the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Feb 2021
An overlooked freshwater source contributed to the extreme freshening event in the eastern subpolar North Atlantic after 2014
Abstract. Outflows of low-salinity waters from the Arctic to the upper layers of the subpolar North Atlantic (SPNA) are central in redistributing freshwater from river runoff, melting sea ice, and precipitation. They act to reduce shallow, as well as deep, convection; thereby affecting both biological production and the Atlantic Meridional Overturning Circulation. The two main sources of low-salinity water to the SPNA are the flows through the Canadian Arctic Archipelago and through the Denmark Strait. A potential additional source of low-salinity water is the shelf/slope region south of Iceland, mainly fed by Icelandic runoff. Normally this water passes into the Nordic Seas, but in some periods, it may instead flow into the upper layers of the central parts of the Iceland Basin in the eastern SPNA. This low-salinity water has previously been overlooked as a freshwater supply to the SPNA. Using a range of observational data sets, we show that the conditions for a diversion of this water mass from the south Iceland shelf into the Iceland Basin were favourable during the 2014–2018 period. In those years the Iceland Basin became extraordinarily fresh, characterized by surface salinity lower than previously seen in a 120-year long time series. The event is thought to have been mainly caused by unusual winter wind stress patterns that diverted freshwater from the western SPNA to the eastern basin and caused a zonal shift of the subpolar front. Here, we show that the low-salinity signal near the surface was locally reinforced in the central Iceland Basin by anomalous diversion of low-salinity water originating in the shallow shelf areas south of Iceland and that this can help explain why the surface salinity of the Iceland Basin became so exceptionally low. The diversion was generated by anomalous wind conditions over the Iceland Basin and caused slightly enhanced freshening of the warm waters crossing the Greenland-Scotland Ridge from the SPNA into the Nordic Seas. The low-salinity Icelandic-source water also increased the near-surface stratification and reduced the depth of convection in the Iceland Basin during two consecutive winters with reduced nutrient renewal of near-surface waters as a consequence. Although especially pronounced after 2014, this extra freshwater input probably occurs more generally, which may help explain why the central Iceland Basin may be an oligotrophic region, as has previously been suggested.
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RC1: 'Comment on os-2021-14', Anonymous Referee #1, 05 Mar 2021
Review of “An overlooked freshwater source contributed to the extreme freshening event in the eastern subpolar North Atlantic after 2014” by Hansen et al.
This manuscript describes a connection between Iceland-sourced freshwater and the freshening of the eastern subpolar gyre from 2014-2018. This freshening event has been previously identified by Holliday et al. (2020) as the largest freshening event in the past 120 years, thus the topic is of great interest to the oceanographic community, and is suitable for publication in Ocean Science Discussions. The authors of the present study use satellite altimetry, surface drifters, repeat hydrographic sections, and mooring data to support their claims of increased Iceland-sourced freshwater in the Iceland basin. The compilation of this many sources of data clearly took a lot of effort, and deserves recognition.
The idea that freshwater from Iceland can have an appreciable impact on the large-scale salinity structure of the subpolar North Atlantic is certainly a novel idea that is worth researching. However, the Iceland-sourced freshwater could be overlooked for good reason – possibly because it is too small to have an effect on the large-scale. It is a tall task to convincingly demonstrate that the oceanographic community has largely ignored an important phenomenon, and it requires strong evidence to do so.
I did not find that this paper presented such evidence. I found that the authors’ central thesis lacked quantification (and thus was difficult to understand and prove/disprove), and the results were insufficient to support the authors’ claims. I explain these points further below.
Lack of quantification: From the outset, it was unclear what signal(s) the paper is attempting to explain. Holliday et al. (2020) describes a freshening of the upper 1000 m of the Iceland Basin and its forcing mechanisms, without mention of freshwater emanating from Iceland. In particular, Holliday et al. (2020) find a remarkable agreement between the magnitude of the salinity increase of the Scotian Shelf/Gulf of Maine region with the magnitude of the salinity decrease of the eastern SPG, implying that the whole of the freshening in the eastern SPG can be explained by freshwater coming from the Grand Banks region. Yet this paper seems to assume that the mechanisms presented in Holliday et al. (2020) are insufficient to explain the freshening. What evidence is there of this? How much of the freshwater signal from Holliday et al. (2020) is unexplained? And can the magnitude of the freshwater flux from Iceland explain the unexplained portion?
Later in the text, the authors focus on the near-surface freshening as evidence of the Iceland-sourced contribution. But the authors acknowledge that the OSNAP moorings they use to track the vertical structure of the salinity anomalies lack surface instruments, so it is again unclear what signal the authors are trying to explain. If indeed surface salinity is of interest (and it appears that it is), why not use satellite sea-surface salinity, which has near global coverage from 2009-present? From this, one could construct a time series of surface salinity for a given region, and then do an analysis of the relative roles of various mechanisms.
Results insufficient to support claims: I will address each of the data sources individually:
- Satellite altimetry – the authors use satellite altimetry to demonstrate that there is a mode of variability in which the height of the central Iceland Basin is anti-correlated to the height of the shelf/slope region. This mode is especially strong in the 2014-2018 period, indicating that the cyclonic surface circulation around the Iceland Basin was strengthened during this period, “representing enhanced anticlockwise circulation that might bring water from the shelf/slope region south of Iceland in southwest-ward direction towards the western Iceland Basin” (line 176). However, there are two issues with this logic: (a) if the water were to follow the SSH isolines indicated in either Fig 2 or 3a, then the fresh water would flow almost directly into the Irminger Sea and not affect the salinity of the Iceland Basin, and (b) there is no indication that this mode is at all related to the shelf-basin exchange (geostrophic or ageostrophic) around Iceland. The authors acknowledge “the near surface flow… has no indication of an average flow from the Icelandic shelf/slope region into the central Iceland Basin” (line 156) and “it seems likely that the MDT and ADT are not sufficiently accurate in the shelf/slope region south of Iceland to reflect the actual flow…” (supplementary materials). Given these statements, the motivation for looking at altimetry to document shelf-basin exchange is unclear. What information does altimetry provide? Could adding the Ekman velocities onto the surface geostrophic velocities from altimetry be more informative?
