the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Dec 2023
Estimation of ice melt, freshwater budget, and their multi-decadal trends in the Baffin Bay and Labrador Sea
Abstract. The Labrador Sea and contiguous Baffin Bay play an important role in the formation of the upper layer of the North Atlantic Deep Water, an essential component of the Atlantic Meridional Ocean Circulation. The hydrography of these two seas is strongly influenced by the melting of sea-ice and glacier-ice, which has likely been affected by long-term climate changes. In this study, we use historical data of ocean temperature and salinity from 1950 to 2022 to estimate the summer freshwater volume (SFV) in Baffin Bay and the Labrador Sea, establish climatologies, and assess the impact of multi-decadal climate change. The SFV climatology (1956 km3) and the summer freshwater budget (2286 km3) estimated from various components are in good agreement. Sea ice and glacial melt account for 37 % and 26 % of the freshwater budget, respectively. SFV climatologies before and after 1995 reveal an increase in Baffin Bay (+226 km3) because of enhanced glacier melting, and a decline (-112 km3) in the Labrador Sea because of recent sea ice volume decreases. The time series of Labrador Sea SFV and total freshwater content are uncorrelated at the multi-decadal scale possibly because the influx of freshwater from the Beaufort Sea dominates the long-term variability.
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RC1: 'Comment on egusphere-2023-2492', Anonymous Referee #1, 13 Jan 2024
This manuscript looks at the freshwater budget in Baffin Bay and on the Labrador shelf of the Labrador Sea. Beyond the budget, the paper looks at the relevant sources and processes, from precipitation to ice melt (land and sea-ice) and runoff. The authors also use a nice construct to examine summer freshwater volume to help understand the sources in the budget. The authors show a freshening of Baffin Bay. The results on the Labrador Shelf are less clear, but the authors speculate on the role of a Beaufort Gyre release in the changes.
This is a well written paper, topical and interesting, considering an important subject. The figures are generally of an acceptable quality. Understanding Arctic freshwater changes, the role of ice melt (sea-ice and glaciers, plus Greenland) and oceanic impact is a key topic, especially given the location of the study regions near to the deepwater formation sites in the Atlantic. Therefore, the work is definitely worth publishing. That said, prior to that, I do have some concerns about aspects of the methodology and interpretation of the results that should be dealt with before publication. I would thus recommend major revisions.
Additionally, I wonder about the choice of the journal. Given the importance of ice processes (land ice melt, sea-ice melt, sea-ice transport), The Cryosphere may be an obvious choice. Yet, the results are linked in with ocean data analysis, and the focus of the analysis is changes in freshwater content in the marine system. These results are very relevant to the oceanographic community, and I worry they may get missed by many potential readers by not being in a more ocean focused journal.
Major Concerns:
- My first major issue is the identification of the study regions. Baffin Bay is fine. But I don’t feel like the authors can state they are studying the Labrador Sea. They are studying the Labrador Shelf. That is where all their data is (e.g. figure 3). They have effectively discarded all the data from the interior of the Labrador Sea. They basically have no data from the West Greenland Current System. I would have loved to have seen a freshwater content map, from the data, for the entire Labrador Sea. But it is not present. This doesn’t distract from the quality or the importance of the study – the Labrador Current is a key conduit for high-latitude freshwater. But the discussion needs to make clear what is being studied, and then use appropriate labels – such as the Labrador Shelf.
- Additionally, the line that splits the author’s study region between Baffin Bay and the Labrador Shelf doesn’t make sense to me. The pink or red line (depending on the figure) at 62N is not an appropriate boundary between the two regions. As the authors schematic in figure 1 shows with its yellow line, Davis Strait is the appropriate boundary between the two regions. The area south of Davis Strait and north of 62N is part of the Labrador Sea and contains much of the circulation that is exchanging water from the West Greenland Current system to the Labrador Current, and so the processes acting there are not related to Baffin Bay. The authors need to recalculate their results based on a proper regional definition.
- With respect to the data used, are the authors sure they have all the relevant data? I would have expected more for both the Labrador Shelf, as well as the west Greenland shelf. For example, looking at the data numbers given in figure 9a, have the authors accessed all the AZMP data (DFO, Canada) from the Labrador Shelf. Similarly, there are 6 sections across the West Greenland Current that were regularly occupied in summer through most of the authors study period (GINR, Greenland). I understand that authors can’t be expected to necessarily get all available data, but given the importance of those two shelves, I wonder about the importance of missing data sources. At the very least, a discussion is needed.
