the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Apr 2023
The Iceland-Faroe warm-water flow towards the Arctic estimated from satellite altimetry and in situ observations
Bogi Hansen
Karin Margretha Húsgarð Larsen
Hjálmar Hátún
Steffen Malskær Olsen
Andrea Martina Ulrike Gierisch
Svein Østerhus
Sólveig Rósa Ólafsdóttir
Abstract. The inflow of warm and saline Atlantic water to the Arctic Mediterranean (Nordic Seas and Arctic Ocean) between Iceland and the Faroes (IF-inflow) is the strongest Atlantic inflow branch, in terms of volume transport, and associated with a large transport of heat towards the Arctic. The IF-inflow is monitored on a section east of the Iceland-Faroe Ridge (IFR) by use of Sea Level Anomaly (SLA) data from satellite altimetry, a method that has been calibrated by in situ observations gathered over two decades. Monthly averaged surface velocity anomalies calculated from SLA data were strongly correlated with anomalies measured by moored Acoustic Doppler Current Profilers (ADCPs) with consistently higher correlations when using the reprocessed SLA data released in December 2021 rather than the earlier version. In contrast to the earlier version, the reprocessed data also had the correct conversion factor required by geostrophy. Our results show that the IF-inflow crosses the IFR in two separate branches. The Icelandic branch is a jet over the Icelandic slope with average surface speed exceeding 20 cm s−1, but it is narrow and shallow with an average volume transport less than 1 Sv (106 m3 s−1). Most of the Atlantic water crosses the IFR close to its southernmost end in the Faroese branch. Between these two branches, water from the Icelandic branch turns back onto the ridge in a retroflection with a recirculation over the northernmost bank on the IFR. Combining multi-sensor in situ observations with satellite SLA data, monthly mean volume transport of the IF-inflow has been determined from January 1993 to December 2021. The IF-inflow is part of the Atlantic Meridional Overturning Circulation (AMOC), which is expected to weaken under continued global warming. Our results show no weakening of the IF-inflow. Annually averaged volume transport of Atlantic water through the monitoring section had a statistically significant (95 % confidence level) increasing trend of (0.12 ± 0.10) Sv per decade. Combined with increasing temperature, this caused an increase of 13 % in the heat transport, relative to 0 °C, towards the Arctic of the IF-inflow over the 29 years of monitoring. The near-bottom layer over most of the IFR is dominated by cold water of Arctic origin that may contribute to the overflow across the ridge. Our observations confirm a dynamic link between the overflow and the Atlantic water flow above. The results also provide support for a previously posed hypothesis that this link may explain the difficulties in reproducing observed transport variations of the IF-inflow in numerical ocean models, with consequences for its predictability under climate change.
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Bogi Hansen et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-828', Anonymous Referee #1, 07 Jun 2023
General comments:
This paper describes the Iceland-Faroe warm-water flow towards the Arctic Mediterranean by using multiple sources of data, including satellite altimetry, ADCPs, PIES, drifters, and hydrographic data. The authors investigate two branches of the inflow, calculate heat and volume transports, and put their results into the wider context of the AMOC (as the inflow is part of it), detailing their analysis methods.
The paper fits into the scope of this journal and is a relevant contribution to the literature. It presents new data, analysis and methods. Substantial conclusions are reached (the highlighted text in section 7 works well to get the main message per section across) but overall conclusions could be pulled together at the very end in a more concise manner. Scientific methods and assumptions are valid and clearly outlined, the results are sufficient to support the interpretations, and the conclusions are traceable. Title, abstract and references are all appropriate. The structure could use some potential reorganization (detailed below). Some figures could also get improved by some minor adjustments. The paper includes a lot of tables (14), could the authors have a critical review if all of these are needed in the main results sections or if some could get moved to the methods and/or an appendix (see comment below)? The paper has the potential to be valuable to other researchers, it is well-written, below are a few suggestions with regard to organization and for some minor edits, which would hopefully improve the manuscript. I would recommend this paper to be published after some minor changes.
Specific comments:
Organization:
- The results sections 5 and 6 (having multiple sub-sections) would benefit from a brief introductory paragraph for each of those sections (before 5.1 and 6.1).
- The results sections include a lot of detail on methods used (for example section 6.1 which refers back to section 3 and Table 3 a few times; section 6.2 following on from 6.1; parts of section 6.3 (for example Fig 12)). Would the paper flow better if some sections got moved into the Methods section? Maybe that will disturb the flow of the paper but maybe the authors could consider this.
- Tables: a total of 14 tables are in the main part of the manuscript, as mentioned above, could these get critically evaluated to see if some could get moved to the methods section and/or an appendix (Tables 3, 4 include a lot of detail that it already suitably summarized in the text and could be included in an Appendix for example).
- Finish the paper with one strong paragraph summarizing the main results in a final Conclusions section (7.7).
Clarifications:
- Lines 145-146: What is the seasonal spread of the CTD cruises? Have you applied standard QC to the CTD data, if so, mention it.
- Lines 169-174: What type of ADCP was used?
- Line: 175: “either 10 m or 25 m” state what this depends on.
- Line 207: “necessary additional information” – be specific what this it.
- Lines 362-363: the average surface velocity is likely underestimated by the extrapolation method – could you give an indication or estimate of how much this would be?