- Surface drifters – the authors use 11 surface drifter tracks that crossed from the Iceland shelf into the western Iceland Basin between 1995 and 2018 to demonstrate that Icelandic shelf water influences the western Iceland basin. But these drifters represent a small portion (1/8) of the total surface drifters that crossed into the Iceland shelf region during this period, thus the western Iceland basin is not a primary pathway for the freshwater around Iceland. There is the possibility that at certain times, this pathway is more important than others (potentially important between 2014-2018), but the authors acknowledge that “With only 11 drifters taking this path, it would be hard to determine temporal variations” (line 207). The authors then attempt to tie the surface drifter pathways to the altimetry EOF, but their argument falls flat because the altimetry EOF does not indicate shelf-basin exchange around Iceland (i.e. the motivation for using the drifters). Thus I believe the surface drifters demonstrate the opposite of what the authors contend: that the majority of the freshwater on the Iceland shelf does not flow into the Iceland Basin, and that it either goes eastward into the Nordic Seas or westward into the Irminger Sea. The small number of drifters that flow into the western Iceland basin are a small percentage of a small freshwater flux from Iceland (~5 mSv, no reference salinity provided), and therefore likely represent a very small quantity.
- Extended Ellett line – the authors contend that the salinity structure in the upper 200 m across the Iceland basin indicates an input from Icelandic-sourced freshwater. The salinity structure varies between years, and during periods of low salinity (2015-2017), the salinity of the central Iceland basin was lower than the eastern and western boundaries (in contrast to the other years, which increased almost monotonically from Iceland to Scotland). But again, this result seems to fly in the face of the authors’ argument that the freshwater is coming off Iceland – if the input from the Iceland shelf is confined to the western Iceland basin, as indicated by the surface drifters, then shouldn’t the salinity of the western Iceland basin decrease the most? The low salinities in the central Iceland basin (in regions of northward velocities) indicates that these low salinity waters are sourced from the south, rather than the north.
- OSNAP moorings – the argument here is that the high-frequency variability in the vertical salinity structure cannot be explained by far-field forcing from the western boundary and instead requires more local forcing (line 300). However, I do not follow this logic. The Iceland basin is full of eddies and small-scale structure that advect property anomalies – why can’t these cause the high-frequency salinity variability seen at the OSNAP moorings? If the argument from Holliday et al. (2020) were that a ‘pool’ of freshwater came off the Newfoundland/Labrador Shelf and moved coherently into the eastern SPG, then I would agree with the authors that the vertical structure of this salinity anomaly would be eroded by the time it arrived to the eastern SPG. But I don’t believe that’s the argument in Holliday et al. (2020).
- Hydrographic sections south of Iceland – I found the timeseries of freshwater thickness compelling, but given the infrequent coverage of the hydrographic sections, these data should only be considered ancillary, and not central to the argument. In other words, I would be more convinced of the authors’ arguments presented in this section if the altimetry and drifter data were stronger. Furthermore, the lack of velocity data diminishes what can be inferred from these sections, particularly regarding the strength of the freshwater fluxes. It was unclear to me even which direction these freshwater fluxes at each line are directed considering that Section IH “seems to be located in a divergence zone between eastward- and westward-flowing waters” (line 276). Does this imply that the freshwater flux at Section ST is eastward? How does that align with Fig. 1a? And what evidence is there to multiply the salinity fields by a constant 10 cm/s velocity (line 273)?
Despite my reservations about this manuscript, I am not convinced that the Iceland-sourced freshwater does not play a role in the large-scale salinity structure of the subpolar North Atlantic. In particular, Figs. 2 and 3 from Holliday et al. (2020) indicate that there were fresh anomalies in 2014-2016 south of Iceland that seemed to contribute to the freshening of the eastern SPG. Whether those fresh anomalies are sourced from Iceland is unclear, as well as why the anomalies appear to move against the cyclonic subpolar circulation into the eastern SPG, but it does deserve further study. This mechanism is particularly important given how much water mass transformation occurs in the Iceland basin, and the potential role of meltwater from Iceland.
Introducing an entirely new concept to a well-developed field takes strong evidence. It’s possible that freshwater from Iceland is indeed overlooked but to prove that it is, the authors need to quantify its impact and rigorously demonstrate that it plays an important role.
To provide more guidance, I am including more specific comments below:
- Line 48-49 “most of the entrainment into overflow also occurs in the eastern SPNA” are you referring to most of the water mass transformation across the isopycnals of maximum overturning?
- Line 63 where is the Faxafloi line? Please mark this on Fig 1a.
- Line 85 – what is the reference salinity for the 5 mSv of freshwater carried by the Iceland Coastal Current?
- Line 87 – “In addition to this” – what does “this” refer to?
- Section 2 – Is it necessary to have so many subpoints? Can all the data be summarized in a single paragraph?
- Line 131 – it would be instructive if Supplementary Table S1 also included the seasonal and interannual timing of these cruises.
- Line 148 – Please elaborate on “This makes the result independent of any assumptions used in generating the MDT…”. To what assumptions are you referring?
- 2 – If the black line delineates the contour that separates the flow north and south of the Faroes, why doesn’t it intersect the Faroes?
- Line 154 and elsewhere – replace “anticlockwise” with “cyclonic” (or “clockwise” with “anticyclonic”).
- Line 157 – what is meant by ‘distorted’? The average flow pattern incorporates all time scales, including synoptic variability. The horizontal resolution of the SSH will not resolve the mesoscale, but why does that mean it’s distorted?
- Line 165 – what is meant by long time scales? Fig 3c outlines the seasonal component of this mode… does the seasonality come into the argument at all?
- 3c – it would be useful to use a box-and-whisker plot here to show the median, quartile ranges, outliers, etc. Currently, it is tough to determine the strength of the seasonal cycle purely from the overlapping markers.
