the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jan 2026
Water mass modification of the warm Atlantic Inflow towards the Arctic across the Iceland-Faroe Ridge
Abstract. The warm Atlantic inflow across the Iceland-Faroe Ridge (IFR) is the strongest of the three branches carrying warm water to the Nordic Seas and further into the Arctic. This branch (IF-inflow) carries almost 50 % of the total Atlantic water inflow. The Atlantic water crossing the IFR mainly flows through the Iceland Basin with no indication of a significant contribution from the Rockall Trough. The salinity variations of the Atlantic water approaching the IFR are well explained as a delayed response to the intrusion of the Subpolar Gyre into the Iceland Basin while the temperature variations may be seen as a combined response to the gyre and global warming. The flow corridor feeding the IFR with Atlantic Water gets narrower and migrates southeastward to the rim of the Iceland Basin when the Subpolar Gyre extends into the basin. As the Atlantic water crosses the IFR, it is cooled by at least 1 °C and freshened by at least 0.1 g kg-1. Satellite-tracked drifters that crossed the IFR exhibited prolonged residence times on the western flank of the IFR together with a high level of eddy kinetic energy. This indicates that transformation occurs in this area. We show that transformation of the water mainly is caused by mixing with Arctic water masses, rather than by air-sea-interactions, and that most of the modification takes place upstream of the ridge crest. The Atlantic water changes character from an almost barotropic to a much more baroclinic flow over and northeast of the ridge enabling water masses, otherwise constrained to follow isobaths, to cross the ridge. We find that the cooling and freshening of the Atlantic water across the IFR are relatively constant throughout the whole period from 1993 to 2023, except for the last few years. The associated freshening implies that the salinity difference between IF-inflow water and the deep waters northeast of the ridge has been reduced by roughly 20–30 % after crossing the ridge, which weakens the potential for dense-water formation. Updated transport estimates show a slight strengthening for this AMOC branch. From 1993 to 2023, the volume transport of the IF-inflow increased by (12±7) %, while the heat transport relative to 0 °C increased by (16±8) %.
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Status: open (until 26 Mar 2026)
- RC1: 'Comment on egusphere-2026-359', Anonymous Referee #1, 02 Mar 2026 reply
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RC2: 'Comment on egusphere-2026-359', Anonymous Referee #2, 05 Mar 2026
reply
Review of “Water mass modification of the warm Atlantic Inflow towards the Arctic across the Iceland-Faroe Ridge” by Eriksdóttir et al.
Eriksdóttir et al. investigate how the Atlantic water inflow branch across the Iceland-Faroe Ridge (IFR) approaches the ridge and how the properties of the flow are modified during the crossing, using observations from drifter, CTD stations, and satellite data. They also present an updated time series of the Atlantic water transport at the standard section N north of the Faroe Islands. The authors focus mainly on the long-term variability and its causes.
This study fits well into the scope of Ocean Science and will be a useful addition to the literature for this area, and the findings have implications for the large-scale circulation. The authors have generally done a good job in addressing many aspects of the Atlantic water flow toward the IFR, but some of the analyses and arguments lack robustness to make the results more convincing. In particular, I’m missing uncertainties for many of the profiles and time series, which would help addressing how significant the observed changes are. While the manuscript reads well, the overall structure can be improved by linking sections better and making the text more concise, especially in the abstract. The figures are mainly clear and support the argumentation, but some can be improved to be more intuitive and help guide the interpretation. Overall, I think this manuscript would benefit from some moderate revision before it can be accepted for publication. Please see my specific comments below.
General comments:
- Regarding structure, I don’t quite see the difference between inter-annual and long-term variation. After Section 3.3. about “inter-annual variations”, I was wondering why you get these two periods from the EOF. You address the causes of both the changes in properties and flow later in Section 4.2 (now “long-term variations”), but it’d help a lot to get some “road map” information at the end or beginning of each section on where you will continue with the analysis. Some of this information is currently provided in the introduction (L82-105). I think that section can be shortened, i.e., you only provide a general outline of the content of the paper and refer to the actual section numbers in the introduction, while the details (and findings) should be reserved for when you get to the actual sections.
- Uncertainties: The profiles and time series are generally presented without uncertainties (e.g., Figs. 6, 9). Adding standard errors to your mean lines would help assessing whether the differences are significant (both for the figures and Table 1, where you report absolute differences).
Specific comments:
- The abstract is relatively long and reads like a summary, it could be more concise highlighting the main findings. It is unclear what belongs to the background/established knowledge and where the results of this paper start (typically: “Here we show …” or similar). The motivation for and implications of the study could be more explicit.