- Lines 683-684: Here you use more decimals than in the rest of the paper, be consistent, 11 cm s-1 and 12 cm s-1 .
- Sections 7.2 and 7.4 need highlights in bold italic.
Figures:
- It would be useful if the lat/lon range in the figures were consistent (except for 1a which requires a larger area for context) for ease of comparison (especially for Figs 5a and 5b).
- Please make the colour bars distinct for ease of interpretation (Figs 4, 10).
- Are the figures compatible with colour-vision deficiencies? (using not just colours but dashed/dotted lines for example)
- Fig 3a: could this figure get rotated to have depth on the y-axis to match Fig 3b or is there a specific reason for this orientation?
- Fig 6: Mark (by a horizontal line?) where the extrapolation starts on each of the four profiles.
- Fig 7: make the four sub-plots consistent with regard to x-lim, number of bars for ease of comparison.
Technical corrections:
- Line 35: Define Greenland-Scotland-Ridge acronym (GSR) at first use and then use acronym throughout.
- Line 41: include abbreviation “IFF” here.
- Line 43-44: check the grammar of that sentence.
- Line 143: “Materials and Methods”
- Line 146: “many CTD profiles” – be specific: “between 98 and 155 CTD profiles”
- Line 229: only use “MDT” (acronym already defined earlier)
- Fig 2a: The label for A2 is hidden behind N01, reposition it so it is visible; label for A4 is missing in the figure
- Fig 5b: the blue track is hard to see, choose a different/more distinct colour/line style/width.
- Line 643: change to “Comparison of in situ observations with old and new altimetry data”
- Line 725: “some”
Citation: https://doi.org/10.5194/egusphere-2023-828-RC1 -
AC1: 'Reply on RC1', Bogi Hansen, 15 Jun 2023
Referee 1
We thank the referee for a very constructive and helpful review. Below, we detail how each referee comment has been addressed and how the manuscript has been modified accordingly.
Manuscript structure
As suggested by both referees, we have tried to re-structure the manuscript to enable better flow. Five tables have been moved to a new Appendix B and Figure 12 has been moved to the Materials and methods section together with associated text. In addition, the Introduction (Sect. 1) has been organized into four sub-sections, mainly to put more emphasis on the definition of Atlantic water for transport calculation.
In the following, we address specific comments from Referee 1 where quoted text from the old or the new version of the manuscript is in italic.
Organization:
Referee: The results sections 5 and 6 (having multiple sub-sections) would benefit from a brief introductory paragraph for each of those sections (before 5.1 and 6.1).
Reply: Has been done
Referee: The results sections include a lot of detail on methods used (for example section 6.1 which refers back to section 3 and Table 3 a few times; section 6.2 following on from 6.1; parts of section 6.3 (for example Fig 12)). Would the paper flow better if some sections got moved into the Methods section? Maybe that will disturb the flow of the paper but maybe the authors could consider this.
Reply: Fig. 12 and associated text has been moved to Sect. 2.1. Together with five tables being moved to the new Appendix B, this should help streamlining the manuscript. Unfortunately, we have not been able to identify any additional re-structuring that could get a better flow.
Referee: Tables: a total of 14 tables are in the main part of the manuscript, as mentioned above, could these get critically evaluated to see if some could get moved to the methods section and/or an appendix (Tables 3, 4 include a lot of detail that it already suitably summarized in the text and could be included in an Appendix for example).
Reply: A new appendix was added: “Appendix B: Supplementary tables” and 5 tables moved from the main text to this appendix: Table 1, Table 3, Table 5, Table 10, and Table 14. The associated text was modified to de-emphasize these tables.
Referee: Finish the paper with one strong paragraph summarizing the main results in a final Conclusions section (7.7).
Reply: Has been done. The heading of Sect. 7 has been modified accordingly.
Clarifications:
Referee: Lines 145-146: What is the seasonal spread of the CTD cruises? Have you applied standard QC to the CTD data, if so, mention it.
Reply: The text: “mainly in February, May, August–September, or November” has been added. Also, the following sentence was added: “Initially, an EG&G CTD was used but since 1996, this was replaced by a SeaBird 911+. Water samples were acquired for salinity calibration and all the data have been quality controlled.”
Referee: Lines 169-174: What type of ADCP was used?
Reply: The following sentence has been added: “Three different ADCP models from Teledyne RD Instruments have been used: 150 kHz Broadband, 75 kHz Broadband, and Long Ranger.”
Referee: Line: 175: “either 10 m or 25 m” state what this depends on.
Reply: We have added the text: “depending on bottom depth and ADCP model”
Referee: Line 207: “necessary additional information” – be specific what this it.
Reply: The paragraph has been reorganized and it has been specified that it is the lack of regular CTD data that prevent a more reliable extrapolation to the surface.
Referee: Lines 362-363: the average surface velocity is likely underestimated by the extrapolation method – could you give an indication or estimate of how much this would be?
Reply: We do not have any evidence, on which to base an objective estimate and would prefer not to guess. Instead, we now write: “the shape of the profile indicates that the average surface velocity is likely to be underestimated by the extrapolation method, although it is difficult to estimate by how much.”.
Referee: Lines 683-684: Here you use more decimals than in the rest of the paper, be consistent, 11 cm s-1 and 12 cm s-1 .