- Additionally, for the data used, there is no discussion of the quality control. Given the focus on data analysis, a detailed discussion is needed. Even the global databases used have data of variable quality. I also wonder about the authors choice to exclude profiles that don’t have water colder than 0C in the profile. What is special about 0C, given it is not the freezing point for seawater? Depending on the salinity, water as warm as 2-4C (see any of the Yashayaev papers, for example) will mix down into a winter mixed layer in the Labrador Sea interior. More discussion is needed at the least, as well as potentially some sensitivity analysis.
- At the start of section 3 (Results), the authors state they are performing their climatological analysis over the period 1950-2022. Yet, very little of their data is available over that full period. Only the oceanographic data includes data over that full range, and even that has limited numbers of observations (often coarser bottle data) in the early years of the period. Precipitation covers 1965-2022. The Ice discharge data covers 1986-2022 and despite the authors stating it should cover well the full 1950-2022 period, I would expect the ice discharge to be biased high given increases in recent years. Sea-ice volume is in the end, based on the 2011-2022 period. And the ice volume fluxes are based on 2016-2022, a period of mobile, and likely thinner ice. The authors do try to account for these uncertainties. But they have to impact the results. At the very least I’d like to see all these items and time periods brought back and interpreted (and clearly mentioned) in the discussion. Additionally, the authors break their analyses up into before and after 1996 periods. Even though most of the data is only for the latter period. Again, clarify what is being used in the analysis for the early period, and discuss those limitation more.
- Finally, the authors suggest that the remote source of freshwater is the Beaufort Gyre. And refer to a few papers that discuss a Beaufort Gyre release. Although this is possible, it is not the only source (and not the most likely, at least in this reviewer’s opinion). There is no signal of such a release from the observational Arctic gateway straits (e.g. Haine, Curry papers). There is significant freshwater export out of Hudson Strait, as the authors hint at (and see references listed below in the additional points). But there is also significant freshwater export from the Arctic through Fram Strait (that has seen increases – e.g. Karpouzoglou et al, 2022). And then this freshwater is augmented by melt from Greenland all along the its east coast. And this freshwater gets into the Labrador Sea (e.g. Myers, 2005; Gillard et al., 2016; Luo et al., 2016; Dukhovskoy et al., 2016, Schulze-Chretien and Frajka-Williams, 2018, etc.) and may also feed into Baffin Bay, or circulate around to the Labrador shelf (Gou et al., 2022). Holliday et al (2020) discusses significant sub-polar freshening from the Labrador Shelf, linking it to atmospheric changes, rather than a Beaufort Gyre release.
- In the end, I agree with the authors final comment, much of this is beyond their study – but if that is the case, the authors need to not push the Beaufort Gyre release ‘story’ as heavily, and give the other possibilities greater weight (with a much better literature review of this topic in the discussion).
Additional Points:
- The title feels long-winded, a bit disjoint and doesn’t really make clear the key ideas and results in the paper. Maybe something indicating the paper is looking at the evolution of the freshwater budgets in Baffin Bay/Labrador Shelf, and the key processes/drivers that set the budget and its change.
- Line 26: The authors discuss the role of the Labrador Sea in the overturning circulation. Are the authors aware of the recent OSNAP results? I think this text could be better written given what we know about overturning versus ventilation in the Labrador Sea.
- The discussion of the Labrador Sea circulation could be modernized. The general view is now that there is not a separate West Greenland and Irminger Current in the Labrador Sea, but one current carrying multiple water masses. Given the inshore branch of the Labrador Current is mentioned, shouldn’t there be a mention of the West Greenland Coastal Current – both of which carry low salinity and glacial melt water.
- Line 40: The authors state “the Labrador Current then feeds the Labrador Sea shelf and interior with its fresh surface waters”. Does it feed the interior? Or does most of that water come from the West Greenland Current system, and/or flowing north from the Grand Banks/Flemish Cap region. Given the focus on freshwater, the discussion of freshwater processes in the region in the introduction feels a bit superficial.
- Line 56: Newer references available
- Figure 1: The red circle for the deep convection region in the Labrador Sea seems significantly too large, and extending too far to the north.
- I was surprised to see no mention in the introduction of any of the studies based on observations at Davis Strait. All of Cuny et al (2005) and Curry et al (2010, 2014) discuss transport between Baffin Bay and the Labrador Sea and include estimates of volume, freshwater and sea-ice transport, that would seem relevant for evaluation of the authors results.
- Figure 2 caption: The red line is at the bottom of the winter mixed layer, not the top.