- Fig. 1: Please use a different color for land (green is part of the color map you use). Please mark the standard stations on the two sections.
- L83, 348, etc.: You introduce many abbreviations, but are not consistently using them throughout the manuscript. For readers not very familiar with the region, the manuscript becomes more difficult to read the more abbreviations you use. Please reduce the amount of abbreviations you use and be then consistent with their use for the rest of the manuscript.
- L139-141/Fig. 2: Perhaps this would be easier to see if you expanded the boundaries of Fig. 2.
- Fig. 2: Is the semi-transparent gray shading necessary? It is difficult to see (I was confused at first what to look for) and changes the perception of the colors. Also, perhaps use a discrete color map with increments every 1 cm, as it is very difficult to read off the Δh = 10 cm.
- L156-160: Why do you use the deepest point in the IFR and not the average depth for D? I also cannot follow the sentence in L159/160: You read 10 cm off Fig. 2 as a starting point for the calculation, so how can this provide support for Fig. 2?
- Fig. 3: This figure is not very clear, perhaps because the spaghetti lines overlap each other and do not show preferred pathways, if there are any. Have you tried to either plot the trajectories as semi-transparent lines or grid the drifter data on a fine grid (as in Fig. 10 or finer) and plot a heat map?
- Fig. 4: Panels c) and d) have very similar ranges of the color bars. For easier comparison, please use the same range on both panels.
- L201: Please explain why you subtract the spatial average of all grid points in the domain before computing the EOF. EOFs aim to capture the spatiotemporal variability; the spatial structure will not be conserved if you artificially force the field (with arbitrary domain boundaries) to have zero spatial mean at each time step. The anomaly for each grid point is normally computed by removing the temporal mean at each grid point before the EOF analysis. What difference to the result would that approach make?
- L223-230: A more common approach would be to use composites of the years with the highest and lowest pc1 for Fig. 5. This would help to exclude the years that are not extreme (such as 2003), while you still can state that these composites correspond to an early and late period.
- L245: For the properties upstream of the IFR, T_IB and S_IB, you could use profiles from Argo floats to supplement with data from the region south of the ridge, between stations I and V. If your argumentation holds, the time series from I and V should envelop the measurements in between.
- L257: Please add a reference for the large vertical and horizontal variability or show a plot of standard deviation for the mean properties in the supplementary.
- L260/Fig. 6: You’re (for good reasons) not comparing apples with apples here. For a closer comparison, you could consider also showing the T and S time series from the N-section as a 3-year running mean.
- L265: This is an example of where you could add some “road map” info, such as “We quantify this cooling and freshening in Section 5.1.” or similar.
- L296-301: The ocean is not as well mixed as the atmosphere. A global SST anomaly will likely not be representative for the upper 500m of the water column (what T_IB was constructed to be), nor will it have the correct timing for each region on Earth. Your current analysis simplifies these aspects too much, and a lag analysis does not seem trustworthy. Perhaps ocean heat content in the subpolar North Atlantic would be a better parameter to investigate. In any case, the time series of the global SST anomaly does not imply causality; other multidecadal variability, such as the Atlantic Multidecadal Variability, could also affect the long-term variability of T_IB and pc1.
- L337: It might not make much of a difference, but it is probably more appropriate to use the median instead of the mean crossing times, as the mean is more sensitive to extreme values for a potentially skewed distribution of crossing times.
- L340: To make it easier for readers who are not familiar with the method used by Larsen et al. (2012), please describe the method briefly (and keep the reference for details). For example, please specify what you mean by heat flux (sensible and latent heat fluxes, or also radiation?). Please also comment on the following: 1) The air-sea interaction only affects the mixed layer, but here you use the upper 500m for stations V and I, so you probably wouldn’t expect a high percentage anyway? 2) What are the seasonal differences in the flow during the IFR crossing? I expect that the result of the calculation is sensitive to the season of the crossing, with more air-sea interaction in winter than in summer.
- L345: “the only explanation” sounds a bit absolute, considering there is no estimate of the uncertainty associated with your estimates and method.
- L346: “well-established process” – I agree, but you could be explicit about the intermittency of the process (it is first alluded to later in Section 5.3.3).