- 3d – is this panel a zoom in on panel b? If so, this should be highlighted in panel b (maybe draw a rectangle in panel b around the bounds of panel d, or alternatively just make a note of this in the caption for panel d).
- Line 175 – It should be noted that this sentence refers to reduced cyclonic circulation, rather than an actual anticyclonic circulation in the Iceland basin.
- Line 210 – why is only longitude considered?
- Lines 207-214 – the argument that the altimetry and surface drifters are well correlated would be significantly strengthened if there were an accompanying figure. Can you produce a set of maps of altimetry and surface drifter tracks for ~4 time steps? Or alternatively show the two curves that are correlated? I am surprised that there is such good agreement between these data sets considering that the Ekman velocities are not added into the surface geostrophic velocity field. Would these comparisons improve if the Ekman velocities were added?
- Line 230 – remove comma after ‘months’.
- Figure S7b – at mooring M2, why does the salinity decrease from 50 m to 100 m when it increases in panel a and Fig. 6b?
- Lines 250-253 – It is not clear why the freshwater content is valid here. Schauer and Losch (2019) discuss the arbitrary use of reference salinities in the calculation of freshwater transport, and their arguments hold whether the freshwater came from ‘pure freshwater’ or not. The only way around this issue is to use a closed volume in which the mass budget is balanced. Another method is to report the freshwater fluxes relative to two reference salinities to demonstrate their sensitivity to the choice of reference.
- 7 – why is 200 m chosen as a bottom limit?
- Line 285 – There is no evidence presented here that the drifters followed different paths in different years, and the authors admit as much (line 207).
- Line 351 – how is the importance of the precipitation trend assessed relative to other mechanisms? Given that there is no explanation of what signal the authors are trying to explain, it is hard to assess whether the precipitation is small or large comparatively.
- Line 356 – This explanation of the errors in the MDT near land and steep topography should go in section 3.1. This is much more clear than the current explanation on lines 147-149.
- Line 361 – Two drifters followed this pathway in 2015 from table S2, but earlier the authors admit that they cannot use the drifters for temporal variability due to insufficient coverage. Along these lines, there were three drifters that followed this pathway in 1996 when the principal component was near zero.
- Line 364 – “we find it most likely that the Icelandic freshwater source was the primary cause of the near-surface freshening events in the 2015-2017 period.” I don’t know of any evidence to support this claim, particularly in regard to the ranking of roles that various mechanisms played.
Citation: https://doi.org/10.5194/os-2021-14-RC1 - AC2: 'Reply on RC1', Bogi Hansen, 09 Jun 2021
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RC2: 'Comment on os-2021-14', Anonymous Referee #2, 13 Apr 2021
General comments
This paper uses observational data (satellite altimetry, trajectory of surface drifters, in situ salinity measurements) to suggest an overlooked freshwater source off the Iceland coast. The authors argue that this addition of low-salinity waters likely contributed to the recent freshening in the eastern subpolar North Atlantic that was described by a previous study (Holliday et al. 2020). Despite the title referring to “freshwater source”, the manuscript does not argue for a source of freshwater, but rather a phenomenon where fresher surface waters off the coast of Iceland were diverted into the Iceland Basin due to an anomalous circulation pattern south of Iceland.
The analysis starts by evaluating satellite altimetry data to define the common characteristics of geostrophic flow in the easter subpolar gyre as well as its temporal variability in the recent years. By applying an EOF analysis on the satellite altimetry over the eastern SPG the authors identify an enhanced cyclonic circulation after 2014. They also use satellite-tracked drifter data and correlate it with the temporal variability of the principal component in the satellite altimetry EOF analysis. This is interpreted by the authors as that during the enhanced anticlockwise circulation the drifter trajectories originating from the Iceland shelf are preferably directed southwestwards (into a region of anomalous freshening). They then looked at various salinity observations from hydrographic sections in the region (Extended Ellett Line, OSNAP morning and sections over the south-Icelandic shelf) to describe a possible addition of freshwater into the eastern subpolar North Atlantic derived from the Icelandic coast via a change in the upper circulation.
The study includes a comprehensive analysis of various data sets and observations and I acknowledge the attempt to connect these diverse datasets with each other. However, these results do not convincingly show the relevance of such a freshwater source to contribute to the overall freshening. Therefore this complicates without suitable justification the story that Holliday et al. (2020) laid out to explain the freshening.
The main issue with this study is the lack of quantification. This type of analysis essentially requires a budget analysis in order to provide a clear freshwater estimate and establish that the redirection of low-salinity water off the Iceland shelf is a relevant signal in the eastern subpolar North Atlantic. The analysis presented here does not meet current standards given that there is now readily available data that goes beyond mooring and hydrographic ship data. In particular it is surprising that Argo profiling data has not been utilized to complement the other salinity observations. Especially for recent time periods, salinity profiles from Argo floats should provide a better picture of the spatiotemporal variability in that regions. There are also a number of gridded salinity products derived from Argo and other profile data as well as satellite-derived surface salinity estimates which are publicly and readily available. In particular, the use of ocean reanalysis products (e.g., SODA, ECCOv4) would be essential in such investigation as these allow closed budget analyses that can establish underlying mechanisms. It is essential to include these analyses, which then can be compared to the present hydrographic data to see if the picture is still consistent with the freshening and hypothesized pathway of freshwater over the Iceland Basin.
In summary, I am not able to recommend the manuscript for publication at this stage. Much more rigorous quantitative data needs to be presented in order to demonstrate that freshwater from the Iceland shelf is relevant in describing variability in the salinity and stratification in the eastern subpolar North Atlantic. This will require major revisions, with both new (and robust) analysis as well as substantial rewriting of the text.
Specific comments
Title: The title is misleading since it describes a “freshwater source”, but in fact the hypothesized mechanism is a change in circulation.
Line 34: What is meant by the term “buffering region”?
Lines 43-46: This statement needs references. I am not aware that the subpolar North Atlantic is nutrient limited and would expect it to be largely light limited. Thus, a reduction in vertical mixing could also lead to higher productivity due to a decrease in light limitation.