- Line 156: Why is the 500 m isobath used? That seems quite deep for a definition of a continental shelf and would seem to include a significant part of the waters off the shelf along the upper slope beyond the shelf-break.
- Figures: The authors don’t examine the results in Hudson Bay. That is fine. But then I’d recommend masking out the relevant region in all figures, so results that are not being considered are not shown.
- Figure 3 caption: “The gray bars represent the total number of data (except in the Labrador Basin and Hudson Bay) for a given year”. Not sure what the words in brackets are for – likely saying points in those regions are not included in the numbers. If so, could be worded better. Or better yet, just remove those areas from the study and then there is no need to mention such aspects.
- Figure 5: Units not given/mentioned.
- Line 281: “Exhibit similar variations”. What is the correlation coefficient? And is it significant?
- Figure 6: Do panels a and b both use the same color bar? If so, state in the caption given no color bar is given for panel b.
- Section 2.4.1: The authors give values for ice area and volume fluxes through the various gateway straits. This information is of value (e.g. numerical model evaluation), and might be easier for readers to access if given in a table than just listed in the text.
- Line 350: west of Greenland (east of Greenland isn’t in the authors study domain).
- Line 395: Might be worth referring to some relevant Hudson Bay/Hudson Strait papers (e.g. Straneo and Saucier, 2008; Ridenour et al., 2019, 2021, Babb et al., 2021).
- Table 1 could be made to look nicer. Some of the component names roll from one line to the next. No differentiation between rows in the table. Time periods of data sources could be clearly mentioned (better than the present attempt with foot notes).
- Line 435: In the discussion of the Labrador Shelf (not Labrador Sea), the authors discuss how the summer freshwater decreases. One additional concern here is that the Labrador shelf is a conduit from Baffin Bay/Hudson Strait to the Grand Banks, where the freshwater is exported offshore. The timescale of this transit is fairly short. How does this impact the result. Is it possible signals transit this area and are missed/aliased by the authors analysis that only looks at part of the year?
Citation: https://doi.org/10.5194/egusphere-2023-2492-RC1 -
RC2: 'Comment on egusphere-2023-2492', Igor Yashayaev, 05 Mar 2024
Please see my full report in the attached file.
Even though I highlighted both novelty and potential impact, there are serious issues in the present version, especially on the data validation and processing sides, and lacking connection with convection region that need to be reworked from scratch. Seasonality and shifts in spatial distribution add significant noise, even biases in data. Same goes to data quality of specific measurements. Restrictions on data selection. like salinity threshold may add a bias to the years with data crossing that threshold, etc. Different, conceptually different methods should be used for data analysis. 1996 was used as a split point between two periods. Even if one of the factors changed its sign in that year, it does not mean the ocean would respond in the same year. I am not sure this selection is justified. I would suggest three periods.
Overall, the work is on a very important subject and the data part can be done much better and with stronger oceanographic logic in the background.
Best of luck!
Igor
Status: closed
-
RC1: 'Comment on egusphere-2023-2492', Anonymous Referee #1, 13 Jan 2024
This manuscript looks at the freshwater budget in Baffin Bay and on the Labrador shelf of the Labrador Sea. Beyond the budget, the paper looks at the relevant sources and processes, from precipitation to ice melt (land and sea-ice) and runoff. The authors also use a nice construct to examine summer freshwater volume to help understand the sources in the budget. The authors show a freshening of Baffin Bay. The results on the Labrador Shelf are less clear, but the authors speculate on the role of a Beaufort Gyre release in the changes.
This is a well written paper, topical and interesting, considering an important subject. The figures are generally of an acceptable quality. Understanding Arctic freshwater changes, the role of ice melt (sea-ice and glaciers, plus Greenland) and oceanic impact is a key topic, especially given the location of the study regions near to the deepwater formation sites in the Atlantic. Therefore, the work is definitely worth publishing. That said, prior to that, I do have some concerns about aspects of the methodology and interpretation of the results that should be dealt with before publication. I would thus recommend major revisions.
Additionally, I wonder about the choice of the journal. Given the importance of ice processes (land ice melt, sea-ice melt, sea-ice transport), The Cryosphere may be an obvious choice. Yet, the results are linked in with ocean data analysis, and the focus of the analysis is changes in freshwater content in the marine system. These results are very relevant to the oceanographic community, and I worry they may get missed by many potential readers by not being in a more ocean focused journal.