Reply: We used the extra decimal to indicate that the difference between the two numbers was small, but the sentence has been changed so that it now reads: “For the whole interval between A3 and A8, the average offset based on the new MDT is only 4 % smaller than the ADCP-based value.”
Referee: Sections 7.2 and 7.4 need highlights in bold italic.
Reply: Section 7.2 now has three highlighted sentences and Section 7.4 has two.
Figures:
Referee: It would be useful if the lat/lon range in the figures were consistent (except for 1a which requires a larger area for context) for ease of comparison (especially for Figs 5a and 5b).
Reply: Old Fig. 5a (new Fig. 6a), Fig. 5b (new Fig. 6b), Fig.14, and Fig. 16 now have the same map boundaries and the same basic map.
Referee: Please make the colour bars distinct for ease of interpretation (Figs 4, 10).
Reply: The colour bars of old Fig. 4 (new Fig. 5) and Fig. 10 (new Fig. 11) have been made distinct by introducing lines at the same intervals as the contour lines.
Referee: Are the figures compatible with colour-vision deficiencies? (using not just colours but dashed/dotted lines for example).
Reply: Old Fig. 6 (new Fig. 7) and Fig. 9b (new Fig. 10b) have been modified by making some of the lines dashed.
Referee: Fig 3a: could this figure get rotated to have depth on the y-axis to match Fig 3b or is there a specific reason for this orientation?
Reply: Old Fig 3a (new Fig. 4a) has been rotated so that the depth axis is vertical as in Fig 3b.
Referee: Fig 6: Mark (by a horizontal line?) where the extrapolation starts on each of the four profiles.
Reply: The extrapolation start is now marked by a black circle on each profile on the new Fig. 7a.
Referee: Fig 7: make the four sub-plots consistent with regard to x-lim, number of bars for ease of comparison.
Reply: A new consistent figure (new Fig. 8) has been made.
Technical corrections:
Referee: Line 35: Define Greenland-Scotland-Ridge acronym (GSR) at first use and then use acronym throughout.
Reply: The Greenland-Scotland-Ridge is only referred to twice in addition to this, and towards the end of the manuscript. We have therefore instead replaced the one reference to “GSR” by the full name: “Greenland-Scotland Ridge”.
Referee: Line 41: include abbreviation “IFF” here.
Reply: Has been done.
Referee: Line 43-44: check the grammar of that sentence.
Reply: The sentence has been split into two sentences and clarified.
Referee: Line 143: “Materials and Methods”
Reply: Has been corrected.
Referee: Line 146: “many CTD profiles” – be specific: “between 98 and 155 CTD profiles”
Reply: Has been specified.
Referee: Line 229: only use “MDT” (acronym already defined earlier)
Reply: Has been done.
Referee: Fig 2a: The label for A2 is hidden behind N01, reposition it so it is visible; label for A4 is missing in the figure
Reply: The figure has been modified to correct this.
Referee: Fig 5b: the blue track is hard to see, choose a different/more distinct colour/line style/width.
Reply: The track has been made thicker and the background has also been changed, which should enhance the contrast (new Fig. 6b).
Referee: Line 643: change to “Comparison of in situ observations with old and new altimetry data”
Reply: Has been changed
Referee: Line 725: “some”
Reply: Has been corrected.
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RC2: 'Comment on egusphere-2023-828', Anonymous Referee #2, 08 Jun 2023
Review on the manuscript
«The Iceland-Faroe warm-water flow towards the Arctic estimated from satellite altimetry and in situ observations” by Hansen et al. submitted to Ocean Science.
The authors estimate the transport toward the Arctic between Iceland and the Faroes using a combination of ADCPs and satellite altimetry. There are several steps in which the sea level anomalies (SLA) are correlated with ADCP-measurements to produce a method such that the transport toward the Arctic can be estimated with SLA only. It is a substantial contribution to the scientific progress within the scope of Ocean Science. The manuscript is well written and structured, and the figures are of good quality.
The manuscript is however very technical with several analysis and many correlations are presented. The manuscript will benefit from moving some of these analysis/tables to supplementary material or to an appendix. This will make it easier for the reader to follow the steps and analysis. My main conclusion is that the manuscript can be published in Ocean Science with minor/moderate revision.
Major comment
The transport estimates are calculated by integration the velocities down to the depth where T=4 oC that they define as the base of the Atlantic layer. However, the properties of the AW change with time (e.g., fig 15b), and the authors should give some estimates of the sensitivity of results if other depths are used as the lower limit. E.g., what will the transport estimates be when using T=3 oC or T=5 oC as the lower limit of the depth integration.
Minor comments
Is table 1 needed? Fig. 2. shows the bottom depth of the area/stations and it is already written that the CTD-stations are taken 3-4 times a year. If deleted, the sentence on lines 146-147 can also be deleted.
Line 211-212: “… the extrapolation factor may be modified to account for these.” Please, specify. Was this done?
Figure 6. It would be interesting to see the variance of the cross-ridge velocities for the four ADCPs, e.g., include standard deviation in the figure.
Line 371: “They document that…” Who are they?
Line 405-413. With the regression analysis for u=a*dH+b, the values a will determine the strength of the variability while b determines the bias. Thus, it is the variability of the surface velocity that is underestimated.
