Physiographic Controls on Landfast Ice Variability from 20 Years of Maximum Extents across the Northwest Canadian Arctic

Round 1

Reviewer 1 Report

The manuscript describes landfast ice variability in the Northwest Canadian Arctic. 

The manuscript is solid and well written and deserves to be published.

1. Unclear the usage of the words "Physiographic controls". This is a very broad term, you need to clearly explain the meaning in the context of the research.
2.  The choice of the study area should be better explained. Why the entire Arctic is not covered, the data are available for the whole region.
3.  The choice of the time period should be justified. Any specific hypothesis on variability over the last 20 years? 

 

Author Response

Reviewer 1

The manuscript describes landfast ice variability in the Northwest Canadian Arctic.

The manuscript is solid and well written and deserves to be published.

• Unclear the usage of the words "Physiographic controls". This is a very broad term, you need to clearly explain the meaning in the context of the research.

As advised, we have now added a definition of ‘Physiographic Controls’ and its direct meaning relating to our paper. We have clarified this within the abstract and specifically defined its meaning within the introduction to add further context and enhance the flow of the manuscript.

L22-25: Static physiographic controls (namely topography and bathymetry) were found to influence MLIE change across regional sites, but no association was found with dynamic environmental controls (storm duration, mean air temperature, freezing and thawing degree day occurrence).

L55-60: The influence of physiographic controls on MLIE have been limited by a lack of spatial and temporal observational data at a sufficiently resolved scale. Physiographic controls, in this context, can be defined by the various controls on landfast ice from static settings such as topography or bathymetry, contrasted against dynamic environmental drivers such as temperature or storminess.

 

• The choice of the study area should be better explained. Why the entire Arctic is not covered, the data are available for the whole region.

The study area tracked the Northwest Canadian Arctic coastline instead of the entire Arctic as this area had not yet been explored for its spatial and temporal variability in maximum landfast ice extent, only sea ice specifically. Furthermore, many pan-Arctic studies have failed to attain the level of detail of the maximum landfast ice extent edge and distance from coastline that is achieved within this study at regional and sub-regional scale, this level of detail and increased understanding of landfast ice behaviour it provided would not have been possible at a pan-Arctic scale.

We have now included this explanation within the study area section in the manuscript, see below.

L124-133: The Northwest Canadian Arctic coastline represents a varied complex topographic setting (68^oN - 78^oN and 115^oW - 145^oW; Figure 1). The low-lying deltaic coasts at the mouth of the Mackenzie contrast with steep cliffs and slumps of the outer islands, whereas the topographic setting of the Amundsen Gulf consists of complex bays, channels, and straits. This topographic variety is less evident in the relatively uniform steep cliffs of both West and North Banks Island. The Northwest Canadian Arctic is a critical area of the Arctic where landfast ice extents remain poorly understood but influence both vital shipping routes and many coastal communities. Understanding the detailed patterns and controls on landfast ice extent in the Northwest Canadian Arctic enables an essential new level of detail, highlighting four regional sections: the Mackenzie Delta, the Amundsen Gulf, West Banks Island and North Banks Island. These regions were chosen to account for a distinction in topographic setting, bathymetry, temperature, windspeed and direction; allowing both static and dynamic controls on ice variability and its wider implications to be explored.

 

• The choice of the time period should be justified. Any specific hypothesis on variability over the last 20 years?

The 20-year time frame was chosen to evaluate the rapid changes since the beginning of the 21^st century (2000-2019), highlighted in previous sea ice research. Specifically, temperature rise over this period has been well documented and the 20 year duration enables any significant year-on-year trends to be identified.

L11-12: Despite its significance, there remains a paucity of data on the spatial variability of landfast ice and limited understanding of the environmental processes controls since the beginning of the 21st century.

L54-55: Maximum landfast ice extent (MLIE) changes have varied more than landfast ice thickness across the Northwest Passage since the beginning of the 21st century [1].

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript deals with an interesting topic that is interesting from environmental, ecological and societal perspectives. The manuscript has a good structure and a sufficient number of high-quality figures. As such, it can be recommended for publication after minor revision.

1. P3 L125: -> 68^oN - 78^oN and 115^oW - 145^oW

Similarly, P6 L202 - 203

2. P3 L148: Add a reference to “GEBCO 2021 Grid”.
3. Please show the bathymetry in Figure 4. So the reader can see the result more clearly.
4. P9 L304: “Figure 4Ai” -> “Figure 4A(i)”

P9 L305: “Figure 4Aii” -> “Figure 4A(ii)”

5. P8 L278: It should be section 3.2.

P9 L307: It should be section 3.3.

