the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Aug 2024
Circulation of Baffin Bay and Hudson Bay waters on the Labrador Shelf and into the subpolar North Atlantic
Abstract. In the coming decades increasing amounts of freshwater are predicted to enter the subpolar North Atlantic from Greenland and the Arctic. If this additional freshwater reaches the regions where deep convection occurs, it could potentially dampen ventilation and the formation of deep waters. In this study we use a surface drifter dataset spanning the period 1990–2023 to investigate the pathways followed by waters originating from Davis Strait and Hudson Strait on the Labrador Shelf and into the interior subpolar North Atlantic. Recent drifter deployments in the region allow for an improved understanding of the circulation on the Labrador Shelf, in particular its northern part, where prior data was sparse. We show that waters originating from Davis and Hudson Strait remain on the shelf as they flow downstream, until they reach the Newfoundland Shelf. This confirms that very little exchange take place between the Labrador Shelf and interior Labrador Sea. Decomposing the Labrador Shelf into five regions, we further describe typical pathways for these waters and show that extensive exchanges take place between the coastal and shelfbreak branches of the Labrador Current. Our results suggest that if increasing amounts of freshwater reach the Labrador Shelf, it would not directly affect the Labrador Sea convection region; instead, it would lead to the formation a salinity anomaly off the Grand Banks, which could then circulate around the subpolar North Atlantic.
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RC1: 'Comment on egusphere-2024-2541', Anonymous Referee #1, 13 Sep 2024
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General comments
This article examines the export of ocean drifters from the continental shelf in the Northwest Atlantic to the Labrador Sea and subpolar gyre. The goal of the article is to examine potential freshwater pathways from a variety of Arctic sources such as Hudson Bay, Davis Strait, Baffin Bay, and Greenland ice sheets. The authors use Surface Velocity Program (SVP) drifters which have a long holey-sock drogue centered at 15m depth. The authors find that, of the drifters analyzed, most are exported off the shelf at a few key locations. For drifters originating from Davis Strait and Hudson Bay, the main export region was near the Grand Banks on the Newfoundland Shelf. They conclude that fresh water originating from Davis Strait and Hudson Bay is unlikely to have a direct impact on the Labrador Sea and deep convection because it would be entrained into the North Atlantic Current and subpolar gyre before entering the Labrador Sea.
The article is well-written and covers scientific questions that are of interest to the readership of Ocean Sciences, that is, what is the fate of fresh water from the Arctic and what is its connection to the Labrador Sea. The application of observed drifter data is a novel approach in addressing this question, however, I would argue that there are limitations when using this dataset. Those limitations will be described in more detail below. For that reason, I believe some of the conclusions are too strong given the analysis performed and I would recommend a major revision.
Below, I have provided some specific comments that, once addressed in a revised manuscript, would lead to an improvement. I would also be open to considering a rebuttal if the authors disagree with my position.
Specific comments
1. Sparse data and sampling biases – Drifter tracks are very chaotic. For instance, two drifters of the same type deployed at the same place and time will eventually separate, especially when considering the spatial and temporal time scales discussed in this study. So, a small group of drifter tracks cannot represent all the possible pathways between areas of interest. A large sample size is needed. There are only 11 drifters that originate from Davis Strait, so statements like “On the other hand, freshwater that stems from Baffin Bay and Davis Strait, or from most of the West Greenland ice sheet, can only enter the interior subpolar North Atlantic further downstream: at the Grand Banks of Newfoundland” (Lines 461-462) are not supported.
I appreciate the authors’ ingenuity and efforts to increase the sample size by including drifters introduced to the system downstream of the main fresh water sources considered. This is a nice dataset, and it is encouraging to see scientists apply this data to investigate important questions. My main concern, however, is that the conclusions are presented without an adequate description of the limitations of the data and methods.
2. Sampling depth of drifters and representativeness of fresh water – After reading this article, I am left wondering how representative these drifters are of freshwater pathways. The drifters are drogued at 15m and so they represent near-surface currents, however, I am wondering how deep the fresh water extends in these areas and if there is any vertical shear in the currents. For instance, would fresh water closer to the surface take a different path or, similarly, would drifters drogued at a shallower depth give the same result? I recognize the authors are working with the data that is available and a comparison of between the SVPs and other drifter types is not possible. Perhaps though, a description of the seasonal stratification could help the readers interpret the results.
Furthermore, fresh water mixes laterally. Drifters at the shelf edge or meandering back and forth across the export boundary may indeed represent a flux of fresh water from the shelf to the interior oceans through mixing. Even though these fluxes might be much smaller than other sources, like eddies shedding from the West Greenland Current, they might still be important as suggested in Clements et al., 2023. The drifter data analysis presented in this article does not account for this type of exchange.