Line 439-440: According to Figure 6, zero velocity are not reached at several ADCP-locations. Is the vertical integration done to bottom when the velocity does not reach zero?
Table 7. The mean values of Deq and Leq are presented but I assume that there might be large temporal variation. If standard deviations or errors can be included, this will give some indications on the sensitivity of the method.
Figure 11. Is the altimetric offset calculated from the averaged surface velocities from ADCPs?
Line 532: Please include a reference for the depth of AW = 4 oC.
Line 573: see my comment on Line 532.
Line 772: “… surface AND deeper…”
Line 776: See my major comment.
Appendix
Eq. A1: why 500 m? Why not use depth of the Atlantic layer or bottom depth?
Citation: https://doi.org/10.5194/egusphere-2023-828-RC2 -
AC2: 'Reply on RC2', Bogi Hansen, 15 Jun 2023
Referee 2
We thank the referee for a very constructive and helpful review. Below, we detail how each referee comment has been addressed and how the manuscript has been modified accordingly.
Manuscript structure
As suggested by both referees, we have tried to re-structure the manuscript to enable better flow. Five tables have been moved to a new Appendix B and Figure 12 has been moved to the Materials and methods section together with associated text. In addition, the Introduction (Sect. 1) has been organized into four sub-sections, mainly to put more emphasis on the definition of Atlantic water for transport calculation.
In the following, we address specific comments from Referee 2 where quoted text from the old or the new version of the manuscript is in italic.
Referee: The manuscript is however very technical with several analysis and many correlations are presented. The manuscript will benefit from moving some of these analysis/tables to supplementary material or to an appendix. This will make it easier for the reader to follow the steps and analysis.
Reply: Five of the tables have been moved to a new Appendix B. Also, Fig. 12 and associated text has been moved to Sect. 2.1.
Major comment
Referee: The transport estimates are calculated by integration the velocities down to the depth where T=4 oC that they define as the base of the Atlantic layer. However, the properties of the AW change with time (e.g., fig 15b), and the authors should give some estimates of the sensitivity of results if other depths are used as the lower limit. E.g., what will the transport estimates be when using T=3 oC or T=5 oC as the lower limit of the depth integration.
Reply: The discussion of Atlantic water in the Introduction has been enhanced to include the sensitivity and the effect of changing AW properties. In connection with this, the Introduction has been split into four subsections, for emphasis, with the discussion of Atlantic water as Sect. 1.3.
Minor comments
Referee: Is table 1 needed? Fig. 2. shows the bottom depth of the area/stations and it is already written that the CTD-stations are taken 3-4 times a year. If deleted, the sentence on lines 146-147 can also be deleted.
Reply: Table 1 has been moved to the new Appendix B.
Referee: Line 211-212: “… the extrapolation factor may be modified to account for these.” Please, specify. Was this done?
Reply: “may be modified” has been changed to: “was modified”
Referee: Figure 6. It would be interesting to see the variance of the cross-ridge velocities for the four ADCPs, e.g., include standard deviation in the figure.
Reply: A graph showing standard deviation has been added (new Fig. 7b) as well as a sentence referring to it in the text.
Referee: Line 371: “They document that…” Who are they?
Reply: The words “They document” have been replaced by “This implies”.
Referee: Line 405-413. With the regression analysis for u=a*dH+b, the values a will determine the strength of the variability while b determines the bias. Thus, it is the variability of the surface velocity that is underestimated.
Reply: We have added a sentence: “This might be due to a large bias, b in Eq. (3), for this site but inspection of individual daily velocity profiles does not support that (Fig. 9 in Hansen et al., 2018).”
Referee: Line 439-440: According to Figure 6, zero velocity are not reached at several ADCP-locations. Is the vertical integration done to bottom when the velocity does not reach zero?
Reply: The sentence before Eq. (7) has been changed to: “Sites IA, IB, and IE have inflow throughout the water column, on average (Fig. 7a), and the integration is down to the bottom. For site IW, we only integrate down to the depth, z = z0, where the average cross-ridge velocity becomes zero:”
Referee: Table 7. The mean values of Deq and Leq are presented but I assume that there might be large temporal variation. If standard deviations or errors can be included, this will give some indications on the sensitivity of the method.
Reply: For ADCP site IW, we now write: “LEq = (12 ± 4) km, where the uncertainty is determined by the uncertainty of αReg”. For the other sites on the IFR, we write: “For these sites, the relative uncertainty of LEq is higher (between 38 % and 75 %), and LEq has been set equal to the interval width, L.”. For DEq, we could not find any objective way to derive any uncertainty estimate. We have, however, added a sentence to Sect. 7.4 (old line number 832): “Also, the many uncertainties involved make the numbers in the bottom row of Table 4 rough estimates.”
Referee: Figure 11. Is the altimetric offset calculated from the averaged surface velocities from ADCPs?
Reply: The caption for Fig. 11 (new Fig. 12) has been clarified by the following text: “Optimized values for the Altimetric offset, Uk0, in each altimetry interval are shown by the thick continuous red line. The value for U20 is based on ADCP NI (Table B2). U30 and U40 are based on linear combinations of surface velocities from two or three ADCPs (Sect. 3). U50 and U60 are combined estimates from NC and NH (Table B2) and the geostrophic method. U70 is based on the geostrophic method.”
Referee: Line 532: Please include a reference for the depth of AW = 4 oC.
