the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Nov 2021
Changes in the Surface Salinity Gradient and Transport of the Irminger Current: The Climate Perspective
Abstract. Here we use a new analysis schema, the Freshening Length, to study the transport in the Irminger Current on the east and west sides of Greenland. The Freshening Length schema relates the transports on either side of Greenland to the corresponding surface salinity gradients by analyzing climatological data from a data assimilating global ocean model. Surprisingly, the warm and salty waters of the Current are clearly identified by a salinity maximum that varies nearly linearly with distance along the Current’s axis. Our analysis of the climatological salinity data based on the Freshening Length schema shows that only about 20 % of the transport east of Greenland navigates the southern tip of Greenland to enter the Labrador Sea in the west. The other 80 % disperses into the ambient ocean. This independent quantitative estimate based on a 37-year long record complements seasonal to annual field campaigns that studied the connection between the seas east and west of Greenland more synoptically. A temperature-salinity analysis shows that the Irminger Current east of Greenland is characterized by a compensating isopycnal exchange of temperature and salinity, while west of Greenland the horizontal convergence of less dense surface water is accompanied by downwelling/subduction.
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RC1: 'Comment on os-2021-100', Anonymous Referee #1, 21 Nov 2021
Summary of review
This study presents a diagnostic, which is referred to as Freshening Length, to infer along-track changes in the Irminger Current transport around the South-Greenland coast. Based on the Freshening Length, the authors conclude that 20% of the original water mass in the Irminger Current at the eastern side are transported along the southern coast of Greenland to the western side, while 80% are dispersed into the ambient ocean.
Overall, I find this study quite thin. Further analyses may be needed to justify publication. From the current version, I am not able conclude that the study represents a substantial contribution to the journal.
For instance, the manuscript includes four figures and only two of these are original. Figure 1 contains maps adapted from other studies and Figure 2 shows climatological fields of the sea surface temperature and salinity from the publicly available SODA dataset, along with the points at the locations that were used to diagnose the transport changes.
I would recommend publication only if (1) additional analyses are added, or (2) the authors can demonstrate more clearly what the overall value of their analysis is.
Additional analyses could include an exploration of the processes along the current section by which the transport exchanges occur, such as eddies, or further dynamical implications of their study, or temporal changes over or the investigated period and links to larger-scale ocean or atmospheric variability. In any case, I would recommend that the authors further demonstrate the potential information that can be gained from the Freshening Length.
General comments
1) The scientific writing could be improved. Some paragraphs are difficult to follow. In particular, many paragraphs could be shortened and the sentences could be written in a more concise way. Phrases like "it appears" (line 31) do not sound scientific. Overall, I think the amount of text is not in proportion to the amount of information it includes. Therefore, I would recommend shortening of the text.
2) I did not understand why the Freshening Length is important. The climatological map of the sea surface salinity (Figure 2b) already shows there is a gradual freshening along the current. This is expected since freshwater from the Greenland coastal currents is gradually added along its path. Given that the results are expected already from the climatology map, what information is gained from the additional quantification by the Freshening Length?
Other diagnostics, like the freshwater column, which is the integrated freshwater anomaly over depth relative to a pre-defined reference salinity, or the freshwater volume, have been used in numerous earlier studies. These diagnostics have been applied to distinct scientific questions. I am not convinced of the overall value of the Freshening Length unless the authors can demonstrate a clear use or application of the Freshening Length that other diagnostics or just visual inspection of the climatological sea surface salinity map are not able to provide.
3) Based on my understanding of this study, the main result is the statement that 20% of the water in the Irminger Current travel around the southern tip of Greenland. Yet, why is it important to know how much water travels around the southern tip of Greenland as a coherent current? I do not think that the analysis provides meaningful information about the AMOC since the loss of transport that is calculated may be compensated for by other currents and eddies. I cannot see a clear connection to the AMOC from this analysis.
4) The analysis is focused on grid point averages around selected points along the salinity maximum. Yet, the current may be broader at some locations than at others, in which case the diagnostic does not describe the transport in the Irminger Current but is sensitive to how the current is defined. A considerable fraction of the transport could also occur in eddies or in the boundary currents like the Greenland Coastal Currents. These are not captured by focusing on a narrow current with pre-defined width.
