic regime, and thus more transpolar drift in 2021) (and, of course, with a little lag).

There were three other papers by Wefing et al. on Fram radionuclide sections (at least the Wefing et al 2019 and 2022 are with binary model, and not the TDD one (and the 2021 paper only subsurface); So, the originality of this paper would be the  use of TDD on the different sections… (but I have not read the 2025 Wefing et al paper: is it on line, as no journal mentioned; on the other hand probably not so relevant here, as dealing with central Arctic?). The other originality is the one of extending the model to the surface data, but that is based on some assumptions to take into account salinity variations and propotion of Pacific water (TTD model already used by Wefing et al (2021), but not for the surface; here there is a salinity normalization and some assumptions on the proportion of Pacific water for the fallout component in 236U).

After consulting these earlier papers, I was surprised that the 129I and 236U 2018 and 2019 Fram Strait data from Wefing et al (2022) are not really mentioned / included here, at least in the discussion. Is it because there were not enough stations in 2018-2019 (mostly EGC and not very far on the shelf? (or that many stations do not have the pair of measurements) or is there is some doubts related to the 2018 and 2019  238U measurements? Altogether, why are they not included. After all: 2016, 2018, 2019, 2020 and 2021 would (apparently) make for a more consistent time series than having the gap between 2016 and 2020, and help to better figure out whether the conclusions derived by comparing 2020 and 2021, also hold for the earlier years.

The use of a source function (NCC and other branches; Figure 1b) is key for the analysis. It is based on some reasonable assumptions as discussed in Casacuberta et al (2018) and Wefing et al (2021). However, should the error bars that are plotted here (Figure 1b) be reevaluated (maybe to phrase it differently: how much confidence does one have in these error bars?)

Furthermore, 2020 and 2021 sampling separated by almost a month seasonally (later in 2020, thus with likely more vertical mixing/vertical diffusion of the surface signal (and more melt of sea ice): could this explain the absence of the shallow subsurface peak in that year, compared to 2021 (and maybe also some of the near-surface differences between the two years).

It is very helpful to have these sections in two successive years. It would have been better if there had been more 2021 (and 2020) data collected in the SCC Atlantic water inflow west of Svalbard, to ascertain that there are only little differences in these water masses between the two years, as expected (otherwise, I wonder how one can fully eliminate that the difference beween the two sections do not result from data/measurement errors in one of the two sections).

Comments:

1. 154: ‘… and adding data…’
2. 155: one needs to add where those 236U samples were measured. According to Figure A1, I guess that it is at the Danish Technological Institute in Aarhus.
3. 243: remove ‘than’ thus ‘estimates of … updated from the ones used in…’
4. 332: this is not stated, but I believe that the argument for choosing the 55% for fGF is that it is the one for which most data of all years fit onto the TDD grid (and not outliers on the plots). On the other hand, why would not it vary between the three years (after all, the later comment on the circulation change between 2020 and 2021 could also have an impact on Pacific water proportion). Could you get an agreement for specific years, such as 2020, with a different fraction? To summarize: what motivates the choice to retain a common constant fraction? (I realize that the nutrient approach is not 100% proof, in particular due to the denitrification on shelves, but any idea why this seems not to work at all? I am still puzzled by the statement)

Section 3.4: seems that the scatter is really large in the different TDD parameters (for example in 3.4.1 Mid-depth Arctic Atlantic water). Greenland slope, indeed seems more homogeneous / consistent (and overall larger gammas…)

Figure 1 is fine for focus on Fram Strait. I would still place other arrows across Canadian archipelagos for completeness.

Figure 8:  I find that the slope is rather different from 1 (even negative in 2020 between tracer age and gamma (but of course, as commented, these are for larger delta, and I understand the argument on the influence of delta, and the possibility of misinterpreting the binary model results). But I am still puzzled with this huge scatter in gamma (and such large gammas in this surface  Polar water I). First, I found that a bit counter-intuitive (but long (and varying) storage in Beaufort gyre could explain it? Probably OK with the later discussion)

Other minor comments:

1. 476: instead of ‘discrepancy’ use ‘difference’
2. 545: after ‘largely’ a word missing.
3. 555: I am sorry that water isotopes (nutrients) are not used here as this would help see more directly (with salinity) whether there is a shift to more Amerasian contribution in 2020 (the reference suggests 2019 and not necessarily 2020?) (and water isotopes would help separate brine signals associated with ice formation and meteoric input). Actually, nutrients were used but suggested not to be conclusive. I would suggest that nutrient data be shown (in Supp mat). I am aware that this is not 100% proof (due in particular to denitrification processes along the way). Maybe those, as well as alkalinity and CDOM (among the parameters that are likely to have been collected/measured during the cruises) would help provide supportive information (I understand that neodymium or gallium might provide further evidence, but I doubt that they were measured on most sections).

