the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jan 2024
An emerging pathway of Atlantic Water to the Barents Sea through the Svalbard Archipelago: drivers and variability
Abstract. The Barents Sea, an important component of the Arctic Ocean, is experiencing shifts in ocean currents, stratification, sea-ice variability, and marine ecosystems. Inflowing Atlantic Water (AW) is known to be a key driver of change. Although AW predominantly enters the Barents Sea via the Barents Sea Opening, other pathways exist but remain relatively unexplored. Summer climatology fields of temperature in the last century compared to 2000–2019 indicate warming in the trench Storfjordrenna and the shallow banks Hopenbanken and Storfjordbanken in the Svalbard Archipelago, and shoaling of AW, extending further into the "channel" between Edgeøya and Hopen islands. This region emerges as a pathway of AW into the northwestern Barents Sea. One year-long records from a mooring deployed between September 2018 and November 2019 at a saddle in this channel, show the flow of Atlantic-origin waters into the Arctic domain of the northwestern Barents Sea. The average current is directed eastward, into the Barents Sea, but is dominated by large variability throughout the year. Here, we investigate this variability on time scales from hours to months. Wind forcing mediates the currents and the water and heat exchange through the channel through geostrophic adjustment to Ekman transport. The main drivers for the AW inflow and the cross-saddle transport of positive temperature anomalies are persistent strong semidiurnal tidal currents, intermittent wind-forced events, and wintertime warm water intrusions forced by upstream conditions. We propose that similar topographic constraints where the Polar Front acts like a barrier may become more important for AW inflow and heat exchange in the future. The ongoing warming and possible shoaling of AW together with changes in the large-scale weather patterns would likely increase inflow and heat transport through the processes identified in this study.
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RC1: 'Review on the manuscript egusphere-2023-3080', Anonymous Referee #1, 21 Feb 2024
Summary
In this manuscript the authors suggest an existence of newly emerging pathway for the warm and saline Atlantic Water entering the Arctic Ocean via the northwestern Barents Sea. They propose that the new corridor is located between Edgeøya and Hopen, the islands situated southeast of Spitsbergen, in the easternmost, shallow part of Storfjordrenna. The authors present and thoroughly analyse recently collected observations, including a year-long time series from moored instruments spanning 2018-2019, as well as three hydrographic transects from ships during the autumns of 2019-2021. The obtained results are put in a wider climatological context drawing upon a large database of quality-controlled observations spanning nearly a century, from 1930 to 2019. Although the available database primarily covers summer seasons, it provides sufficient information to reconstruct the climatological vertical hydrographic section near the mooring location.
General assessment
The manuscript's topic aligns well with the scope of 'Ocean Science 'and addresses the evolving role of the Barents Sea in the context of the warming Arctic and changing ocean climate, a subject of extensive discussion within the scientific community in recent years. Despite the somewhat limited dataset in terms of both time and spatial coverage (comprising a single shallow mooring and relatively short hydrographic surveys obtained in autumn), the authors made an effort to comprehend and characterise the variability occurring across various time scales in meticulous detail. Through the application of various filtering techniques, the authors have successfully distinguished between slow-acting and fast-acting mechanisms underlying the oscillating currents and observed changes in water temperature.
In my opinion, the manuscript is written and illustrated well enough and makes a reasonable research statement in the previously undocumented area. However, I also found it slightly overloaded with content and details and thus difficult to follow at times. Therefore, I suggest that it can be published after some minor corrections.
Comments and remarks:
Introduction:
Line 28-30: Please add some newer references since the mentioned, although crucial studies, are more than 20 years old. Incorporating some recent studies will help strengthen the manuscript's relevance and ensure that it remains up-to-date with the latest advancements in the field.
Line 59: ‘Summer of 2016 had the warmest and longest-lasting marine heatwave in the Barents Sea’ – you may add ‘so far’.
Line 70: ‘Here, we investigate the physical processes that drive and mediate the inflow of Atlantic-origin water masses’ – I would say that the water masses are rather briefly classified in this paper, if at all. I wonder why: is it problematic to compare the CT and SA with previous classifications made for T&S? Or there is another reason behind it., i.e. you focus on mechanisms rather than structure of the water column itself? In this case, providing a brief overview of water masses may be sufficient (one-two sentences).
Data & methods
Line 78: ‘70 meter deep saddle on Hopenbanken between Storfjordrenna and the interior of the Barents Sea’ – describe here what ‘saddle’ is in bathymetry. Also, what ‘banken’ and ‘renna’ are (instead of indicating it only in the ‘Results’ section).