Major Concerns:
- My first major issue is the identification of the study regions. Baffin Bay is fine. But I don’t feel like the authors can state they are studying the Labrador Sea. They are studying the Labrador Shelf. That is where all their data is (e.g. figure 3). They have effectively discarded all the data from the interior of the Labrador Sea. They basically have no data from the West Greenland Current System. I would have loved to have seen a freshwater content map, from the data, for the entire Labrador Sea. But it is not present. This doesn’t distract from the quality or the importance of the study – the Labrador Current is a key conduit for high-latitude freshwater. But the discussion needs to make clear what is being studied, and then use appropriate labels – such as the Labrador Shelf.
- Additionally, the line that splits the author’s study region between Baffin Bay and the Labrador Shelf doesn’t make sense to me. The pink or red line (depending on the figure) at 62N is not an appropriate boundary between the two regions. As the authors schematic in figure 1 shows with its yellow line, Davis Strait is the appropriate boundary between the two regions. The area south of Davis Strait and north of 62N is part of the Labrador Sea and contains much of the circulation that is exchanging water from the West Greenland Current system to the Labrador Current, and so the processes acting there are not related to Baffin Bay. The authors need to recalculate their results based on a proper regional definition.
- With respect to the data used, are the authors sure they have all the relevant data? I would have expected more for both the Labrador Shelf, as well as the west Greenland shelf. For example, looking at the data numbers given in figure 9a, have the authors accessed all the AZMP data (DFO, Canada) from the Labrador Shelf. Similarly, there are 6 sections across the West Greenland Current that were regularly occupied in summer through most of the authors study period (GINR, Greenland). I understand that authors can’t be expected to necessarily get all available data, but given the importance of those two shelves, I wonder about the importance of missing data sources. At the very least, a discussion is needed.
- Additionally, for the data used, there is no discussion of the quality control. Given the focus on data analysis, a detailed discussion is needed. Even the global databases used have data of variable quality. I also wonder about the authors choice to exclude profiles that don’t have water colder than 0C in the profile. What is special about 0C, given it is not the freezing point for seawater? Depending on the salinity, water as warm as 2-4C (see any of the Yashayaev papers, for example) will mix down into a winter mixed layer in the Labrador Sea interior. More discussion is needed at the least, as well as potentially some sensitivity analysis.
- At the start of section 3 (Results), the authors state they are performing their climatological analysis over the period 1950-2022. Yet, very little of their data is available over that full period. Only the oceanographic data includes data over that full range, and even that has limited numbers of observations (often coarser bottle data) in the early years of the period. Precipitation covers 1965-2022. The Ice discharge data covers 1986-2022 and despite the authors stating it should cover well the full 1950-2022 period, I would expect the ice discharge to be biased high given increases in recent years. Sea-ice volume is in the end, based on the 2011-2022 period. And the ice volume fluxes are based on 2016-2022, a period of mobile, and likely thinner ice. The authors do try to account for these uncertainties. But they have to impact the results. At the very least I’d like to see all these items and time periods brought back and interpreted (and clearly mentioned) in the discussion. Additionally, the authors break their analyses up into before and after 1996 periods. Even though most of the data is only for the latter period. Again, clarify what is being used in the analysis for the early period, and discuss those limitation more.
- Finally, the authors suggest that the remote source of freshwater is the Beaufort Gyre. And refer to a few papers that discuss a Beaufort Gyre release. Although this is possible, it is not the only source (and not the most likely, at least in this reviewer’s opinion). There is no signal of such a release from the observational Arctic gateway straits (e.g. Haine, Curry papers). There is significant freshwater export out of Hudson Strait, as the authors hint at (and see references listed below in the additional points). But there is also significant freshwater export from the Arctic through Fram Strait (that has seen increases – e.g. Karpouzoglou et al, 2022). And then this freshwater is augmented by melt from Greenland all along the its east coast. And this freshwater gets into the Labrador Sea (e.g. Myers, 2005; Gillard et al., 2016; Luo et al., 2016; Dukhovskoy et al., 2016, Schulze-Chretien and Frajka-Williams, 2018, etc.) and may also feed into Baffin Bay, or circulate around to the Labrador shelf (Gou et al., 2022). Holliday et al (2020) discusses significant sub-polar freshening from the Labrador Shelf, linking it to atmospheric changes, rather than a Beaufort Gyre release.
- In the end, I agree with the authors final comment, much of this is beyond their study – but if that is the case, the authors need to not push the Beaufort Gyre release ‘story’ as heavily, and give the other possibilities greater weight (with a much better literature review of this topic in the discussion).
