the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Atmospheric highs drive asymmetric sea ice drift during lead opening from Point Barrow
MacKenzie E. Jewell
Jennifer K. Hutchings
Cathleen A. Geiger
Abstract. Throughout winter, sea ice leads open episodically from headlands along the Alaskan coast under the winds of passing weather systems. As leads extend offshore into the Beaufort Sea, they produce ice velocity discontinuities that are challenging to represent in models. Here, we investigate how synoptic wind patterns form large-scale leads originating from Point Barrow, Alaska and influence Pacific Arctic sea ice circulation. We identify 135 leads from January–April 2000–2020 and generate an ensemble of lead opening sequences by averaging atmospheric conditions, ice velocity, and lead position across events. On average, leads open as the winds of migrating high-pressure systems drive differing ice-coast interactions across Point Barrow. Southward winds compress the Beaufort ice pack against the coast east of Point Barrow over several days, slowing sea ice drift. As offshore winds develop in the west, a lead opens and separates the western ice pack from the coast. The eastern ice pack remains in contact with the coast, drifting at half the rate of western ice despite similar wind speeds. As a result, sea ice drifts asymmetrically along the Alaskan coast during these events. Most events occur under north or east-northeast winds, and wind direction relative to the coast controls patterns of opening and ice drift. These findings highlight how coastal boundaries modify the response of the consolidated ice pack to wind forcing in winter. Observed connections between winds, ice drift, and lead opening provide effective test cases for sea ice models aiming to capture realistic ice transport during these recurrent events.
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MacKenzie E. Jewell et al.
Status: final response (author comments only)
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RC1: 'Comment on tc-2023-9', Sascha Willmes, 27 Feb 2023
Summary
The presented paper aims to evaluate typical dynamic conditions throughout the process of lead formation at Point Barrow. The authors construct an ensemble average lead sequence from MODIS thermal-infrared satellite imagery and derive the associated daily atmospheric conditions and sea ice motion. From this combined data set they find a typical synoptic condition over the Beaufort Sea region during lead opening that is mainly characterized by SLP above average. This pattern appears to cause a strong zonal asymmetry in sea ice drift north of the Alaskan coast, which in combination with coastal interactions, drives the break-up of sea ice with the typical pattern found at Point Barrow. The authors conclude that wind direction and coastal geometry are key controls of lead formation in the Beaufort Sea during wintertime.
General comments and decision
The paper represents an interesting study on sea-ice dynamics in the Beaufort Sea during winter and its drivers in the atmosphere. The analysis and the presentation of results are scientifically sound and certainly provide new insight into the causes of sea-ice variability in the Arctic and sea ice - atmosphere as well as sea ice – coastal interactions in general. The study nicely adds up to some other recent publications about what drives the formation of leads in the Arctic and thereby contributes to an improved understanding of the Arctic climate system.
I suggest the paper to be published after mostly minor corrections that I am listing below.
Specific comments
My only major annotation is that the process of the ensemble lead sequence calculation lacks some information to the reader. Although the obtained leads and their patterns are well described in Appendix A1 and B2, it would surely improve the paper if for one exemplary lead sequence the associated satellite images were shown additionally to demonstrate how the observations make up the ensemble. In this context, I am a bit surprised about how exactly the long time series (Fig. A1) was extracted. The authors mention that “each acquired thermal MODIS image was visually analyzed to document the sea ice activity in the region”. But that would mean that more than 7000 MODIS composites (3x daily, 120 days, 20 years) were individually screened for the presence of leads? I think that adding the above-mentioned example for some scenes would help clearing this issue. In this context, I also recommend adding a simple graphical demonstration of how the mentioned active contour model (2.4) does extract a lead from the thermal infrared image (raw image and derived lead).
Minor comments:
L 22: “within O (550 km)” What is meant? I guess a technical correction is necessary here.
Figure 1: A small inset or subfigure with an overview map (whole Arctic) might be useful.
Section 2.4 can be shortened I think. Especially the first two paragraphs seem a bit misplaced.
LL 128-130: “However, … Point Barrow”. Unclear what is meant here.
L 156: “200 m”. Is that a fixed value determining the minimum width of a lead to become apparent in a MODIS image? Wouldn’t that depend on the contrast between lead temperature and surrounding temperature rather than on width only?
Figure 5: I find it a bit confusing that the lead in the DLO subplot disappears in DLO+1. It might make the reader think that the leads last for one day only.
L319: “average speeds” … please add “of sea ice drift”
L323, L329: These numbers (0.2%, 0.3%) are really small. How does that relate to the effect size? The shown spatial patterns underline that the effect is definitely important, but some discussion about this might help here.