Line 95: ‘the measured current were in fairly good agreement with Arc5km2018’ – what does ‘fairly good’ mean here? Also, change ‘current’ for ‘currents’.
Line 99: ‘Wind speed and direction in the region are taken from the ERA5 reanalyses’ - have you considered or tried to use CARRA reanalysis? Any thoughts on this?
Line 120-121: ‘the multitaper method from Percival and Walden (1993) with Slepian data tapers (e.g., Slepian, 1978; Thomson, 1982). To analyse the time-variability of the spectral components, wavelet transforms following Lilly and Olhede (2009) with generalised Morse wavelets (Olhede and Walden, 2002) were used’ – adding a brief explanation about the general purpose of using the methods mentioned (rationale behind and how they contribute to the study's objectives) in the manuscript could enhance the reader's understanding and contribute to smoother reading.
Line 134: ‘bottom half of the water column’ – why? Do you aim to separate the surface layer circulation or rather ADCP data quality away from transducer was a limiting factor here?
Results:
I advise the authors to consider prioritising the most significant results and simplifying the presentation of detailed analyses, particularly in Section 3.4. The abundance of details makes it challenging for readers to follow the discussion effectively. Instead of presenting all available analyses, the authors might consider using a simple table with numerical data to present these results. Additionally, referring large part of the results solely to Figure 8 can be difficult for readers.
Line 152: ‘salinity has increased by 0.05 g/kg to 0.35g/kg’- it’s interesting how 34.4 isohaline deepened, and that negative T anomaly is noticeable in this location as well (Fig 4b-c) in the last decades in the Olga Basin. Is it an effect of more intense mixing? Or perhaps this is a signal of the Polar Front sharpening and concurrent steepening of isohalines/isopycnals? As we know from ‘Introduction’ section (Lines 38-42) the amount of inflowing freshwater has decreased in the northern Barents Sea…
Line 161: ‘At this time, AW was present in Storfjordrenna and close to the mooring’ – again, I miss the water mass classification in this manuscript. As the AW definition was not mentioned in the Introduction and Data & methods part, it’s necessary to introduce it here before you start to describe the results. As you use CT and SA, it will be good to define AW in these scales.
Line 198: ‘to 34 cm s−1 near the surface’ - when exactly? It will be good to find it in the plot - one cannot see the local maxima near the surface.
Discussion:
Winds, ice, tides, and AW inflows as well as upstream conditions definitely work together to shape the seasonal and interannual variability. However, attempting to address all the complex factors in one comprehensive article is probably ambitious task. I found the discussion too broad, it’s understandable given the extensive range of results presented in previous sections. However, I miss a clear statement regarding which mechanisms are deemed most responsible for the observed high interannual variability.
Line 323: ‘water column has not yet cooled down at that time’ – indeed, the observed differences between mid-October and November suggest a potential regime shift from Atlantic Water (AW) dominance to Arctic Water (AW) dominance during that time of the year. But of course, it could also be an exceptional year with more/warmer inflowing AW as you wrote.
Figures:
In general, I think a few plots and subplots would be better with some additional information that would make them more self-explanatory. Increasing the thickness of lines, enlarging markers, and reducing clutter can improve the visibility of key data points. It feels like this manuscript has many more figures than only 12 (plus these in appendices).
Figure 3: isotherm 0 deg C (light grey) - too like 200 m isobath - can it be changed?
Figure 7f-g: Perhaps some simple legend naming the lines can be added to read these plots without checking back and forward with the caption?
Figure 8: This plot seems a bit too complicated to me. Also, it would profit from adding some legends.
Figure 9: Why is there no current anomalies arrow in the subplot c)? The reference unit vectors for wind and currents are missing, as well.
Figure 12: ‘Sea ice concentration (black, left vertical axis) and sea surface temperature (red, right vertical axis)’ - from which product - write it here, also - add years on the subplots - it would be easier to look at.
Citation: https://doi.org/10.5194/egusphere-2023-3080-RC1 -
RC2: 'Comment on egusphere-2023-3080', Anonymous Referee #2, 26 Apr 2024
In presented manuscript the Authors propose a future pathway of Atlantic Water towards the Barents Sea, taking place across the channel between Edgeøya and Hopen islands. As it appears to be smaller than other existing AW pathways, the Authors emphasize the role of this inflow on the Olga Basin, reservoir of the coldest and thickest Arctic Water layer, and impact on East Spitsbergen Current important for the shelf and fjords west of Svalbard. Under this assumption, the Authors provides comprehensive analysis of interannual and seasonal variability of hydrographic conditions in the region based on historical and new data during late autumn as well as year-long data from mooring.