5) The analysis only considers the climatological mean over a 37-year period. I do not think there is a substantial gain in such an analysis. It would be more interesting to look at the time variability of the transport and investigate the involved dynamical processes.
6) Part of the freshwater transport around Greenland's coast occurs as sea ice. I am not sure if the authors accounted for this. Melting of sea ice along the way may also influence the salinity and hence the Freshening Length.
Specific comments
Title: I find the title misleading and difficult to understand. The study does not investigate "changes in the surface salinity gradient". It should either read "changes in the sea surface salinity along the current" or "the sea surface salinity gradient along the current" but not "changes in the gradient" (which would correspond to the second rather than a first derivative). This mistake is repeated later, for instance in the abstract.
Also, I am not sure what is meant by "the climate perspective" in the title.
line 4, "surprising": I do not find it surprising that the Irminger Current can be identified based on salinity maxima, given that it represents a saline current system around the fresher subpolar gyre.
line 12: "A temperature-salinity analysis shows that the Irminger Current east of Greenland is characterized by a compensating isopycnal exchange of temperature and salinity, while west of Greenland the horizontal convergence of less dense surface water is accompanied by downwelling/subduction."
This sentence is misleading and confusing. It suggests that less dense water is subducted beneath denser water.
line 79: The SODA data set contains very irregular measurements in time. Thus, it is likely biased towards the recent period.
line 120: The title and figures refer to the sea surface salinity but the Freshening Length itself seems to be integrated over the full current depth. I find this confusing. To avoid misunderstandings, it would be great if the authors could clarify this in the text and if necessary, adjust the title.
In case the analysis is restricted to the surface, the Freshening Length would not be a meaningful indicator of the transport fraction that travels around Greenland, as part of the freshwater could be mixed down to depth.
line 178: The conclusion, that the transport loss occurs in eddies is not supported by the preceding sentence. It is not clear from the sentence or the paragraph why the transport loss should occur in eddies. The writing here could be more precise.
line 193: I understand that the Freshening Length is only robust on climatological mean data. However, considering the large interannual and decadal variability in the subpolar region, it is questionable if the application of the Freshening Length to only the climatological average contains meaningful information.
line 195: "...the Freshening Length estimate of the transport is much more robust and informative than direct estimates based on velocity profiles..."
I strongly disagree with this sentence and the entire paragraph. Of course, the method used always depends on the question that needs to be answered. Still, I would argue that direct estimates based on velocity profiles are generally preferred to calculate transports.
Figure 1: in panel b, the labels are difficult to read
Figure 4: I am not sure why the red and blue lines are fitted to the points. What information is gained from doing this?
Citation: https://doi.org/10.5194/os-2021-100-RC1 -
AC1: 'Reply on RC1', Nathan Paldor, 28 Jan 2022
The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-100/os-2021-100-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Nathan Paldor, 28 Jan 2022
-
RC2: 'Comment on os-2021-100', Anonymous Referee #2, 20 Dec 2021
This paper introduced an analysis method, Freshening Length, and applied this to determine the changes of Irminger Current on the both sides of Greenland using SODA reanalysis data.
I have four major concerns/comments for their consideration:
- Many times the authors stated in the paper that the West Greenland Current (WGC) is a continuation of the Irminger Current. In fact before becoming the WGC, the Irminger Current has merged with East Greenland Current on the east side, referred to as EGC/IC (or EGIC) (e.g., Cuny et al, 2002; Sutherland and Pickart, 2008). I would suggest the authors to improve the introduction with more up-to-date references.
- The author determined the Irminger Current by the maximum salinity at surface. To do that, the authors seem to believe the Irminger Current only contain Irminger Water and this water situates at surface. However, the EGC/IC and WGC contain multiple water masses vertically and the Irminger Water resides away from the surface (~ 500m depth, Pacini et al., 2020). As such, the results shown in the paper can reflect neither the change of Irminger Current nor Irminger Water.