- L354-356: Please state the period over which the observations were taken or refer to the methods section where you introduce the data. Is the distribution of profiles over time somewhat constant for the three regions, or would it be best to restrict the period to more recent years? Else your result may be biased considering the long-term changes in T_IB and S_IB (Fig. 8). Perhaps also consider to augment your data set of CTD profiles (at least for the upstream region, potentially also the flank region) by including profiles from Argo floats. This would reduce the uncertainties on your mean profile, as these profiles are usually not synoptic (10 days in between) compared to cruise sections.
- L361: Here you use the standard stations, as opposed to the transport-averaged T and S properties from before. With which station would a transport-averaged profile align most?
- Figs. 10 & 11: I suggest to combine these two figures into one. In panel 10a, do you mean “average” time spent in each grid cell? Please also mark the outline of the regions defined in Fig. 9 (upstream, flank, crest) to help the comparison and interpretation. Perhaps also just refer to the “flank” instead of the “western flank” (or consistently “western flank”, in all text and figures) – I was initially confused as the residence time appears higher on the central flank until I realized that you called the entire region “western flank” in L355, but only “flank” in Fig. 9.
- L400: Please add a reference for the Taylor-Proudman theorem.
- Supplementary Fig. 4: You refer to Fig. 3.7a in the caption – do you mean Fig. 9a?
- Supplementary Fig. 5: Please add isopycnals to the panels. To make the figure more intuitive and independent from the caption, you could also add titles/labels “I”, “V”, and “KR” to the panels.
- L447: Instead of assuming a fixed 1:3 ratio, you can compute it using the mixing line in a TS-diagram for each year.
- L458: This is the same formulation as in L23. Instead of saying that things have changed, please be explicit about what has changed (less cooling and more freshening, if I interpret Fig. 14 correctly?).
- L479: Bretones et al. (2022) may not be the most fitting reference here. They used a single model simulation and focused mainly on the far future, at the expense of details on the present or near future. Also, other models show a weakening in the Nordic Seas before a strengthening (e.g., Årthun et al., 2023, https://www.nature.com/articles/s41467-023-37846-6). In any case, I suggest tp reword to “some model simulations are consistent with our result” or similar – your observational results should provide the baseline, not the model simulations.
- L484: Please specify that the cooling refers to a reduction over space, not over time. Just before you were talking about temporal changes, which could lead to confusion.
- L511&513: Again, this sounds as if this is a recent change (in time, not space) for dense-water formation and freshening, so please reword.
Technical edits/suggestions:
- L10: There are many different definitions of the Arctic, many of them include the Nordic Seas. Perhaps best to be specific here and use “Arctic Ocean”.
- L14: Be consistent about the capitalization of “Atlantic water”/ “Atlantic Water” throughout the manuscript.
- L23: It is unclear whether the freshening refers to the “last few years” or the entire period.
- L25: Perhaps be explicit about the dense-water formation occurring “downstream”or “in the Nordic Seas”.
- L55: “deeper than 300m” → Do you mean “located below 300m depth”?
- L57: remove “A” before “high fishing activity”.
- L90: It is unclear why the citation is placed here, perhaps it should rather be at the end of the sentence or an indirect citation.
- L117-125: remove the sentence “The standard stations and sections are shown in Fig. 1” and add “(Fig. 1)” at the end of the sentences in L119 and L123 instead.
- L120: Perhaps use “Marine and Freshwater Research Institute (MFRI) in Iceland” as MFRI is the better known abbreviation.
- L125: The TEOS-10 standard suggests to capitalize Conservative Temperature and Absolute Salinity, which you have done sometimes, but not always.
- L135: “The background colours indicate the MDT” This sentence belongs into the caption (or remove it altogether).
- L142: “Subpolar Gyre”
- L147 (and similar captions): “a thicker 1000 m contour”
- L151: “south Icelandic slope”
- L173-174: You normally use “data” as plural, so change to “support” and “they also indicate”
- L181/182: Is there a reason for why you use “Ekman drift” instead of the more common “Ekman transport”?
- L199: You haven’t introduced the Rockall-Hatton Plateau yet, perhaps it could be marked on Fig. 1 or you explain what regions it encompasses.
- L204: move “(Fig. 4a)” to the end of the sentence, it interrupts the flow.
- L246 & L406: “overflow water”
- L256: You probably mean “Atlantic water core”.
- L276: Instead of “similarity between the cyan and black curves”, please name the properties you’re comparing to make the text easier to follow for the reader.
- L334: Either “Western Boundary” or “western boundary”
- L359: “time, this”
- L497: no comma between “find” and “that”
- L510: “to a projected”
Citation: https://doi.org/10.5194/egusphere-2026-359-RC2
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- 1
This is an important analysis of a crucial portion of the AMOC of the upper limb. I am happy with much of this study, but have some points that should be addressed. They are outlined as follows:
Line 38: Reference for entrained ambient waters?