Reply: We now refer to the new Sect. 1.3.
Referee: Line 573: see my comment on Line 532.
Reply: We now refer to the new Sect. 1.3.
Referee: Line 772: “… surface AND deeper…”
Reply: Has been corrected
Referee: Line 776: See my major comment.
Reply: Hopefully, the new Sect. 1.3 with information on Atlantic water clarifies here.
Appendix
Referee: Eq. A1: why 500 m? Why not use depth of the Atlantic layer or bottom depth?
Reply: Our reason for doing this is that the deep boundary of the Atlantic layer is in general sloping. This was not well explained, however, so now the following text has been inserted before Eq. (A1): “Within each altimetry interval, k (spanning Ak–Ak+1), Uk(z,t) is the eastward velocity at depth z and time t, horizontally averaged within the interval. The contribution to Q(t) from this interval is found by integrating (summing) the velocity down to the deep boundary of the Atlantic layer (bottom or 4 °C isotherm) and multiplying by the interval width. The deep boundary is, however, in general not horizontal. To account for this, we introduce a parameter Wk(z,t), which is the width of Atlantic water within altimetry interval k at depth z and time t. With this definition, the volume transport is:”. The paragraph after Eq. (A1) has been modified accordingly.
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AC2: 'Reply on RC2', Bogi Hansen, 15 Jun 2023
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-828', Anonymous Referee #1, 07 Jun 2023
General comments:
This paper describes the Iceland-Faroe warm-water flow towards the Arctic Mediterranean by using multiple sources of data, including satellite altimetry, ADCPs, PIES, drifters, and hydrographic data. The authors investigate two branches of the inflow, calculate heat and volume transports, and put their results into the wider context of the AMOC (as the inflow is part of it), detailing their analysis methods.
The paper fits into the scope of this journal and is a relevant contribution to the literature. It presents new data, analysis and methods. Substantial conclusions are reached (the highlighted text in section 7 works well to get the main message per section across) but overall conclusions could be pulled together at the very end in a more concise manner. Scientific methods and assumptions are valid and clearly outlined, the results are sufficient to support the interpretations, and the conclusions are traceable. Title, abstract and references are all appropriate. The structure could use some potential reorganization (detailed below). Some figures could also get improved by some minor adjustments. The paper includes a lot of tables (14), could the authors have a critical review if all of these are needed in the main results sections or if some could get moved to the methods and/or an appendix (see comment below)? The paper has the potential to be valuable to other researchers, it is well-written, below are a few suggestions with regard to organization and for some minor edits, which would hopefully improve the manuscript. I would recommend this paper to be published after some minor changes.
Specific comments:
Organization:
- The results sections 5 and 6 (having multiple sub-sections) would benefit from a brief introductory paragraph for each of those sections (before 5.1 and 6.1).
- The results sections include a lot of detail on methods used (for example section 6.1 which refers back to section 3 and Table 3 a few times; section 6.2 following on from 6.1; parts of section 6.3 (for example Fig 12)). Would the paper flow better if some sections got moved into the Methods section? Maybe that will disturb the flow of the paper but maybe the authors could consider this.
- Tables: a total of 14 tables are in the main part of the manuscript, as mentioned above, could these get critically evaluated to see if some could get moved to the methods section and/or an appendix (Tables 3, 4 include a lot of detail that it already suitably summarized in the text and could be included in an Appendix for example).
- Finish the paper with one strong paragraph summarizing the main results in a final Conclusions section (7.7).
Clarifications:
- Lines 145-146: What is the seasonal spread of the CTD cruises? Have you applied standard QC to the CTD data, if so, mention it.
- Lines 169-174: What type of ADCP was used?
- Line: 175: “either 10 m or 25 m” state what this depends on.
- Line 207: “necessary additional information” – be specific what this it.
- Lines 362-363: the average surface velocity is likely underestimated by the extrapolation method – could you give an indication or estimate of how much this would be?
- Lines 683-684: Here you use more decimals than in the rest of the paper, be consistent, 11 cm s-1 and 12 cm s-1 .
- Sections 7.2 and 7.4 need highlights in bold italic.
Figures:
- It would be useful if the lat/lon range in the figures were consistent (except for 1a which requires a larger area for context) for ease of comparison (especially for Figs 5a and 5b).
- Please make the colour bars distinct for ease of interpretation (Figs 4, 10).
- Are the figures compatible with colour-vision deficiencies? (using not just colours but dashed/dotted lines for example)
- Fig 3a: could this figure get rotated to have depth on the y-axis to match Fig 3b or is there a specific reason for this orientation?
- Fig 6: Mark (by a horizontal line?) where the extrapolation starts on each of the four profiles.
- Fig 7: make the four sub-plots consistent with regard to x-lim, number of bars for ease of comparison.
Technical corrections:
- Line 35: Define Greenland-Scotland-Ridge acronym (GSR) at first use and then use acronym throughout.
- Line 41: include abbreviation “IFF” here.
- Line 43-44: check the grammar of that sentence.
- Line 143: “Materials and Methods”
- Line 146: “many CTD profiles” – be specific: “between 98 and 155 CTD profiles”
- Line 229: only use “MDT” (acronym already defined earlier)
- Fig 2a: The label for A2 is hidden behind N01, reposition it so it is visible; label for A4 is missing in the figure
- Fig 5b: the blue track is hard to see, choose a different/more distinct colour/line style/width.
