the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Patterns of wintertime Arctic sea ice leads and their relation to winds and ocean currents
Günther Heinemann
Frank Schnaase
Abstract. We use a novel sea-ice lead climatology based on satellite observations with 1 km2 spatial resolution to identify predominant patterns in Arctic wintertime sea-ice leads. The causes for the observed spatial and temporal variabilities are investigated using ocean surface current velocities and eddy kinetic energies from an ocean model (FESOM) and winds from a regional climate model (CCLM) and ERA5 reanalysis, respectively. The presented investigation provides clear evidence for the influence of ocean depth and associated currents on the mechanic weakening of sea ice and the accompanied occurrence of sea-ice leads with their characteristic spatial patterns. While the ocean influence on lead dynamics acts on a rather long-term scale (seasonal to inter-annual), the influence of wind appears to trigger sea-ice lead dynamics on shorter time scales of weeks to months and is largely controlled by individual events causing increased divergence.
Sascha Willmes et al.
Status: open (until 16 Apr 2023)
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RC1: 'Comment on tc-2023-22', Jonathan W. Rheinlænder, 09 Mar 2023
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The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2023-22/tc-2023-22-RC1-supplement.pdf
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RC2: 'Comment on tc-2023-22', Daniel Watkins, 28 Mar 2023
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Review of “Patterns of wintertime Arctic sea ice leads and their relation to winds and ocean currents”
Overview
The authors compare a new high resolution wintertime sea ice lead data set against modeled ocean currents from FESOM and wind from CCLM and ERA5. The results show a compelling linkage between small-scale ocean currents associated with gradients in bathymetry and regions with enhanced lead frequency. The influence of winds is examined in terms of monthly averages of velocity and divergence. The manuscript demonstrates that the new lead dataset has broad potential for increasing our understanding of ice processes.
General comments
The role of small-scale ocean currents is often neglected in discussions of ice motion and deformation. This study makes a strong argument that ocean currents, particularly those forced by bathymetry, are important for setting up regions of stronger ice deformation, resulting in regions of higher lead frequency. The results are based on a new dataset as well as new runs of a modern ocean model with high spatial resolution. My main concerns are with the treatment of wind forcing and with the attribution of causality for ocean currents. In the following I will pose a few questions for the authors that I think need to be addressed before the manuscript is ready for publication.
Can the lead fraction dataset distinguish between leads, polynyas, and regions of low sea ice concentration? If not, how does this affect the interpretation of regions marked as having high lead frequency? I question whether the high lead fractions shown in Baffin Bay, the Barents Sea, and the Greenland Ice Tongue are conflating leads with the typically low sea ice concentrations in those regions.
How does variability in the location of the ice edge affect the reported lead fractions? From the example image in Figure 2a, I see that the highest lead frequencies are found in the marginal ice zone and along coasts. Looking at the red arrow in Figure 3a, I am concerned that the lead frequencies reported near Nova Zemlya represent different numbers of years, as there is often open water for a large fraction of that transect.
What role does wind direction have on the lead fraction near coasts? The manuscript states that there is no indication that wind impacts the location of the lead regions. The analysis focuses on monthly averaged values of divergence and wind speed and attempts to correlate wind values with lead fraction in each grid point. I think that the lack of connection between winds and lead fraction results from the analysis method. The effects of winds on sea ice represents an integration of stresses upwind. The geometry of wind stress and coastlines is a critical component of preferential lead formation. In particular, coastal regions are strongly affected by changes in the wind direction even if the wind speed is constant. See for example Lewis and Hutchings 2019, Jewell and Hutchings 2022, and the preprint of Jewell et al. 2023 for analysis of sea ice lead formation in the Beaufort Sea.
How does the choice of monthly averaged winds affect the interpretation? Using monthly averages removes effects of cyclones and most synoptic variability, which are especially important for ice dynamics. Model output is hourly, so this aspect of dynamics is captured partially–why not use higher temporal resolution for the reanalysis? It is possible that the high lead frequency seen along coastlines and near the boundaries of landfast ice corresponds with reversals in the direction of along-shore breezes as the ice is alternately pushed toward and away from the coast. This effect would not be seen in the monthly average wind speed. Consideration of the monthly standard deviation of wind velocity components or wind direction may be useful there.
Can the authors rule out other possible causes for the co-location of high lead fraction and strong ocean currents? I agree with the authors that the cross sections showing enhanced currents implies that currents are a possible cause for enhanced lead formation. However, coastal interaction in combination with higher frequency wind variability (hours to weeks) may also result in higher lead frequency. I think the case could be strengthened by testing other candidate factors, and also by testing the hypothesis that high ocean currents lead to high lead fraction by looking at other regions where ocean currents are high. Are strong gradients in ocean currents always associated with increased lead frequency? If not, can the discrepancy be explained?
Minor comments
49 Artefact → Artifact
95 Shear is a form of deformation, what specifically is meant here? Spreen et al. 2017 discuss total deformation is the divergence and shear added quadratically, which is a standard measurement. Perhaps the authors meant to write “Total deformation”.
Figure 6 – CPE is a useful tool for seeing where both lead frequency and ocean current metrics are anomalous. It would be useful to also see the regions where one quantity is anomalous and the other is not, for example where there is enhanced lead frequency with no corresponding increase in current speeds or EKE. Are there places where a strong boundary current does not appear to affect the LFQ?
Figure 7 – Is it known why coast effects are so strong in C15, and why the coastal winds are so different between ERA5 and C15?
Figure 11 – date index should be in dates for the time series, not in month numbers since some arbitrary start date, as done in Figure 8.
245 I suggest replacing “increasing towards South” with “increase southward”
Citation: https://doi.org/10.5194/tc-2023-22-RC2
Sascha Willmes et al.
Sascha Willmes et al.
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