Section 2: I think this section would read better without the different subsection and instead have a single section with complete paragraphs. Also, I feel there is a lot of important information missing. For example, the EOF analysis is a key method in this study, but it is only described in the supplementary material. A shortened description should be included in the Method section too. As well, what is the calculation method for freshwater thickness? What analysis software has been used (Python, Matlab etc.)?
Line 128-129: How has potential density been derived?
Line 130-134: The location of these sections should be included in the map (Figure 1).
Line 135-137: This is confusing phrasing. ERA5 should be stated as the atmospheric reanalysis product and CDS the repository from which the data has been obtained.
Section 3: Instead of calling the subtitles by the data product, it would be better to have an actual subject that refers to the finding/processes etc. For example, instead of the title “satellite altimetry”, it should be called something like “geostrophic flow pattern”.
Line 157: Need to explain more how the spatial pattern in Figure 2 can be interpreted to identify the flow.
Line 159-163: I think the description of the EOF analysis fits better in Section 2.
Line 160: Please specify the spatial domain that has been used in the EOF analysis (cite lat and lon bounds).
Line 166: “Anomalous behaviour” is unnecessarily vague. Just describe what is anomalous about it.
Figure 3 b-d: Units are missing on the axis labels. Is it "cm" as in panel a?
Lines 174-176: How is the connection made between zero to negative PC values and enhanced clockwise circulation? This connection has not been clearly explained.
Line 203: I’m not sure if the term “streamlines” is appropriate here. This would set the EOF equivalent to a stream function, which I don’t think is the case.
Lines 211-213: It is not obvious to me how a fairly weak correlation between the zonal drift and PC time series leads to that conclusion. Maybe it helps if the authors actually describe the process behind the apparent correlation.
Lines 224-226: The fact that the drop in salinity is confined to the middle of the basin indicates to me that the freshening is sourced from the south, as this region corresponds to the pathway of the northward flowing NAC.
Lines 248-249, Fig 7c: How's the freshwater content calculated? This needs to be included in Section 2.
Line 254: Cite the actual value that is chosen as the reference salinity.
Line 274: The freshwater flux of 5 mSv needs to be put in context. This is quite small compared to any other freshwater flux estimate over the subpolar North Atlantic.
Lines 276-277: Please clarify the relevance of this statement.
Section 4: A majority of the content of this section can be regarded as results. Usually, the Discussion is for bringing up previous studies, raising potential concerns and caveats, and restating the main findings.
Lines 296-299: If the freshening is explained by just deviation in the circulation, then shouldn’t we expect salinification in the downstream region where the Icelandic source usually ends up?
Line 305: This statement need to be supported by quantitative analysis.
Line 308: The phrasing “more normal flow” is odd. It should be clarified what normal is.
Line 324: The phrasing “drained of freshwater” does not make sense in the context of oceanic freshwater content.
Line 332: Clearly describe the steps used to get to the estimate of 0.5 m.
Section 4.3 This suggest only a minor influence of the Icelandic freshwater source and thus contradicts the whole premise of this study.
Line 415: Statement “improves conditions for primary production” needs references.
Line 427: Specify a quantitative criterion to determine convection depth.
Page 20: This is a strange way to end the paper. A section containing Conclusions is missing.
Citation: https://doi.org/10.5194/os-2021-14-RC2 -
AC1: 'Reply to both referees', Bogi Hansen, 27 Apr 2021
Response to comments from both referees
With the comments from both referees posted, it seems clear that neither of the referees finds the manuscript worthy of publication without major revisions. Some of this is no doubt because of unclear explanations in our text. In hindsight, the title should probably have been less provocative, less certain, and more direct – perhaps something like “Runoff from Iceland may have contributed to the extreme surface freshening in the Iceland Basin after 2014”. It was certainly never the intention to claim absolute proof of our hypothesis. Only to argue for its likelihood, which we still feel that we have done. Neither was it the intention to claim that the Icelandic freshwater source was large compared to the western source described by Holliday et al. (2020), as the referees seem to read our text. The aim was to argue that the Icelandic source – although much smaller than the western source – was local (to the Iceland Basin) and surface intensified, which could give the surface salinity of the Iceland Basin the extra downward kick to make it so extreme (e.g., lines 22-25 in the abstract).
Apparently, these messages were not sufficiently clear, for which we apologize. If this was the only problem, more careful rewriting of the text might make the manuscript more acceptable, but you would also expect referees to take this into account. The referees raise more serious questions, however, that are more difficult to address.
One of these questions is in regard to insufficient support. In our manuscript, we presented five types of observational evidence (satellite altimetry, surface drifters, Extended Ellett Line, OSNAP moorings, and Icelandic standard sections). Referee 1 discusses each of these and finds none of them convincing. Instead, he/she recommends using satellite sea-surface salinity. This might indeed be worthwhile but, to our knowledge, these data are not reliable close (50 km) to land (Sea Surface Salinity | NOAA CoastWatch & OceanWatch) and are not very accurate (doi:10.1002/2014JC009961). A brief look at these data indicates extremely high scatter in the Iceland Basin. It does not seem likely that these data can provide evidence of an Icelandic contribution that is much more convincing than the collective observational evidence already presented. A similar argument may be made to the suggestion by Referee 2 to use ARGO data. Again, this might be worthwhile, but we do not find it likely that the relatively few ARGO floats that at any time are present in the Iceland Basin with their preference for bottom depths > 1000 m and high spatio-temporal variability can provide much more convincing evidence.
A second question raised by both referees is in regard to quantification. It seems that the referees want quantification of the Icelandic source relative to the western source discussed by Holliday et al. (2020). This can, of course, be done although with some uncertainty, but we did not include it directly since we (as mentioned above) never intended to claim that the Icelandic source was of comparable magnitude to the western source. However, if this is what the referees mean by lack of quantification, Table 1 and the discussion in Sect. 4.2 do compare the two sources indirectly (summarized in lines 409-412). Our main argument was that runoff from Iceland was sufficient to explain the extra freshening of the surface layer relative to deeper levels. As shown in lines 328-342 in our manuscript, we feel that we have done that quantitatively.