- Line 643: change to “Comparison of in situ observations with old and new altimetry data”
- Line 725: “some”
Citation: https://doi.org/10.5194/egusphere-2023-828-RC1 -
AC1: 'Reply on RC1', Bogi Hansen, 15 Jun 2023
Referee 1
We thank the referee for a very constructive and helpful review. Below, we detail how each referee comment has been addressed and how the manuscript has been modified accordingly.
Manuscript structure
As suggested by both referees, we have tried to re-structure the manuscript to enable better flow. Five tables have been moved to a new Appendix B and Figure 12 has been moved to the Materials and methods section together with associated text. In addition, the Introduction (Sect. 1) has been organized into four sub-sections, mainly to put more emphasis on the definition of Atlantic water for transport calculation.
In the following, we address specific comments from Referee 1 where quoted text from the old or the new version of the manuscript is in italic.
Organization:
Referee: The results sections 5 and 6 (having multiple sub-sections) would benefit from a brief introductory paragraph for each of those sections (before 5.1 and 6.1).
Reply: Has been done
Referee: The results sections include a lot of detail on methods used (for example section 6.1 which refers back to section 3 and Table 3 a few times; section 6.2 following on from 6.1; parts of section 6.3 (for example Fig 12)). Would the paper flow better if some sections got moved into the Methods section? Maybe that will disturb the flow of the paper but maybe the authors could consider this.
Reply: Fig. 12 and associated text has been moved to Sect. 2.1. Together with five tables being moved to the new Appendix B, this should help streamlining the manuscript. Unfortunately, we have not been able to identify any additional re-structuring that could get a better flow.
Referee: Tables: a total of 14 tables are in the main part of the manuscript, as mentioned above, could these get critically evaluated to see if some could get moved to the methods section and/or an appendix (Tables 3, 4 include a lot of detail that it already suitably summarized in the text and could be included in an Appendix for example).
Reply: A new appendix was added: “Appendix B: Supplementary tables” and 5 tables moved from the main text to this appendix: Table 1, Table 3, Table 5, Table 10, and Table 14. The associated text was modified to de-emphasize these tables.
Referee: Finish the paper with one strong paragraph summarizing the main results in a final Conclusions section (7.7).
Reply: Has been done. The heading of Sect. 7 has been modified accordingly.
Clarifications:
Referee: Lines 145-146: What is the seasonal spread of the CTD cruises? Have you applied standard QC to the CTD data, if so, mention it.
Reply: The text: “mainly in February, May, August–September, or November” has been added. Also, the following sentence was added: “Initially, an EG&G CTD was used but since 1996, this was replaced by a SeaBird 911+. Water samples were acquired for salinity calibration and all the data have been quality controlled.”
Referee: Lines 169-174: What type of ADCP was used?
Reply: The following sentence has been added: “Three different ADCP models from Teledyne RD Instruments have been used: 150 kHz Broadband, 75 kHz Broadband, and Long Ranger.”
Referee: Line: 175: “either 10 m or 25 m” state what this depends on.
Reply: We have added the text: “depending on bottom depth and ADCP model”
Referee: Line 207: “necessary additional information” – be specific what this it.
Reply: The paragraph has been reorganized and it has been specified that it is the lack of regular CTD data that prevent a more reliable extrapolation to the surface.
Referee: Lines 362-363: the average surface velocity is likely underestimated by the extrapolation method – could you give an indication or estimate of how much this would be?
Reply: We do not have any evidence, on which to base an objective estimate and would prefer not to guess. Instead, we now write: “the shape of the profile indicates that the average surface velocity is likely to be underestimated by the extrapolation method, although it is difficult to estimate by how much.”.
Referee: Lines 683-684: Here you use more decimals than in the rest of the paper, be consistent, 11 cm s-1 and 12 cm s-1 .
Reply: We used the extra decimal to indicate that the difference between the two numbers was small, but the sentence has been changed so that it now reads: “For the whole interval between A3 and A8, the average offset based on the new MDT is only 4 % smaller than the ADCP-based value.”
Referee: Sections 7.2 and 7.4 need highlights in bold italic.
Reply: Section 7.2 now has three highlighted sentences and Section 7.4 has two.
Figures:
Referee: It would be useful if the lat/lon range in the figures were consistent (except for 1a which requires a larger area for context) for ease of comparison (especially for Figs 5a and 5b).
Reply: Old Fig. 5a (new Fig. 6a), Fig. 5b (new Fig. 6b), Fig.14, and Fig. 16 now have the same map boundaries and the same basic map.
Referee: Please make the colour bars distinct for ease of interpretation (Figs 4, 10).
Reply: The colour bars of old Fig. 4 (new Fig. 5) and Fig. 10 (new Fig. 11) have been made distinct by introducing lines at the same intervals as the contour lines.
Referee: Are the figures compatible with colour-vision deficiencies? (using not just colours but dashed/dotted lines for example).
Reply: Old Fig. 6 (new Fig. 7) and Fig. 9b (new Fig. 10b) have been modified by making some of the lines dashed.
Referee: Fig 3a: could this figure get rotated to have depth on the y-axis to match Fig 3b or is there a specific reason for this orientation?