3. Confusing and contradictory statements – The article contains some statements that are unclear and confusing. For example, Lines 309 – 310 – “By contrast, all the drifters that at some point circulated over the shelf were driven into Flemish pass (in blue), with a small minority going over Newfoundland shelf (in green).” How can all drifters go into Flemish Pass while at the same time a minority go over the Newfoundland shelf?
Furthermore, lines 254-256 state that Hudson Strait/Davis Strait waters are only exported downstream of Flemish Pass contradicts the previous paragraph which describes at least one of the Hudson Strait/Davis Strait drifters crossing the Labrador Shelf boundary. While I understand that drifter remains in the Labrador Current, it shows there is potential for freshwater transport from Hudson Strait into the Labrador Sea through a combination of advection and lateral mixing, as discussed in a previous comment. Given the small sample size of Hudson Strait and Davis Strait drifters, these types of pathways may simply be under sampled with this dataset.
I encourage the authors to clarify these statements and avoid using absolutes like “all” or “only”, unless the analysis has supported their use.
4. Inconsistent naming conventions – I find the naming conventions, especially for the “Newfoundland boxes” first mentioned in Figure 3, to be a little bit confusing and hard to follow. Specifically, in the text that describes Figure 7 on lines 297-310, the boxes are referred to as “Newfoundland box”, “Flemish Pass box” and “Flemish Cap box”, but in Figure 7 they are “Newfoundland 1”, Newfoundland 2”, and “Newfoundland 3”. It would be clearer to use the same naming in the text and figures. A similar comment applies to the Labrador Shelf boxes. In the text they are referred to as inshore and offshore but in Figure 7 numbers are used to label the boxes.
5. Fate of the exported drifters – In several places, the article describes that the drifters exported from the shelf eventually end up in the subpolar North Atlantic (e.g., Lines 484-485). These statements are misleading because Figure 10 shows evidence of some of the drifters entering the subtropical North Atlantic.
6. Line 94 – Could you provide a brief description regarding how the velocities are computed?
7. Line 110 – I am curious about how you determine a drifter is caught in sea-ice. Can you provide a brief description?
8. Line 187 – I am not sure what is meant by “core”. Could you be more descriptive?
Minor comments and typos
- Figure 1b - The duplicate copies of 30' on the y-axis in the figure is confusing because the degrees for those gridlines are not provided and substituting the degrees from a nearby gridline doesn’t make a lot of sense. From what I can gather, these two gridlines are 52 degrees 30' and 57 degrees 30'. Is that right? Is it possible to provide the degrees for those two gridlines so that readers do not have to guess?
- Figure 4 – the units on the colorbar are m/s. I believe this is a mistake and the units should be cm/s because 60 m/s currents are not observed in this area (if anywhere!).
- Figure 5a – The caption describes that a fraction is plotted, but the colorbar describes the number of drifters. This discrepancy should be corrected.
- Figure 8a – The orange track is hard to see. The plot could be clearer by using a thicker line.
- Figure 10 – The axis titles are not consistent with what is described in the caption. The titles indicate Flemish Cap is on the left and Flemish Pass is on the right, but the caption says the opposite. Please correct this discrepancy.
- Figure 10 – Why are there blue data areas south of Flemish Cap in the plot on the right?
- Figure S1b – This figure is difficult to interpret for several reasons. First, because some of the grey tracks are easily confused with the bathymetry contours. Second, the dotted blue track does not appear to have a solid counterpart which makes it difficult to determine when the data gap occurred. Finally, the caption mentions the red track is from Clements et al., 2023. Is it possible to specify the WMOID of the relevant drifter track? When I looked at the drifter tracks mentioned in the paper, neither of them have data as far north was what is represented by the orange-red solid line in this plot.
- Figure S3 – this figure needs an improved caption, and some revision. What do the orange arrows represent? Why is Davis Strait still labelled at the top if the figure is means to represent Hudson Strait drifters?
- Line 62, 208, 418 - Colbourne et al., 1997 is not listed in the References.
- Line 65 – Petri and Anderson 1983 should be Petrie and Anderson, 1983.
- Line 89, 91 – The citations to Duyck and De Jong, 2023 need to be clarified because the reference list contains an a and b.
- Line 127 – “corresponding to 750km” – if the segments are 200km apart, wouldn’t 5 segments apart correspond to 1000km?
- Line 133 – “divide the region into five section.” – section should be sections
- Line 171 – “along the 1500m” – the caption in Figure 4 indicates that the 1000m isobath is plotted. Given this paragraph is discussing Figure 4, it would be clearer if the same isobath is referenced in both the text and figure.