Line 124: Standard stations not marked in Fig. 1.
Lines 108-130: I think it’s important to mention how the reader can learn more about how each of these data products are collected and processed. I would encourage adding information to each of these three data sources specifically stating where that information can be found (even if it’s just the same website where you got the data).
Line 150: You should extend the x-axis slightly eastward to close the dashed-white contour south of Faroe.
Line 158: “From Fig. 2, ∆h is 10 cm.” Where? I’m guessing it is between the thick-dashed white contours, but you should specify.
174: Delete comma
Lines 173-178: To be fair, only 3 drifters seem to originate in the Rockall Trough. I would delete the second half of the first sentence, or I would change it to something like “…and it also indicates a clear surface pathway from the IB through the FSC” and delete the last sentence of the paragraph. For the first part of the second sentence, I think you need to mention how many drifters were even in the western part of the RT.
Line 184: Reference? Even if it’s the ones earlier in the paragraph, you should reference again for a statement like this.
Line 185: Word choice on “thus, point at”? Maybe use “suggest” instead?
186-187: Again, there is very little data to directly support this from the drifters. Actually, the data shows that this pathway never occurs, but it’s based on 3 drifters. Instead, there is plenty of data that shows that the water crossing the IFR comes from the IB, which essentially is the same point. I would re-word this concept in those terms.
189: How many different transacts is that? The sentence could be strengthened by saying something like “…from approximately (# of transacts) transacts along 4 different routes between…” I’m guessing it’s hundreds.
223: Does low pc1 (high pc1) also mean that the PC is negative (positive)? If so, clarify. If not, wouldn’t it make more sense to do this analysis as positive or negative PC, rather than by year? (though you could mention that clearly the majority of pc1 will be pre-2008, and vice versa)
223-230: I am just realizing that not all drifter deployment locations are included in the figures. This should be clarified earlier when you first introduce the drifter images. Maybe just in the captions, and it would help to be specific (e.g. the deployment locations of 17 drifters are outside the figure.)
241-249: Since you are using the water properties from I and V to determine what water is going over the ridge, it would be helpful to note the mooring locations on the float figures that show the WB and EB. It could be worth mentioning if all the floats cross the IV section before crossing WB and EB, or what percentage do.
296-301: There must be a better way to remove temperature trend than subtracting a constant value of global SST from the data set. At the very least, it would make more sense to subtract the global SST trend. It would be best to get a much more local trend, though using the trend from TIB would clearly be influenced by a positive trend in the EOF pc1. Perhaps you can find a longer local temperature data set where the EOF pc1 won’t play as much of a role since the gyre would’ve likely gone through a couple cycles? Or make the SST data have a greater spatial range without covering the entire globe (e.g., all SST data within a 10 degree lat and 20 degree lon box centered on the IFR)? Of course, using SST to represent temperature trends in the upper 500m will surely be biased as well. Maybe use the trend in mean Argo data from the top 500m in the northern North Atlantic, and then assume that trend for your entire record (since that data starts in 2004)? None of these are perfect, but I think there’s a better way. My suggestion is to use Argo.
304: I appreciate that this paragraph is trying to address my concerns in my previous comment, but I disagree with this reasoning unless you are referring to the EOF pc1 reinforcing the temperature trend. In that case, I agree, but you should try to find the balance between removing the local temperature trend while avoiding having too much of the EOF pc1 in the trend. Hence my suggestions above. You could even evaluate if your chosen method mostly isn’t being influenced by the gyre expansion/contraction by doing another EOF on it.
358-9: Confusingly worded sentence. I suggest re-writing it.
366: To what extent west does the upstream of the IFR region reach?
376-391: I am confused on this analysis. Is 10a the mean time spent in each grid cell of the drifters that entered? Clearly not all 101 drifters spent time in every grid cell, but also multiple surely spent time in the same grid cells. I think I understand 10b, but you still may want to clarify. Is it the number of drifters that spent time in each grid cell?
393: Wouldn’t EKE from altimetry be more robust? More samples and a different data source. You could do both with figures 11a and 11b.
433: Unbold “mean temperature”
447: Why 1:3? Do we have evidence that the resulting water mass should have these properties? Otherwise, it looks like determining a mixing ratio to match a desired result; if this is the intent, then it would be better to analyze the two water masses and find the exact mixing ratio.
480: Do you mean “more ocean” instead of “the ocean?”