Additional Points:
- The title feels long-winded, a bit disjoint and doesn’t really make clear the key ideas and results in the paper. Maybe something indicating the paper is looking at the evolution of the freshwater budgets in Baffin Bay/Labrador Shelf, and the key processes/drivers that set the budget and its change.
- Line 26: The authors discuss the role of the Labrador Sea in the overturning circulation. Are the authors aware of the recent OSNAP results? I think this text could be better written given what we know about overturning versus ventilation in the Labrador Sea.
- The discussion of the Labrador Sea circulation could be modernized. The general view is now that there is not a separate West Greenland and Irminger Current in the Labrador Sea, but one current carrying multiple water masses. Given the inshore branch of the Labrador Current is mentioned, shouldn’t there be a mention of the West Greenland Coastal Current – both of which carry low salinity and glacial melt water.
- Line 40: The authors state “the Labrador Current then feeds the Labrador Sea shelf and interior with its fresh surface waters”. Does it feed the interior? Or does most of that water come from the West Greenland Current system, and/or flowing north from the Grand Banks/Flemish Cap region. Given the focus on freshwater, the discussion of freshwater processes in the region in the introduction feels a bit superficial.
- Line 56: Newer references available
- Figure 1: The red circle for the deep convection region in the Labrador Sea seems significantly too large, and extending too far to the north.
- I was surprised to see no mention in the introduction of any of the studies based on observations at Davis Strait. All of Cuny et al (2005) and Curry et al (2010, 2014) discuss transport between Baffin Bay and the Labrador Sea and include estimates of volume, freshwater and sea-ice transport, that would seem relevant for evaluation of the authors results.
- Figure 2 caption: The red line is at the bottom of the winter mixed layer, not the top.
- Line 156: Why is the 500 m isobath used? That seems quite deep for a definition of a continental shelf and would seem to include a significant part of the waters off the shelf along the upper slope beyond the shelf-break.
- Figures: The authors don’t examine the results in Hudson Bay. That is fine. But then I’d recommend masking out the relevant region in all figures, so results that are not being considered are not shown.
- Figure 3 caption: “The gray bars represent the total number of data (except in the Labrador Basin and Hudson Bay) for a given year”. Not sure what the words in brackets are for – likely saying points in those regions are not included in the numbers. If so, could be worded better. Or better yet, just remove those areas from the study and then there is no need to mention such aspects.
- Figure 5: Units not given/mentioned.
- Line 281: “Exhibit similar variations”. What is the correlation coefficient? And is it significant?
- Figure 6: Do panels a and b both use the same color bar? If so, state in the caption given no color bar is given for panel b.
- Section 2.4.1: The authors give values for ice area and volume fluxes through the various gateway straits. This information is of value (e.g. numerical model evaluation), and might be easier for readers to access if given in a table than just listed in the text.
- Line 350: west of Greenland (east of Greenland isn’t in the authors study domain).
- Line 395: Might be worth referring to some relevant Hudson Bay/Hudson Strait papers (e.g. Straneo and Saucier, 2008; Ridenour et al., 2019, 2021, Babb et al., 2021).
- Table 1 could be made to look nicer. Some of the component names roll from one line to the next. No differentiation between rows in the table. Time periods of data sources could be clearly mentioned (better than the present attempt with foot notes).
- Line 435: In the discussion of the Labrador Shelf (not Labrador Sea), the authors discuss how the summer freshwater decreases. One additional concern here is that the Labrador shelf is a conduit from Baffin Bay/Hudson Strait to the Grand Banks, where the freshwater is exported offshore. The timescale of this transit is fairly short. How does this impact the result. Is it possible signals transit this area and are missed/aliased by the authors analysis that only looks at part of the year?
Citation: https://doi.org/10.5194/egusphere-2023-2492-RC1 -
RC2: 'Comment on egusphere-2023-2492', Igor Yashayaev, 05 Mar 2024
Please see my full report in the attached file.
Even though I highlighted both novelty and potential impact, there are serious issues in the present version, especially on the data validation and processing sides, and lacking connection with convection region that need to be reworked from scratch. Seasonality and shifts in spatial distribution add significant noise, even biases in data. Same goes to data quality of specific measurements. Restrictions on data selection. like salinity threshold may add a bias to the years with data crossing that threshold, etc. Different, conceptually different methods should be used for data analysis. 1996 was used as a split point between two periods. Even if one of the factors changed its sign in that year, it does not mean the ocean would respond in the same year. I am not sure this selection is justified. I would suggest three periods.
Overall, the work is on a very important subject and the data part can be done much better and with stronger oceanographic logic in the background.
Best of luck!
Igor
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