- I am confused by the Freshening Length. The q needs to be better defined. What is the unit of q? m/s? otherwise the equation doesn’t make sense as the unit of HU is m^2/s. How to determine the H? The L on the east is 5 times more than the one on the west. My understanding is that the less L means freshening in a shorter distance or faster freshening in a certain distance, namely the water freshening is 5-time faster on the west than on the east. I am still confused how it can lead to the conclusion that only 20% of Irminger Current contributes to WGC.
- The authors also stated that the amount of Irminger Current supporting the WGC was previously unknow and they determined that only 20% of Irminger Water rounds Cape Farewell using the Freshening Length. First, the authors should be aware of the studies by Le Bras et al. (2018) and Pacini et al. (2020). The former computed the transport of each boundary current in the east of Greenland using the Overturning in the Subpolar North Atlantic Program mooring data, while the latter reported the transports in the west of Greenland using the multiple-year mooring array on the west side. Their results clearly suggested that the most of the boundary currents in east Greenland flow towards the west Greenland with only a few recirculation at Cape Farewell (interacting with Eirik Ridge). How could the authors explain the discrepancy between the main conclusion of this paper and the compelling observations?
Specific comments:
Line 19. should be Faroe Bank Channel.
Line 20. The connection is not only with Greenland Sea. I would suggest to say Nordic Seas instead.
Line 21. EGC doesn’t have to be fresh, particularly in the deep layer it also contributes the dense overflow water.
Line 26-27. What currents are they talking about? I don’t see the difference. The currents flow continuously northward in Fig. 1a as well.
Figure 1. The two schematics have some discrepancies which add confusions, e.g. EGC and IC are two separate currents in upper panel, while in the bottom panel they are referred as a single merged current – EGIC (green). Why not just show one up-to-date schematic?
Line 80-85. Mark your study region somewhere in Fig.1 or Fig. 2. Label all of the geographic names that were mentioned in the paper, e.g., Cape Farewell.
Line 95-100. How could the mean surface hydrography suggest a cyclonic circulation?
Line 140. It IS important… and did you mean Fig. 1?
I suggest to change the subtitle to Discussion and Conclusions.
Line 155. Downwelling can flux the saltier water towards the coast in the upper layer, while upwelling can transport the fresher water offshore which may influence the water carried by WGC.
Last paragraph. As I said above, the authors should be aware of the published mooring studies. Also the authors can easily check whether their method works out by using the same SODA reanalysis, e.g., compare the volume transports on the both sides of Greenland.
Citation: https://doi.org/10.5194/os-2021-100-RC2 -
AC2: 'Reply on RC2', Nathan Paldor, 28 Jan 2022
The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-100/os-2021-100-AC2-supplement.pdf
Status: closed
-
RC1: 'Comment on os-2021-100', Anonymous Referee #1, 21 Nov 2021
Summary of review
This study presents a diagnostic, which is referred to as Freshening Length, to infer along-track changes in the Irminger Current transport around the South-Greenland coast. Based on the Freshening Length, the authors conclude that 20% of the original water mass in the Irminger Current at the eastern side are transported along the southern coast of Greenland to the western side, while 80% are dispersed into the ambient ocean.
Overall, I find this study quite thin. Further analyses may be needed to justify publication. From the current version, I am not able conclude that the study represents a substantial contribution to the journal.
For instance, the manuscript includes four figures and only two of these are original. Figure 1 contains maps adapted from other studies and Figure 2 shows climatological fields of the sea surface temperature and salinity from the publicly available SODA dataset, along with the points at the locations that were used to diagnose the transport changes.
I would recommend publication only if (1) additional analyses are added, or (2) the authors can demonstrate more clearly what the overall value of their analysis is.
Additional analyses could include an exploration of the processes along the current section by which the transport exchanges occur, such as eddies, or further dynamical implications of their study, or temporal changes over or the investigated period and links to larger-scale ocean or atmospheric variability. In any case, I would recommend that the authors further demonstrate the potential information that can be gained from the Freshening Length.
General comments
1) The scientific writing could be improved. Some paragraphs are difficult to follow. In particular, many paragraphs could be shortened and the sentences could be written in a more concise way. Phrases like "it appears" (line 31) do not sound scientific. Overall, I think the amount of text is not in proportion to the amount of information it includes. Therefore, I would recommend shortening of the text.
