Atmospheric highs drive asymmetric sea ice drift during lead opening from Point Barrow
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.
MacKenzie E. Jewell et al.
Status: open (until 02 Apr 2023)
- RC1: 'Comment on tc-2023-9', Sascha Willmes, 27 Feb 2023 reply
MacKenzie E. Jewell et al.
Model code and software
Routine for extracting sea ice leads from MODIS imagery https://doi.org/10.5281/zenodo.7567150
MacKenzie E. Jewell et al.
Viewed (geographical distribution)
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.
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).
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.