According to Referee 1: “Introducing an entirely new concept to a well-developed field takes strong evidence”. Alternatively, one might argue that a new concept deserves recognition even if it is not 100% proven as long as it does not contradict any basic principles. But that is, of course, a matter of opinion.
In summary, we agree that the manuscript could have been clearer in the text, but we still feel that the evidence presented supports the basic idea of a contribution from Icelandic runoff to Iceland Basin near-surface freshening, although not providing absolute proof. We do not find it likely, however, that we will be able to supply additional evidence that can appear more convincing to the referees.
We have therefore decided not to submit a revised version of the manuscript.
Tórshavn 27 April 2021
Bogi Hansen
Citation: https://doi.org/10.5194/os-2021-14-AC1 - AC3: 'Reply on RC2', Bogi Hansen, 09 Jun 2021
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AC1: 'Reply to both referees', Bogi Hansen, 27 Apr 2021
Interactive discussion
Status: closed
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RC1: 'Comment on os-2021-14', Anonymous Referee #1, 05 Mar 2021
Review of “An overlooked freshwater source contributed to the extreme freshening event in the eastern subpolar North Atlantic after 2014” by Hansen et al.
This manuscript describes a connection between Iceland-sourced freshwater and the freshening of the eastern subpolar gyre from 2014-2018. This freshening event has been previously identified by Holliday et al. (2020) as the largest freshening event in the past 120 years, thus the topic is of great interest to the oceanographic community, and is suitable for publication in Ocean Science Discussions. The authors of the present study use satellite altimetry, surface drifters, repeat hydrographic sections, and mooring data to support their claims of increased Iceland-sourced freshwater in the Iceland basin. The compilation of this many sources of data clearly took a lot of effort, and deserves recognition.
The idea that freshwater from Iceland can have an appreciable impact on the large-scale salinity structure of the subpolar North Atlantic is certainly a novel idea that is worth researching. However, the Iceland-sourced freshwater could be overlooked for good reason – possibly because it is too small to have an effect on the large-scale. It is a tall task to convincingly demonstrate that the oceanographic community has largely ignored an important phenomenon, and it requires strong evidence to do so.
I did not find that this paper presented such evidence. I found that the authors’ central thesis lacked quantification (and thus was difficult to understand and prove/disprove), and the results were insufficient to support the authors’ claims. I explain these points further below.
Lack of quantification: From the outset, it was unclear what signal(s) the paper is attempting to explain. Holliday et al. (2020) describes a freshening of the upper 1000 m of the Iceland Basin and its forcing mechanisms, without mention of freshwater emanating from Iceland. In particular, Holliday et al. (2020) find a remarkable agreement between the magnitude of the salinity increase of the Scotian Shelf/Gulf of Maine region with the magnitude of the salinity decrease of the eastern SPG, implying that the whole of the freshening in the eastern SPG can be explained by freshwater coming from the Grand Banks region. Yet this paper seems to assume that the mechanisms presented in Holliday et al. (2020) are insufficient to explain the freshening. What evidence is there of this? How much of the freshwater signal from Holliday et al. (2020) is unexplained? And can the magnitude of the freshwater flux from Iceland explain the unexplained portion?
Later in the text, the authors focus on the near-surface freshening as evidence of the Iceland-sourced contribution. But the authors acknowledge that the OSNAP moorings they use to track the vertical structure of the salinity anomalies lack surface instruments, so it is again unclear what signal the authors are trying to explain. If indeed surface salinity is of interest (and it appears that it is), why not use satellite sea-surface salinity, which has near global coverage from 2009-present? From this, one could construct a time series of surface salinity for a given region, and then do an analysis of the relative roles of various mechanisms.
Results insufficient to support claims: I will address each of the data sources individually:
- Satellite altimetry – the authors use satellite altimetry to demonstrate that there is a mode of variability in which the height of the central Iceland Basin is anti-correlated to the height of the shelf/slope region. This mode is especially strong in the 2014-2018 period, indicating that the cyclonic surface circulation around the Iceland Basin was strengthened during this period, “representing enhanced anticlockwise circulation that might bring water from the shelf/slope region south of Iceland in southwest-ward direction towards the western Iceland Basin” (line 176). However, there are two issues with this logic: (a) if the water were to follow the SSH isolines indicated in either Fig 2 or 3a, then the fresh water would flow almost directly into the Irminger Sea and not affect the salinity of the Iceland Basin, and (b) there is no indication that this mode is at all related to the shelf-basin exchange (geostrophic or ageostrophic) around Iceland. The authors acknowledge “the near surface flow… has no indication of an average flow from the Icelandic shelf/slope region into the central Iceland Basin” (line 156) and “it seems likely that the MDT and ADT are not sufficiently accurate in the shelf/slope region south of Iceland to reflect the actual flow…” (supplementary materials). Given these statements, the motivation for looking at altimetry to document shelf-basin exchange is unclear. What information does altimetry provide? Could adding the Ekman velocities onto the surface geostrophic velocities from altimetry be more informative?
- Surface drifters – the authors use 11 surface drifter tracks that crossed from the Iceland shelf into the western Iceland Basin between 1995 and 2018 to demonstrate that Icelandic shelf water influences the western Iceland basin. But these drifters represent a small portion (1/8) of the total surface drifters that crossed into the Iceland shelf region during this period, thus the western Iceland basin is not a primary pathway for the freshwater around Iceland. There is the possibility that at certain times, this pathway is more important than others (potentially important between 2014-2018), but the authors acknowledge that “With only 11 drifters taking this path, it would be hard to determine temporal variations” (line 207). The authors then attempt to tie the surface drifter pathways to the altimetry EOF, but their argument falls flat because the altimetry EOF does not indicate shelf-basin exchange around Iceland (i.e. the motivation for using the drifters). Thus I believe the surface drifters demonstrate the opposite of what the authors contend: that the majority of the freshwater on the Iceland shelf does not flow into the Iceland Basin, and that it either goes eastward into the Nordic Seas or westward into the Irminger Sea. The small number of drifters that flow into the western Iceland basin are a small percentage of a small freshwater flux from Iceland (~5 mSv, no reference salinity provided), and therefore likely represent a very small quantity.