Reply: Old Fig 3a (new Fig. 4a) has been rotated so that the depth axis is vertical as in Fig 3b.
Referee: Fig 6: Mark (by a horizontal line?) where the extrapolation starts on each of the four profiles.
Reply: The extrapolation start is now marked by a black circle on each profile on the new Fig. 7a.
Referee: Fig 7: make the four sub-plots consistent with regard to x-lim, number of bars for ease of comparison.
Reply: A new consistent figure (new Fig. 8) has been made.
Technical corrections:
Referee: Line 35: Define Greenland-Scotland-Ridge acronym (GSR) at first use and then use acronym throughout.
Reply: The Greenland-Scotland-Ridge is only referred to twice in addition to this, and towards the end of the manuscript. We have therefore instead replaced the one reference to “GSR” by the full name: “Greenland-Scotland Ridge”.
Referee: Line 41: include abbreviation “IFF” here.
Reply: Has been done.
Referee: Line 43-44: check the grammar of that sentence.
Reply: The sentence has been split into two sentences and clarified.
Referee: Line 143: “Materials and Methods”
Reply: Has been corrected.
Referee: Line 146: “many CTD profiles” – be specific: “between 98 and 155 CTD profiles”
Reply: Has been specified.
Referee: Line 229: only use “MDT” (acronym already defined earlier)
Reply: Has been done.
Referee: Fig 2a: The label for A2 is hidden behind N01, reposition it so it is visible; label for A4 is missing in the figure
Reply: The figure has been modified to correct this.
Referee: Fig 5b: the blue track is hard to see, choose a different/more distinct colour/line style/width.
Reply: The track has been made thicker and the background has also been changed, which should enhance the contrast (new Fig. 6b).
Referee: Line 643: change to “Comparison of in situ observations with old and new altimetry data”
Reply: Has been changed
Referee: Line 725: “some”
Reply: Has been corrected.
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RC2: 'Comment on egusphere-2023-828', Anonymous Referee #2, 08 Jun 2023
Review on the manuscript
«The Iceland-Faroe warm-water flow towards the Arctic estimated from satellite altimetry and in situ observations” by Hansen et al. submitted to Ocean Science.
The authors estimate the transport toward the Arctic between Iceland and the Faroes using a combination of ADCPs and satellite altimetry. There are several steps in which the sea level anomalies (SLA) are correlated with ADCP-measurements to produce a method such that the transport toward the Arctic can be estimated with SLA only. It is a substantial contribution to the scientific progress within the scope of Ocean Science. The manuscript is well written and structured, and the figures are of good quality.
The manuscript is however very technical with several analysis and many correlations are presented. The manuscript will benefit from moving some of these analysis/tables to supplementary material or to an appendix. This will make it easier for the reader to follow the steps and analysis. My main conclusion is that the manuscript can be published in Ocean Science with minor/moderate revision.
Major comment
The transport estimates are calculated by integration the velocities down to the depth where T=4 oC that they define as the base of the Atlantic layer. However, the properties of the AW change with time (e.g., fig 15b), and the authors should give some estimates of the sensitivity of results if other depths are used as the lower limit. E.g., what will the transport estimates be when using T=3 oC or T=5 oC as the lower limit of the depth integration.
Minor comments
Is table 1 needed? Fig. 2. shows the bottom depth of the area/stations and it is already written that the CTD-stations are taken 3-4 times a year. If deleted, the sentence on lines 146-147 can also be deleted.
Line 211-212: “… the extrapolation factor may be modified to account for these.” Please, specify. Was this done?
Figure 6. It would be interesting to see the variance of the cross-ridge velocities for the four ADCPs, e.g., include standard deviation in the figure.
Line 371: “They document that…” Who are they?
Line 405-413. With the regression analysis for u=a*dH+b, the values a will determine the strength of the variability while b determines the bias. Thus, it is the variability of the surface velocity that is underestimated.
Line 439-440: According to Figure 6, zero velocity are not reached at several ADCP-locations. Is the vertical integration done to bottom when the velocity does not reach zero?
Table 7. The mean values of Deq and Leq are presented but I assume that there might be large temporal variation. If standard deviations or errors can be included, this will give some indications on the sensitivity of the method.
Figure 11. Is the altimetric offset calculated from the averaged surface velocities from ADCPs?
Line 532: Please include a reference for the depth of AW = 4 oC.
Line 573: see my comment on Line 532.
Line 772: “… surface AND deeper…”
Line 776: See my major comment.
Appendix
Eq. A1: why 500 m? Why not use depth of the Atlantic layer or bottom depth?
Citation: https://doi.org/10.5194/egusphere-2023-828-RC2 -
AC2: 'Reply on RC2', Bogi Hansen, 15 Jun 2023
Referee 2
We thank the referee for a very constructive and helpful review. Below, we detail how each referee comment has been addressed and how the manuscript has been modified accordingly.
Manuscript structure
As suggested by both referees, we have tried to re-structure the manuscript to enable better flow. Five tables have been moved to a new Appendix B and Figure 12 has been moved to the Materials and methods section together with associated text. In addition, the Introduction (Sect. 1) has been organized into four sub-sections, mainly to put more emphasis on the definition of Atlantic water for transport calculation.
In the following, we address specific comments from Referee 2 where quoted text from the old or the new version of the manuscript is in italic.