Figure 1: green and purple not that well differentiated on left figure. Then, there are black arrows on both panels. No mention on what this refers to (surface + subsurface, maybe). There is also a dashed arrow along Greenland (based on where this is located, has to be surface; probably the dashes refer to intermittent coastal flow)

Figure 2: I have not read Wefing et al (2021) where it is certainly discussed. I was wondering (personal interest; not for the paper)why in the surface layer is the peak before 1990 in 236U so sharp and prominent and preceding by a few years the 129I signal (from La Hague-Sellafield).

Figure 5: I would add 2020 between top and bottom of panels b, and 2021 between top and bottom of panel d

Figure A1: I assume that the bars represent +-1 estimated rms errors.

Figure A2: one needs to again indicate in the caption what is the symbol for each year.

References, lines 924-925: reference incomplete

References, lines 1010-1011: reference incomplete

Review of the manuscript EGUsphere-2026-2020, ‘Enhanced water mass mixing in Fram Strait in 2020 and elevated circulation timescales of Atlantic-derived waters in 2021 based on transient tracers I-129 and U-236’, by Scheiwiller et al.

General comments

The study evaluates mixing and circulation in the Fram Strait utilising the anthropogenic radionuclides I-129 and U-236 applied on binary mixing and transit time distribution approaches. These radionuclides are ideal as transient tracers when applied on studies of especially horizontal spreading of Atlantic-derived waters, but the study also includes an evaluation of Polar waters. The work is very interesting and overall clear, and I recommend publication in Ocean Science after minor revision.

My main concerns are connected to the uncertainties, and how some of the result sections are presented. Detailed comments are given below.

Specific comments

L62: Here you should clarify that this depth range is in the Fram Strait, if this is what you refer to. Or do you mean the Arctic Ocean?

L66: In addition to the density range for AAW you refer to, the temperature maximum is the strongest identifier of this water mass, naturally.

L77: I think you should remove “Polar Water and Arctic Atlantic Water” here, which you just described, leaving the sentence as: Apart from the described outflowing layers, the Fram Strait ….

L156-157: It struck me that if all U-236 data used in this study are from Lin et al. (2023) one would expect Lin to be a co-author. Perhaps this was agreed on but wanted to make a note on this.

L177: The 4% uncertainty mentioned, is that the total measurement uncertainty you assess? This is not clear to me.

Figure 2: The uncertainty shown here (as shaded areas), how large is this, and what is this based on? This is seemingly clearly more than the 4% measurement uncertainty presented, but unless I missed it this is not clearly stated.

L261-263: It would be much helpful if you show numbers instead of just stating how you calculated it, and again I am not sure if your stated 4% is the total measurement uncertainty. Please clarify.

L265-268: This limitation indicate that the total uncertainty may be rather significant. This may be difficult to quantify but should at least be acknowledged.

Figure 3: The inset figures are hard to read due to too small fonts, on all axes and legends.

L277-278: I believe Tanhua et al. (2009) was one of the first studies adopting the TTD approach in the Arctic Ocean. You may consider adding this.

L330-331: How do the results of Dodd et al. compare with earlier studies of Jones et al.? I am not sure if Jones et al. have presented results for the Fram Strait but know they have for the East Greenland Current. If they did it could be interesting to mention how they agree.

Section 3.1: There are several issues with this section. In general, there are lots of details that are difficult to keep track of and seems to describe a very patchy distribution; for example, L352-355. Figure 4 adds to this, with some symbols being very similar (like the stations at –12 and –8°E), and the legend displaying stations in a non-sorted (by longitude) way. In addition, the legends should be placed in the lower right corner to not covering any data points. I started to wonder if the results would not be better presented as a section, but then this is done in Section 3.2/Figure 5. Furthermore, the station at –17°E was only sampled in 2021, but nothing is said about that (unless I missed it).  You also refer to depths down to 1000 and 2500 m, but this is not shown in Figure 4. I would ask the authors to re-evaluate this section and how the results are presented. Could instead the main results be presented in a table, and avoid too patchy and confusing results? (Patchiness is of course often the case in nature, but then this should be spelled out clearly.) I would even suggest considering moving Figure 4 to the Appendix, and to merge sections 3.1 and 3.2. In a table, if this would be chosen, the results could be organised according to water masses.

Figure 5: You may want to consider another colour scale with higher contrasts; the blues and greens are rather similar. In the caption it should say “Panels b and d show cross-sections…”.

L403-408: Consider rephrasing parts of the text; now it is basically repeated information: “near-vertical s₀ structure” and “the front was more or less vertically aligned, with the 27.2 kg m^-3 s₀ isopycnal extending straight down …”.

Figure 6: It is unclear to me why there are so few data shown for Central West (panel c), since there are more data in Figure 4? You may want to consider removing this panel. You could just describe this in the text instead, for the Central West region that is. (This would be in L426.) You should also sort the order of stations in the legends; this helps the reader (now this is only done in panel b).

L416-419: Also in this paragraph I get lost in details. Consider if this could be presented more clearly.

L443-444: It may seem counter-intuitive that the presence of younger waters would result in the width of the TTD increase to values up to 140 years. Perhaps you could clarify this.  