- Line 107 – “Funk Island Deep” is not labelled in Figure 1, rather Funk Island Bank is. Is it possible to label Funk Island Deep for the readers who are unfamiliar with the region?
Citation: https://doi.org/10.5194/egusphere-2024-2541-RC1 -
RC2: 'Comment on egusphere-2024-2541', Anonymous Referee #2, 15 Sep 2024
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This paper looks at the circulation of low salinities water on the Labrador Shelf and explores the origins of said waters, focussing on those waters from Hudson Bay and Baffin Bay. The authors use multiple drifter data sets covering 1990-2023 to examine these questions. Although there are some data sparsity issues, the authors are able to pull together a very complete analysis. The authors shows that waters from the northern straits remain on the Labrador shelf, with offshore exchange beginning on the Newfoundland Shelf. Different pathways from these waters are shown, including that there is significant exchange between the inshore and shelfbreak component of the Labrador Current. Thus, the authors conclude, that freshwater anomalies in the Labrador Current will initially be transported to the area off the Grand Banks, rather than directly to the convective regions of the Labrador Sea.
This is a timely and well written paper, topical and adding novel data to the question of freshwater pathways in the western Labrador Sea. The figure quality is generally high as well. Thus, this paper will be an excellent addition to the literature. I would suggest minor revisions, with details provided below.Data density: The authors use the available drifter data and do a good job putting it together. Yet, figure 2 does show most of the data is from off the shelf, while the analysis focusses on freshwater on the shelf. I think some greater discussion of this limitation, especially in the discussion, would be good. As an aside, part of me wishes this analysis could be augmented with model virtual float data, in a joint study/paper, rather than seeing the observational and modelling work in separate papers (I know this is beyond the authors scope for this paper, but I still want to make the comment for long-term thought).
Bathymetry: At multiple places, the authors refer to the bathymetry. This incudes in figures, such as number 1 and 2. And in there calculations when they define various boxes related to water depth. Yet I don’t see any reference, anywhere in the manuscript, for the source of this topographic data. It needs to ben mentioned and reference. As well, given the potentially issues with bathymetric quality in this region, might the authors want to add some discussion of whether there results are sensitivity to uncertainties in the data.The section/box definitions: The authors divide their study region north-south using 5 sections. Those seem logical choices. They also discuss using 10 boxes, to divide up the sections into east-west definitions as well. This is where I am a bit confused. Have they divided the area between sections into two long boxes, covering the inner and outer shelf? Yet the pink boxes in figure 3 are small – are they then the corners of the boxes? Yet, some sections have different number of pink boxes. So, I will admit to some confusion. This needs to be more clearly explained. And ideally, the actual 2D boxes need to shown on some figure. Additionally, given the authors do at least discuss waters inflowing from the West Greenland Current, why no sections/boxes on that side of the Labrador Sea? It might also be interesting to have a north-south section across the mouth of Hudson Strait, with some division for the inflowing and outflowing components.
Quantitative Freshwater Export Estimates: Given the focus on freshwater, can you make any estimate of freshwater transport by the floats, and/or offshore in the different regions? This would be of value in comparing this work with the results from other analyses and approaches.
Line 30: Is there any evidence for weaken of deep water formation yet? The paper would benefit from a short discussion of this topic.Lines 43, 46: I see at least 2 mispelled author surnames, in different papers – Saucier and Pennelly.
Line 46: For the Hudson Bay origin, might Florindo-Lopez et al also be a good reference to add here.
Figure 2 shows 2 coloured boxes, but I don’t see them defined in the caption. I would also like to see the time period mentioned in the caption to remind the reader. Also I have trouble seeing the grey bathymetry lines. A different colour would help.
Figures such as 2 and 4 could use discrete colour bars, like in figure 3 (and others).
Line 95: Saglek Bank
Line 215: The Flemish Pass section is close to the 47N section studied in detail by the group from the University of Bremen. Might be useful to bring in and reference some of that literature.
Figure 6: It looks like some of the Labrador Curent trajectories loop back on themselves. Is that related to the floats entering/leaving eddies?
Do you see eddies propagating down the shelf break in the data set?
Discussion: I think the paper could be strengthened with some suggestions of future studies, based on gaps or limitations in the present work. At the very least, it sounds like application of additional drifters in the northern Labrador Sea would likely be of value.
Citation: https://doi.org/10.5194/egusphere-2024-2541-RC2
Model code and software
Code - Circulation of Baffin Bay and Hudson Bay waters on the Labrador Shelf and into the subpolar North Atlantic Duyck Elodie https://doi.org/10.5281/zenodo.13255612
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