Referee: The manuscript is however very technical with several analysis and many correlations are presented. The manuscript will benefit from moving some of these analysis/tables to supplementary material or to an appendix. This will make it easier for the reader to follow the steps and analysis.
Reply: Five of the tables have been moved to a new Appendix B. Also, Fig. 12 and associated text has been moved to Sect. 2.1.
Major comment
Referee: The transport estimates are calculated by integration the velocities down to the depth where T=4 oC that they define as the base of the Atlantic layer. However, the properties of the AW change with time (e.g., fig 15b), and the authors should give some estimates of the sensitivity of results if other depths are used as the lower limit. E.g., what will the transport estimates be when using T=3 oC or T=5 oC as the lower limit of the depth integration.
Reply: The discussion of Atlantic water in the Introduction has been enhanced to include the sensitivity and the effect of changing AW properties. In connection with this, the Introduction has been split into four subsections, for emphasis, with the discussion of Atlantic water as Sect. 1.3.
Minor comments
Referee: Is table 1 needed? Fig. 2. shows the bottom depth of the area/stations and it is already written that the CTD-stations are taken 3-4 times a year. If deleted, the sentence on lines 146-147 can also be deleted.
Reply: Table 1 has been moved to the new Appendix B.
Referee: Line 211-212: “… the extrapolation factor may be modified to account for these.” Please, specify. Was this done?
Reply: “may be modified” has been changed to: “was modified”
Referee: Figure 6. It would be interesting to see the variance of the cross-ridge velocities for the four ADCPs, e.g., include standard deviation in the figure.
Reply: A graph showing standard deviation has been added (new Fig. 7b) as well as a sentence referring to it in the text.
Referee: Line 371: “They document that…” Who are they?
Reply: The words “They document” have been replaced by “This implies”.
Referee: Line 405-413. With the regression analysis for u=a*dH+b, the values a will determine the strength of the variability while b determines the bias. Thus, it is the variability of the surface velocity that is underestimated.
Reply: We have added a sentence: “This might be due to a large bias, b in Eq. (3), for this site but inspection of individual daily velocity profiles does not support that (Fig. 9 in Hansen et al., 2018).”
Referee: Line 439-440: According to Figure 6, zero velocity are not reached at several ADCP-locations. Is the vertical integration done to bottom when the velocity does not reach zero?
Reply: The sentence before Eq. (7) has been changed to: “Sites IA, IB, and IE have inflow throughout the water column, on average (Fig. 7a), and the integration is down to the bottom. For site IW, we only integrate down to the depth, z = z0, where the average cross-ridge velocity becomes zero:”
Referee: Table 7. The mean values of Deq and Leq are presented but I assume that there might be large temporal variation. If standard deviations or errors can be included, this will give some indications on the sensitivity of the method.
Reply: For ADCP site IW, we now write: “LEq = (12 ± 4) km, where the uncertainty is determined by the uncertainty of αReg”. For the other sites on the IFR, we write: “For these sites, the relative uncertainty of LEq is higher (between 38 % and 75 %), and LEq has been set equal to the interval width, L.”. For DEq, we could not find any objective way to derive any uncertainty estimate. We have, however, added a sentence to Sect. 7.4 (old line number 832): “Also, the many uncertainties involved make the numbers in the bottom row of Table 4 rough estimates.”
Referee: Figure 11. Is the altimetric offset calculated from the averaged surface velocities from ADCPs?
Reply: The caption for Fig. 11 (new Fig. 12) has been clarified by the following text: “Optimized values for the Altimetric offset, Uk0, in each altimetry interval are shown by the thick continuous red line. The value for U20 is based on ADCP NI (Table B2). U30 and U40 are based on linear combinations of surface velocities from two or three ADCPs (Sect. 3). U50 and U60 are combined estimates from NC and NH (Table B2) and the geostrophic method. U70 is based on the geostrophic method.”
Referee: Line 532: Please include a reference for the depth of AW = 4 oC.
Reply: We now refer to the new Sect. 1.3.
Referee: Line 573: see my comment on Line 532.
Reply: We now refer to the new Sect. 1.3.
Referee: Line 772: “… surface AND deeper…”
Reply: Has been corrected
Referee: Line 776: See my major comment.
Reply: Hopefully, the new Sect. 1.3 with information on Atlantic water clarifies here.
Appendix
Referee: Eq. A1: why 500 m? Why not use depth of the Atlantic layer or bottom depth?
Reply: Our reason for doing this is that the deep boundary of the Atlantic layer is in general sloping. This was not well explained, however, so now the following text has been inserted before Eq. (A1): “Within each altimetry interval, k (spanning Ak–Ak+1), Uk(z,t) is the eastward velocity at depth z and time t, horizontally averaged within the interval. The contribution to Q(t) from this interval is found by integrating (summing) the velocity down to the deep boundary of the Atlantic layer (bottom or 4 °C isotherm) and multiplying by the interval width. The deep boundary is, however, in general not horizontal. To account for this, we introduce a parameter Wk(z,t), which is the width of Atlantic water within altimetry interval k at depth z and time t. With this definition, the volume transport is:”. The paragraph after Eq. (A1) has been modified accordingly.
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AC2: 'Reply on RC2', Bogi Hansen, 15 Jun 2023
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