Section 3.4.2: Some of this section suffers the same issues as some earlier sections in that there are so many results/numbers presented next to each other that one loses track. Especially the first paragraph. For this para is also not clear to me if you refer to all regions, or specific parts? I would consider, also here, to present the results in a table instead.

Figure 7. In this figure, and several of the following ones you write out both “Gamma” and “G”, as well as “Delta” and “D”, which are the same things. (It’s like writing “Metre” and “m”. G is “mean age” as you write in the method section, but never use in the figures, and D is “width” of the TTD, so why don’t you use this instead, or simply only G and D? Another, but tiny remark on the figure is to move the panel letters to the lower right corner where they are not covering any data points.

L488-489/Figure 8: Possibly I am misunderstanding something here, but I can’t see that the data points shown in Fig. 8 generally aligns along the one-to-one line. Instead, the general distribution seems to be along the x axis, and thus along a rather constant tracer age at different mean ages. I agree that the agreement is better at lower mixing, but the initial statement is confusing. Please clarify.

Figure 10: There is a mismatch with the order of the panels and what is stated in the caption. The panel order is now: Delta, Gamma, Delta/Gamma, t_mode.

The dashed lines are, I suppose, added to help illustrating the changes of the average values, but this is not mentioned in the caption. Are those lines needed? The black crosses disappear somewhat on top of the dark blue markers but then you may consider another colour on the crosses?

L591: Where is this shown (U-236 values)? In Fig. 9? If so please refer to that figure.

L596: It is not easy to see where those samples are taken from in the map, and also since there is no depth info in the figure. Of the three points I would think you refer to only one is clearly elevated (with U-236 of ~40, while the other two (~20-25) are similar to one sample in 2016.

L601-603: Even if you conclude that conservative temperature may not be a good indicator you first compare two samples with a mean value, and one of the sample values is very close to the average (–1.51 with –1.49°C). Consider removing, or rephrasing.

L607-608: From Fig. 9 it is not easy to see where the samples are taken from.

L610-611: Can you highlight this in a figure in the Appendix, or relate to the T/S figure in Fig. 5?

L617: You should refer to figure showing this. Fig. 5 perhaps?

L619: Is the age range truly “substantial”? You may want to illustrate this by presenting a range in the text. From looking at the figures alone this is not entirely clear to me.

L634: If you are referring to t_mode this should be Fig. 10 d,e.

L636-637: Is this increase significant? Here also the sampling density plays in. For the AAW layer it seems the sampling density was lower in 2020. If this would have been higher it is not unlikely that the mean value would have been higher too, and then the “shift” would not have been clearly observed in 2021. I don’t really follow the “extreme values” mentioned here. Do you mean in t_mode, in 2020? Are they really “extreme”?

L682: Instead of just writing “very old ages” present some values/range. This would also highlight what the transit time is to the Fram Strait from the Canada Basin.

Appendix: There are several issues with the figures in the Appendix. These are mostly of technical character, but bring it up here:

A2: The colours/markers need to be defined. Add legend or/and in caption.

A4: The isolines should be defined/described, similar to what was done in Fig. 3, and add values to the isolines. This should in any case also be done here to make it easier for the reader. Without these explanations it is very difficult to interpret the results.

A5: Similar comments as for A4.

A6: As pointed out earlier remove “Gamma” and “Delta” and just use the symbols.

For most of the selected isolines the values are not seen, only partly for 27.2. You should state in the caption which ones are presented and see to that the values are visible in the figure.

Nothing is shown for the deeper parts (panels g-i), and it is not clear if this is only shown for 2021? From this I would clearly suggest that you remove the lower panels.

Technical comments

L40: occupying, instead of occupy.

L48: …in the Polar Water…

L67: ... entered the Arctic Ocean via …

L82: … 78.5°N and 80°N, respectively (…

L330-331: … Pacific Water fraction in the Fram Strait of …

L532: …waters entering with the WSC, …

L568: CDOM should ideally be defined; Oxygen-18 is, I believe, not typically written as “delta-Oxygen-18”, but “oxygen-18” (the short version in parenthesis is correct); “Neon and Neodynium” are not written with a capitol letter (i.e., should be “neon and neodymium).

L572 + L587: … Arctic Ocean …

L587: Remove “again”.

L597-598: The wording is strange: “influenced to old source waters.” Please rephrase.

L620 + 624: … Arctic Ocean …

L669: This needs some rephrasing. Either “… mainly from the Pacific Ocean.”, or, “… mainly of Pacific origin.”.

L731: Typo on “members”.

References

Tanhua, T., Jones, E.P., Jeansson, E., Jutterström, S., Smethie, W.M., Jr., Wallace, D.W.R., Anderson, L.G., 2009, Ventilation of the Arctic Ocean: Mean ages and inventories of anthropogenic CO₂ and CFC-11. Journal of Geophysical Research, 114, C01002. https://doi.org/10.1029/2008JC004868