L364: What is exactly meant with “streamline”?
L435: To me it was not really clear what is meant with “a synoptic center aligns with a known center of action”.
L 439: “O (500 km)” also in L 481.
The Discussion (4) is very extensive and can be shortened, I think. Some arguments seem to repeat.
LL 522-528. The description in this paragraph was not clear to me.
Section 5: Is also very extensive, could maybe be shortened.
Technical corrections
None.
Citation: https://doi.org/10.5194/tc-2023-9-RC1 - AC1: 'Reply on RC1', MacKenzie Jewell, 09 May 2023
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RC2: 'Comment on tc-2023-9', Anonymous Referee #2, 29 Mar 2023
Review Mckenzie
This paper analyses the atmospheric conditions during observed sea-ice lead openings from Point Barrow from 2000 to 2020. The authors use the ERA5 reanalysis to generate an atmospheric composite describing the mean atmospheric state during a lead opening. This is augmented by the observed ice drift from the Polar Pathfinder sea-ice motion product. The authors analyse the mean atmospheric and sea-ice state during a lead opening, concluding that such events are primarily driven by strong winds associated with an anti-cyclone over the Beaufort Sea, driving the ice along the Canadian and Alaskan coast, causing a lead to open at Point Barrow, which acts as a focal point for the stresses in the ice. They also analyse the ice response to the different wind directions observed when the lead opens up.
The paper is interesting, well-written, and well deserving of publication in The Cryosphere. The approach is novel, interesting, and well-suited to analyse the lead formation, both of Point Barrow and in general. The paper is very informative, and there is much information there. It is also well-written and readable. I only have one general and a few specific comments on the paper and recommend publication once those are addressed.
General comment:
There's an anomaly in SLP associated with lead openings (e.g. figure 4). But this doesn't seem very dynamically relevant. So the anomaly in the SLP gradient (e.g. figure 7) should be highlighted instead.
Specific comments:
L52: New paragraph at "Landfast ice ..."L52: No need for brackets explaining landfast ice
L63: Change "translating" to "traversing" (for example).
L92: MODIS lead detection is impressive under ideal conditions. But I would have liked to learn more about its ability to detect leads under less-than-ideal conditions. There is no mention of cloud cover problems, for example.
L235: Interesting that the winds strengthen after lead opening. It sounds like a selection bias, but how that would work is not immediately apparent. Should be addressed in the discussion.
L276: Mention that \alpha is essentially the Nansen number (if you ignore ocean currents). As it stands, its appearance here can seem a bit random.
L330/Paragraph: Figure 8(b) needs a better explanation. Why do you take the projection of the ensemble drift onto the climatological one? What does this show us? You say it reveals "one of the most striking features of the ensemble event sequence", but this is lost on me. I feel like I do not understand something important here.
L344: I found this to be the most interesting section! Seeing how the leads open along the wind streamline was particularly interesting. This indicates that the wind direction and topography combination is the controlling factor in lead formation off Point Barrow and that ice strength should have a relatively small impact. This also indicates that modelling such events should be pretty straightforward, but this is not the case. It also contradicts the results of Rheinlænder et al. (2022), who found that thinner ice broke up more easily. So there's food for thought here, which is highly appreciated.
Presentation-wise, I would have liked to see the mean wind field rather than the isolines you show. It's confusing that the in the easterly case, the lead opens perpendicular to the isobars, but this is actually along the wind streamline. I would also note that a more significant role of ice dynamics is indicated where the lead deviates from the wind streamline. Finally, I would remove the westerly case. You say you include it for completeness, but with such few members of your ensemble, I think it's safer to leave it out.
L421: This paragraph belongs in the introduction rather than here.
L420: This is a nice and interesting discussion. But I would start by looking at the mean state (the ensemble means) and then discuss that there are variations from those. That order makes more sense. Simply moving sections around a bit would do.
L470: "… and in summer …" - I guess the "and" should not be there.Citation: https://doi.org/10.5194/tc-2023-9-RC2 -
AC2: 'Reply on RC2', MacKenzie Jewell, 09 May 2023
We would like to thank Referee #2 for their consideration of our manuscript and for providing constructive comments and suggestions. With their feedback the presentation of the manuscript will greatly improve. Please find our response to the comments in the attached pdf file.
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AC2: 'Reply on RC2', MacKenzie Jewell, 09 May 2023
MacKenzie E. Jewell et al.
Supplement
Model code and software
Routine for extracting sea ice leads from MODIS imagery MacKenzie Jewell https://doi.org/10.5281/zenodo.7567150
MacKenzie E. Jewell et al.
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