- Extended Ellett line – the authors contend that the salinity structure in the upper 200 m across the Iceland basin indicates an input from Icelandic-sourced freshwater. The salinity structure varies between years, and during periods of low salinity (2015-2017), the salinity of the central Iceland basin was lower than the eastern and western boundaries (in contrast to the other years, which increased almost monotonically from Iceland to Scotland). But again, this result seems to fly in the face of the authors’ argument that the freshwater is coming off Iceland – if the input from the Iceland shelf is confined to the western Iceland basin, as indicated by the surface drifters, then shouldn’t the salinity of the western Iceland basin decrease the most? The low salinities in the central Iceland basin (in regions of northward velocities) indicates that these low salinity waters are sourced from the south, rather than the north.
- OSNAP moorings – the argument here is that the high-frequency variability in the vertical salinity structure cannot be explained by far-field forcing from the western boundary and instead requires more local forcing (line 300). However, I do not follow this logic. The Iceland basin is full of eddies and small-scale structure that advect property anomalies – why can’t these cause the high-frequency salinity variability seen at the OSNAP moorings? If the argument from Holliday et al. (2020) were that a ‘pool’ of freshwater came off the Newfoundland/Labrador Shelf and moved coherently into the eastern SPG, then I would agree with the authors that the vertical structure of this salinity anomaly would be eroded by the time it arrived to the eastern SPG. But I don’t believe that’s the argument in Holliday et al. (2020).
- Hydrographic sections south of Iceland – I found the timeseries of freshwater thickness compelling, but given the infrequent coverage of the hydrographic sections, these data should only be considered ancillary, and not central to the argument. In other words, I would be more convinced of the authors’ arguments presented in this section if the altimetry and drifter data were stronger. Furthermore, the lack of velocity data diminishes what can be inferred from these sections, particularly regarding the strength of the freshwater fluxes. It was unclear to me even which direction these freshwater fluxes at each line are directed considering that Section IH “seems to be located in a divergence zone between eastward- and westward-flowing waters” (line 276). Does this imply that the freshwater flux at Section ST is eastward? How does that align with Fig. 1a? And what evidence is there to multiply the salinity fields by a constant 10 cm/s velocity (line 273)?
Despite my reservations about this manuscript, I am not convinced that the Iceland-sourced freshwater does not play a role in the large-scale salinity structure of the subpolar North Atlantic. In particular, Figs. 2 and 3 from Holliday et al. (2020) indicate that there were fresh anomalies in 2014-2016 south of Iceland that seemed to contribute to the freshening of the eastern SPG. Whether those fresh anomalies are sourced from Iceland is unclear, as well as why the anomalies appear to move against the cyclonic subpolar circulation into the eastern SPG, but it does deserve further study. This mechanism is particularly important given how much water mass transformation occurs in the Iceland basin, and the potential role of meltwater from Iceland.
Introducing an entirely new concept to a well-developed field takes strong evidence. It’s possible that freshwater from Iceland is indeed overlooked but to prove that it is, the authors need to quantify its impact and rigorously demonstrate that it plays an important role.
To provide more guidance, I am including more specific comments below:
- Line 48-49 “most of the entrainment into overflow also occurs in the eastern SPNA” are you referring to most of the water mass transformation across the isopycnals of maximum overturning?
- Line 63 where is the Faxafloi line? Please mark this on Fig 1a.
- Line 85 – what is the reference salinity for the 5 mSv of freshwater carried by the Iceland Coastal Current?
- Line 87 – “In addition to this” – what does “this” refer to?
- Section 2 – Is it necessary to have so many subpoints? Can all the data be summarized in a single paragraph?
- Line 131 – it would be instructive if Supplementary Table S1 also included the seasonal and interannual timing of these cruises.
- Line 148 – Please elaborate on “This makes the result independent of any assumptions used in generating the MDT…”. To what assumptions are you referring?
- 2 – If the black line delineates the contour that separates the flow north and south of the Faroes, why doesn’t it intersect the Faroes?
- Line 154 and elsewhere – replace “anticlockwise” with “cyclonic” (or “clockwise” with “anticyclonic”).
- Line 157 – what is meant by ‘distorted’? The average flow pattern incorporates all time scales, including synoptic variability. The horizontal resolution of the SSH will not resolve the mesoscale, but why does that mean it’s distorted?
- Line 165 – what is meant by long time scales? Fig 3c outlines the seasonal component of this mode… does the seasonality come into the argument at all?
- 3c – it would be useful to use a box-and-whisker plot here to show the median, quartile ranges, outliers, etc. Currently, it is tough to determine the strength of the seasonal cycle purely from the overlapping markers.
- 3d – is this panel a zoom in on panel b? If so, this should be highlighted in panel b (maybe draw a rectangle in panel b around the bounds of panel d, or alternatively just make a note of this in the caption for panel d).
- Line 175 – It should be noted that this sentence refers to reduced cyclonic circulation, rather than an actual anticyclonic circulation in the Iceland basin.
- Line 210 – why is only longitude considered?
- Lines 207-214 – the argument that the altimetry and surface drifters are well correlated would be significantly strengthened if there were an accompanying figure. Can you produce a set of maps of altimetry and surface drifter tracks for ~4 time steps? Or alternatively show the two curves that are correlated? I am surprised that there is such good agreement between these data sets considering that the Ekman velocities are not added into the surface geostrophic velocity field. Would these comparisons improve if the Ekman velocities were added?
- Line 230 – remove comma after ‘months’.
- Figure S7b – at mooring M2, why does the salinity decrease from 50 m to 100 m when it increases in panel a and Fig. 6b?
- Lines 250-253 – It is not clear why the freshwater content is valid here. Schauer and Losch (2019) discuss the arbitrary use of reference salinities in the calculation of freshwater transport, and their arguments hold whether the freshwater came from ‘pure freshwater’ or not. The only way around this issue is to use a closed volume in which the mass budget is balanced. Another method is to report the freshwater fluxes relative to two reference salinities to demonstrate their sensitivity to the choice of reference.