The paper address important aspects of the Barents Sea region, being a hot-spot in Arctic picture and fall very well in the scope of OS. The data are novel and provide crucial information on largely unexplored region. The study site description is comprehensive, data and methods are carefully selected to extract as much information as possible to provide valuable results and discussion. However, sometimes it appears to be overloaded and difficult to receive, and some things need to be rephrased/restructured to reach broader audience. Nevertheless, I believe that this paper is valuable for scientific community and can serve an important input to understanding the Atlantification of the Barents Sea. Below, I suggest some minor corrections before publication.
Introduction
I find the Introduction very accessible, readable and comprehensive. The only suggestion is to rephrase/create the hypothesis or state research questions to make the goal of the paper more emphasized. From Introduction it appears that this small area (the channel between Edgeøya and Hopen islands) is largely unexplored, which makes your data are novel and of great importance. We also know, that AW in Storfjordrenna is becoming warmer and is observed at shallower depths, so first you can hypothesis that due to the above observations the expansion of AW through the channel may increase leading to formation a future pathway of AW (persistent?) into the Arctic Ocean. Then, you can write about the role of this inflow, e.g. the sentence in line 72 and I think it is also worth mention, that increased AW inflow here can be of great importance for ESC (you mentioned about it in Discussion: line 462), which on the other hands, is crucial for the marine and coastal environment of west Spitsbergen.
Figure 2: Red dots across the saddle are not clearly visible, which makes wrong first impression, that transect in 2019 is only made along the channel. Please fix this. Does the dots representing transect from Storfjordrenna to Olga Basin are the same as transect showed in Figure 3?
Data and methods
This section includes a lot of details, which is good and proves that the Authors carefully and thoughtfully selected data and analysis methods to defined scientific problems. However, the description, despite specific information, makes this section available for rather narrow audience. This is especially true for mooring data. To more clarify, I suggest to state clearly at the beginning of paragraph in line 114, what are the mooring data used for, that those data are utilized to investigate variability in ocean currents and temperature associated with a) let’s say direction of the flow, b) selected time scales, including semidiurnal tides, weather-band processes (explanation/example?) and low-frequency activity (explanation/example?). Therefore, for a) the coordinate system was rotated with -42°… and for b) -28°… . This small rephrasing will help more clearly see and remember how the data will be analyzed. Then you can write about cutoff frequencies for different time scales.
Line 120-121: I think this needs a little explanation of the method and the purpose/justification for its application.
Results
Lines 148-149: What is the reason for choosing this particular isoline (34.7 g kg-1)? Can it be representative for “pure” or Atlantic origin water? As you mentioned in the Introduction, that you investigate the Atlantic origin inflow in this area, it would be useful to define this somehow and follow the idea.
Line 151: Could you more specify Atlantic origin water? I understand that this is rather challenging, especially when you describe seasonal evolution, where water undergoes significant modification and AW just provides heat and salt to the region. However, maybe it would be appropriate to define it/add some description (one sentence) in the Introduction? Or when you say about, for example, “pure” AW, then in brackets you can give some values stating this.
Line 155-157: Figure 5 says that all measurements (Nov 2019, Oct 2020, Nov 2021) were performed across the saddle. However, from Figure 2 I kept in mind (I was pretty sure), that Nov 2019 was preformed from Storfjorde to Olga Basin (red dots). As I mentioned red dots across the saddle in Figure 2 are not clearly visible, which makes wrong first impression. Please fix this.
Line 176: “close to the pure AW properties” means what exactly? This water hasn’t been defined in the manuscript before.
Line 258: Here you give some examples of analyzed time scale, which should be clearly stated in section Data and methods.
Figure 7: Dotted lines representing cutoff frequencies for the semidiurnal tidal band are hardly seen. Is it possible to make them more visible?
Discussion
This section is also overloaded with information and sometimes it is hard to follow. I propose to make some corrections for better reading of the text.
In section 4.1 you want to discuss interannual and seasonal variability in hydrography of the region with asking a question if 2018 – 2019 was a typical year. I suggest to start from discussing only a mooring year, comparing autumns 2018 and 2019, explaining AW inflows. Then you can make comparison with other years (transect across the saddle and time series from Figure 12). At the end you can address the question asked.
Line 333: “However, the mean SST in 2018–2019 was higher than all years between 1981 and 2006” - Does it add anything significant to the discussion?
Figure 12: Please add to the figure caption, that this is from mooring location.
Figure A1 and A2: please add to the captions what magenta diamond indicates.
Citation: https://doi.org/10.5194/egusphere-2023-3080-RC2
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