2) I did not understand why the Freshening Length is important. The climatological map of the sea surface salinity (Figure 2b) already shows there is a gradual freshening along the current. This is expected since freshwater from the Greenland coastal currents is gradually added along its path. Given that the results are expected already from the climatology map, what information is gained from the additional quantification by the Freshening Length?
Other diagnostics, like the freshwater column, which is the integrated freshwater anomaly over depth relative to a pre-defined reference salinity, or the freshwater volume, have been used in numerous earlier studies. These diagnostics have been applied to distinct scientific questions. I am not convinced of the overall value of the Freshening Length unless the authors can demonstrate a clear use or application of the Freshening Length that other diagnostics or just visual inspection of the climatological sea surface salinity map are not able to provide.
3) Based on my understanding of this study, the main result is the statement that 20% of the water in the Irminger Current travel around the southern tip of Greenland. Yet, why is it important to know how much water travels around the southern tip of Greenland as a coherent current? I do not think that the analysis provides meaningful information about the AMOC since the loss of transport that is calculated may be compensated for by other currents and eddies. I cannot see a clear connection to the AMOC from this analysis.
4) The analysis is focused on grid point averages around selected points along the salinity maximum. Yet, the current may be broader at some locations than at others, in which case the diagnostic does not describe the transport in the Irminger Current but is sensitive to how the current is defined. A considerable fraction of the transport could also occur in eddies or in the boundary currents like the Greenland Coastal Currents. These are not captured by focusing on a narrow current with pre-defined width.
5) The analysis only considers the climatological mean over a 37-year period. I do not think there is a substantial gain in such an analysis. It would be more interesting to look at the time variability of the transport and investigate the involved dynamical processes.
6) Part of the freshwater transport around Greenland's coast occurs as sea ice. I am not sure if the authors accounted for this. Melting of sea ice along the way may also influence the salinity and hence the Freshening Length.
Specific comments
Title: I find the title misleading and difficult to understand. The study does not investigate "changes in the surface salinity gradient". It should either read "changes in the sea surface salinity along the current" or "the sea surface salinity gradient along the current" but not "changes in the gradient" (which would correspond to the second rather than a first derivative). This mistake is repeated later, for instance in the abstract.
Also, I am not sure what is meant by "the climate perspective" in the title.
line 4, "surprising": I do not find it surprising that the Irminger Current can be identified based on salinity maxima, given that it represents a saline current system around the fresher subpolar gyre.
line 12: "A temperature-salinity analysis shows that the Irminger Current east of Greenland is characterized by a compensating isopycnal exchange of temperature and salinity, while west of Greenland the horizontal convergence of less dense surface water is accompanied by downwelling/subduction."
This sentence is misleading and confusing. It suggests that less dense water is subducted beneath denser water.
line 79: The SODA data set contains very irregular measurements in time. Thus, it is likely biased towards the recent period.
line 120: The title and figures refer to the sea surface salinity but the Freshening Length itself seems to be integrated over the full current depth. I find this confusing. To avoid misunderstandings, it would be great if the authors could clarify this in the text and if necessary, adjust the title.
In case the analysis is restricted to the surface, the Freshening Length would not be a meaningful indicator of the transport fraction that travels around Greenland, as part of the freshwater could be mixed down to depth.
line 178: The conclusion, that the transport loss occurs in eddies is not supported by the preceding sentence. It is not clear from the sentence or the paragraph why the transport loss should occur in eddies. The writing here could be more precise.
line 193: I understand that the Freshening Length is only robust on climatological mean data. However, considering the large interannual and decadal variability in the subpolar region, it is questionable if the application of the Freshening Length to only the climatological average contains meaningful information.
line 195: "...the Freshening Length estimate of the transport is much more robust and informative than direct estimates based on velocity profiles..."
I strongly disagree with this sentence and the entire paragraph. Of course, the method used always depends on the question that needs to be answered. Still, I would argue that direct estimates based on velocity profiles are generally preferred to calculate transports.