- 7 – why is 200 m chosen as a bottom limit?
- Line 285 – There is no evidence presented here that the drifters followed different paths in different years, and the authors admit as much (line 207).
- Line 351 – how is the importance of the precipitation trend assessed relative to other mechanisms? Given that there is no explanation of what signal the authors are trying to explain, it is hard to assess whether the precipitation is small or large comparatively.
- Line 356 – This explanation of the errors in the MDT near land and steep topography should go in section 3.1. This is much more clear than the current explanation on lines 147-149.
- Line 361 – Two drifters followed this pathway in 2015 from table S2, but earlier the authors admit that they cannot use the drifters for temporal variability due to insufficient coverage. Along these lines, there were three drifters that followed this pathway in 1996 when the principal component was near zero.
- Line 364 – “we find it most likely that the Icelandic freshwater source was the primary cause of the near-surface freshening events in the 2015-2017 period.” I don’t know of any evidence to support this claim, particularly in regard to the ranking of roles that various mechanisms played.
Citation: https://doi.org/10.5194/os-2021-14-RC1 - AC2: 'Reply on RC1', Bogi Hansen, 09 Jun 2021
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RC2: 'Comment on os-2021-14', Anonymous Referee #2, 13 Apr 2021
General comments
This paper uses observational data (satellite altimetry, trajectory of surface drifters, in situ salinity measurements) to suggest an overlooked freshwater source off the Iceland coast. The authors argue that this addition of low-salinity waters likely contributed to the recent freshening in the eastern subpolar North Atlantic that was described by a previous study (Holliday et al. 2020). Despite the title referring to “freshwater source”, the manuscript does not argue for a source of freshwater, but rather a phenomenon where fresher surface waters off the coast of Iceland were diverted into the Iceland Basin due to an anomalous circulation pattern south of Iceland.
The analysis starts by evaluating satellite altimetry data to define the common characteristics of geostrophic flow in the easter subpolar gyre as well as its temporal variability in the recent years. By applying an EOF analysis on the satellite altimetry over the eastern SPG the authors identify an enhanced cyclonic circulation after 2014. They also use satellite-tracked drifter data and correlate it with the temporal variability of the principal component in the satellite altimetry EOF analysis. This is interpreted by the authors as that during the enhanced anticlockwise circulation the drifter trajectories originating from the Iceland shelf are preferably directed southwestwards (into a region of anomalous freshening). They then looked at various salinity observations from hydrographic sections in the region (Extended Ellett Line, OSNAP morning and sections over the south-Icelandic shelf) to describe a possible addition of freshwater into the eastern subpolar North Atlantic derived from the Icelandic coast via a change in the upper circulation.
The study includes a comprehensive analysis of various data sets and observations and I acknowledge the attempt to connect these diverse datasets with each other. However, these results do not convincingly show the relevance of such a freshwater source to contribute to the overall freshening. Therefore this complicates without suitable justification the story that Holliday et al. (2020) laid out to explain the freshening.
The main issue with this study is the lack of quantification. This type of analysis essentially requires a budget analysis in order to provide a clear freshwater estimate and establish that the redirection of low-salinity water off the Iceland shelf is a relevant signal in the eastern subpolar North Atlantic. The analysis presented here does not meet current standards given that there is now readily available data that goes beyond mooring and hydrographic ship data. In particular it is surprising that Argo profiling data has not been utilized to complement the other salinity observations. Especially for recent time periods, salinity profiles from Argo floats should provide a better picture of the spatiotemporal variability in that regions. There are also a number of gridded salinity products derived from Argo and other profile data as well as satellite-derived surface salinity estimates which are publicly and readily available. In particular, the use of ocean reanalysis products (e.g., SODA, ECCOv4) would be essential in such investigation as these allow closed budget analyses that can establish underlying mechanisms. It is essential to include these analyses, which then can be compared to the present hydrographic data to see if the picture is still consistent with the freshening and hypothesized pathway of freshwater over the Iceland Basin.
In summary, I am not able to recommend the manuscript for publication at this stage. Much more rigorous quantitative data needs to be presented in order to demonstrate that freshwater from the Iceland shelf is relevant in describing variability in the salinity and stratification in the eastern subpolar North Atlantic. This will require major revisions, with both new (and robust) analysis as well as substantial rewriting of the text.
Specific comments
Title: The title is misleading since it describes a “freshwater source”, but in fact the hypothesized mechanism is a change in circulation.
Line 34: What is meant by the term “buffering region”?
Lines 43-46: This statement needs references. I am not aware that the subpolar North Atlantic is nutrient limited and would expect it to be largely light limited. Thus, a reduction in vertical mixing could also lead to higher productivity due to a decrease in light limitation.
Section 2: I think this section would read better without the different subsection and instead have a single section with complete paragraphs. Also, I feel there is a lot of important information missing. For example, the EOF analysis is a key method in this study, but it is only described in the supplementary material. A shortened description should be included in the Method section too. As well, what is the calculation method for freshwater thickness? What analysis software has been used (Python, Matlab etc.)?
Line 128-129: How has potential density been derived?
Line 130-134: The location of these sections should be included in the map (Figure 1).
Line 135-137: This is confusing phrasing. ERA5 should be stated as the atmospheric reanalysis product and CDS the repository from which the data has been obtained.
Section 3: Instead of calling the subtitles by the data product, it would be better to have an actual subject that refers to the finding/processes etc. For example, instead of the title “satellite altimetry”, it should be called something like “geostrophic flow pattern”.
Line 157: Need to explain more how the spatial pattern in Figure 2 can be interpreted to identify the flow.
Line 159-163: I think the description of the EOF analysis fits better in Section 2.
Line 160: Please specify the spatial domain that has been used in the EOF analysis (cite lat and lon bounds).
Line 166: “Anomalous behaviour” is unnecessarily vague. Just describe what is anomalous about it.
Figure 3 b-d: Units are missing on the axis labels. Is it "cm" as in panel a?