Figure 1: in panel b, the labels are difficult to read
Figure 4: I am not sure why the red and blue lines are fitted to the points. What information is gained from doing this?
Citation: https://doi.org/10.5194/os-2021-100-RC1 -
AC1: 'Reply on RC1', Nathan Paldor, 28 Jan 2022
The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-100/os-2021-100-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Nathan Paldor, 28 Jan 2022
-
RC2: 'Comment on os-2021-100', Anonymous Referee #2, 20 Dec 2021
This paper introduced an analysis method, Freshening Length, and applied this to determine the changes of Irminger Current on the both sides of Greenland using SODA reanalysis data.
I have four major concerns/comments for their consideration:
- Many times the authors stated in the paper that the West Greenland Current (WGC) is a continuation of the Irminger Current. In fact before becoming the WGC, the Irminger Current has merged with East Greenland Current on the east side, referred to as EGC/IC (or EGIC) (e.g., Cuny et al, 2002; Sutherland and Pickart, 2008). I would suggest the authors to improve the introduction with more up-to-date references.
- The author determined the Irminger Current by the maximum salinity at surface. To do that, the authors seem to believe the Irminger Current only contain Irminger Water and this water situates at surface. However, the EGC/IC and WGC contain multiple water masses vertically and the Irminger Water resides away from the surface (~ 500m depth, Pacini et al., 2020). As such, the results shown in the paper can reflect neither the change of Irminger Current nor Irminger Water.
- I am confused by the Freshening Length. The q needs to be better defined. What is the unit of q? m/s? otherwise the equation doesn’t make sense as the unit of HU is m^2/s. How to determine the H? The L on the east is 5 times more than the one on the west. My understanding is that the less L means freshening in a shorter distance or faster freshening in a certain distance, namely the water freshening is 5-time faster on the west than on the east. I am still confused how it can lead to the conclusion that only 20% of Irminger Current contributes to WGC.
- The authors also stated that the amount of Irminger Current supporting the WGC was previously unknow and they determined that only 20% of Irminger Water rounds Cape Farewell using the Freshening Length. First, the authors should be aware of the studies by Le Bras et al. (2018) and Pacini et al. (2020). The former computed the transport of each boundary current in the east of Greenland using the Overturning in the Subpolar North Atlantic Program mooring data, while the latter reported the transports in the west of Greenland using the multiple-year mooring array on the west side. Their results clearly suggested that the most of the boundary currents in east Greenland flow towards the west Greenland with only a few recirculation at Cape Farewell (interacting with Eirik Ridge). How could the authors explain the discrepancy between the main conclusion of this paper and the compelling observations?
Specific comments:
Line 19. should be Faroe Bank Channel.
Line 20. The connection is not only with Greenland Sea. I would suggest to say Nordic Seas instead.
Line 21. EGC doesn’t have to be fresh, particularly in the deep layer it also contributes the dense overflow water.
Line 26-27. What currents are they talking about? I don’t see the difference. The currents flow continuously northward in Fig. 1a as well.
Figure 1. The two schematics have some discrepancies which add confusions, e.g. EGC and IC are two separate currents in upper panel, while in the bottom panel they are referred as a single merged current – EGIC (green). Why not just show one up-to-date schematic?
Line 80-85. Mark your study region somewhere in Fig.1 or Fig. 2. Label all of the geographic names that were mentioned in the paper, e.g., Cape Farewell.
Line 95-100. How could the mean surface hydrography suggest a cyclonic circulation?
Line 140. It IS important… and did you mean Fig. 1?
I suggest to change the subtitle to Discussion and Conclusions.
Line 155. Downwelling can flux the saltier water towards the coast in the upper layer, while upwelling can transport the fresher water offshore which may influence the water carried by WGC.
Last paragraph. As I said above, the authors should be aware of the published mooring studies. Also the authors can easily check whether their method works out by using the same SODA reanalysis, e.g., compare the volume transports on the both sides of Greenland.
Citation: https://doi.org/10.5194/os-2021-100-RC2 -
AC2: 'Reply on RC2', Nathan Paldor, 28 Jan 2022
The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-100/os-2021-100-AC2-supplement.pdf
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