Lines 174-176: How is the connection made between zero to negative PC values and enhanced clockwise circulation? This connection has not been clearly explained.
Line 203: I’m not sure if the term “streamlines” is appropriate here. This would set the EOF equivalent to a stream function, which I don’t think is the case.
Lines 211-213: It is not obvious to me how a fairly weak correlation between the zonal drift and PC time series leads to that conclusion. Maybe it helps if the authors actually describe the process behind the apparent correlation.
Lines 224-226: The fact that the drop in salinity is confined to the middle of the basin indicates to me that the freshening is sourced from the south, as this region corresponds to the pathway of the northward flowing NAC.
Lines 248-249, Fig 7c: How's the freshwater content calculated? This needs to be included in Section 2.
Line 254: Cite the actual value that is chosen as the reference salinity.
Line 274: The freshwater flux of 5 mSv needs to be put in context. This is quite small compared to any other freshwater flux estimate over the subpolar North Atlantic.
Lines 276-277: Please clarify the relevance of this statement.
Section 4: A majority of the content of this section can be regarded as results. Usually, the Discussion is for bringing up previous studies, raising potential concerns and caveats, and restating the main findings.
Lines 296-299: If the freshening is explained by just deviation in the circulation, then shouldn’t we expect salinification in the downstream region where the Icelandic source usually ends up?
Line 305: This statement need to be supported by quantitative analysis.
Line 308: The phrasing “more normal flow” is odd. It should be clarified what normal is.
Line 324: The phrasing “drained of freshwater” does not make sense in the context of oceanic freshwater content.
Line 332: Clearly describe the steps used to get to the estimate of 0.5 m.
Section 4.3 This suggest only a minor influence of the Icelandic freshwater source and thus contradicts the whole premise of this study.
Line 415: Statement “improves conditions for primary production” needs references.
Line 427: Specify a quantitative criterion to determine convection depth.
Page 20: This is a strange way to end the paper. A section containing Conclusions is missing.
Citation: https://doi.org/10.5194/os-2021-14-RC2 -
AC1: 'Reply to both referees', Bogi Hansen, 27 Apr 2021
Response to comments from both referees
With the comments from both referees posted, it seems clear that neither of the referees finds the manuscript worthy of publication without major revisions. Some of this is no doubt because of unclear explanations in our text. In hindsight, the title should probably have been less provocative, less certain, and more direct – perhaps something like “Runoff from Iceland may have contributed to the extreme surface freshening in the Iceland Basin after 2014”. It was certainly never the intention to claim absolute proof of our hypothesis. Only to argue for its likelihood, which we still feel that we have done. Neither was it the intention to claim that the Icelandic freshwater source was large compared to the western source described by Holliday et al. (2020), as the referees seem to read our text. The aim was to argue that the Icelandic source – although much smaller than the western source – was local (to the Iceland Basin) and surface intensified, which could give the surface salinity of the Iceland Basin the extra downward kick to make it so extreme (e.g., lines 22-25 in the abstract).
Apparently, these messages were not sufficiently clear, for which we apologize. If this was the only problem, more careful rewriting of the text might make the manuscript more acceptable, but you would also expect referees to take this into account. The referees raise more serious questions, however, that are more difficult to address.
One of these questions is in regard to insufficient support. In our manuscript, we presented five types of observational evidence (satellite altimetry, surface drifters, Extended Ellett Line, OSNAP moorings, and Icelandic standard sections). Referee 1 discusses each of these and finds none of them convincing. Instead, he/she recommends using satellite sea-surface salinity. This might indeed be worthwhile but, to our knowledge, these data are not reliable close (50 km) to land (Sea Surface Salinity | NOAA CoastWatch & OceanWatch) and are not very accurate (doi:10.1002/2014JC009961). A brief look at these data indicates extremely high scatter in the Iceland Basin. It does not seem likely that these data can provide evidence of an Icelandic contribution that is much more convincing than the collective observational evidence already presented. A similar argument may be made to the suggestion by Referee 2 to use ARGO data. Again, this might be worthwhile, but we do not find it likely that the relatively few ARGO floats that at any time are present in the Iceland Basin with their preference for bottom depths > 1000 m and high spatio-temporal variability can provide much more convincing evidence.
A second question raised by both referees is in regard to quantification. It seems that the referees want quantification of the Icelandic source relative to the western source discussed by Holliday et al. (2020). This can, of course, be done although with some uncertainty, but we did not include it directly since we (as mentioned above) never intended to claim that the Icelandic source was of comparable magnitude to the western source. However, if this is what the referees mean by lack of quantification, Table 1 and the discussion in Sect. 4.2 do compare the two sources indirectly (summarized in lines 409-412). Our main argument was that runoff from Iceland was sufficient to explain the extra freshening of the surface layer relative to deeper levels. As shown in lines 328-342 in our manuscript, we feel that we have done that quantitatively.
According to Referee 1: “Introducing an entirely new concept to a well-developed field takes strong evidence”. Alternatively, one might argue that a new concept deserves recognition even if it is not 100% proven as long as it does not contradict any basic principles. But that is, of course, a matter of opinion.
In summary, we agree that the manuscript could have been clearer in the text, but we still feel that the evidence presented supports the basic idea of a contribution from Icelandic runoff to Iceland Basin near-surface freshening, although not providing absolute proof. We do not find it likely, however, that we will be able to supply additional evidence that can appear more convincing to the referees.
We have therefore decided not to submit a revised version of the manuscript.
Tórshavn 27 April 2021
Bogi Hansen
Citation: https://doi.org/10.5194/os-2021-14-AC1 - AC3: 'Reply on RC2', Bogi Hansen, 09 Jun 2021
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AC1: 'Reply to both referees', Bogi Hansen, 27 Apr 2021
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Bogi Hansen
Karin Margretha Húsgarð Larsen
Hjálmar Hátún
Steingrímur Jónsson
Sólveig Rósa Ólafsdóttir
Andreas Macrander
William Johns
N. Penny Holliday
Steffen Malskær Olsen
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