Author Response

Reviewer 2

The manuscript deals with an interesting topic that is interesting from environmental, ecological and societal perspectives. The manuscript has a good structure and a sufficient number of high-quality figures. As such, it can be recommended for publication after minor revision.

Thank you for picking up on these errors, they have now been edited.

 

• P3 L125: -> 68^oN - 78^oN and 115^oW - 145^oW Similarly, P6 L202 – 203

These have now been edited, please see L127 and L207-2109

• P3 L148: Add a reference to “GEBCO grid”.

Please see Figure 1 and L156: For static controls, the bathymetric data were generated using GEBCO_21 Grid [53].

• Please show the bathymetry in Figure 4. So, the reader can see the result more clearly.

Thanks for your suggestion, we did initially try this. However, as there are many lines already on Figure 4 (the landfast ice extents), it became cluttered, and we believe complicated the initial message of this figure. Therefore, we ask the reader within text to refer back to the study area Figure 1, which is the same map but is free from the landfast ice extents and therefore allows a detailed view of how the bathymetry differs at and within each region.

• P9 L304: “Figure 4Ai” -> “Figure 4A(i)”
• P9 L305: “Figure 4Aii” -> “Figure 4A(ii)”

Please see L309-312: One of the central clusters (Figure 4A(i)) is between two pinch points, where two headlands create a topographic narrowing in the gulf. The second central cluster of MLIE (Figure 4A(ii)) is south of the large channel, opening into the gulf on the east coast of Banks Island.

• P8 L278: It should be section 3.2.
• P9 L307: It should be section 3.3.

This has now been edited, please see L284 and L313-314.

Author Response File: Author Response.docx

Reviewer 3 Report

This work explores the relationship between the MLIE of Northwest Canadian and the static and dynamic controls, and emphasizes the importance of analyzing landfast ice as a specific component from total sea ice. Overall, I think the work is of importance under the background of dramatic sea iceretreat, and it can provide valuable insight for relevant stakeholders. While interesting results are presented, I have several suggestions to the authors which I think will help improve the manuscript, as follows:

In Figure 3 and Section 3, the average % distance change of MLIE comparing the years 2000 and 2019 directly are used. Due to fluctuations in MLIE, it may not be of great reference value to calculate this indicator directly using the MLIE of the beginning and end years. It may be better to use the trend or the difference between the decadal mean.

In Table 1, I noticed some notable numbers, such as -92 for North Banks Island in 2006 and 86 for Amundsen Gulf in 2000. It may be possible to combine static and dynamic controls to find a reasonable explanation for these prominent values.

In Figure 4B and 5, the label for Y-axis should be added.

Author Response

Reviewer 3

This work explores the relationship between the MLIE of Northwest Canadian and the static and dynamic controls and emphasizes the importance of analysing landfast ice as a specific component from total sea ice. Overall, I think the work is of importance under the background of dramatic sea ice retreat, and it can provide valuable insight for relevant stakeholders. While interesting results are presented, I have several suggestions to the authors which I think will help improve the manuscript, as follows:

Thank you for your thought-provoking suggestions, they are all greatly appreciated. Please see the amendments and justifications regarding your comments below.

• In Figure 3 and Section 3, the average % distance change of MLIE comparing the years 2000 and 2019 directly are used. Due to fluctuations in MLIE, it may not be of great reference value to calculate this indicator directly using the MLIE of the beginning and end years. It may be better to use the trend or the difference between the decadal mean.

This point was a heavy topic of discussion during our process of producing and formatting our results. The fluctuations are calculated based on the average over the study period. However, we highlighted the start (2000) and end (2019) year to indicate why we believe landfast ice needs to be identified and or monitored in its own right. This is because we found many previous sea ice studies, that do not differentiate between pack and landfast ice, have often used average declines over large time periods. This does not work with the high degree of fluctuations picked up on within our landfast ice dataset, we believe the decadal and sometimes multidecadal averages used for sea ice are not representative of the 5-8 year landfast ice cycles we found.

This is highlighted within the results, please see the Figure 3 caption which has been edited to make the point clearer and then also L219-226: Average MLIE distances calculated across the entire study area coastline (Figure 3) declined by 73% in a direct comparison between the individual years of 2000 and 2019. However, MLIE positions were highly dynamic and variable, progressing through both landfast ice rich and poor periods. Across the study area average, the 5 – 8 year cycling from low to high and back to low landfast ice extents show no discernible trend in MLIE over time. This contrasts with relatively continuous decline of sea ice extent noted elsewhere, which comprises of and does not distinguish between pack ice, drifting ice and landfast ice [57–59].

This topic is further discussed in section 4.2. L460-474.

 

• In Table 1, I noticed some notable numbers, such as -92 for North Banks Island in 2006 and 86 for Amundsen Gulf in 2000. It may be possible to combine static and dynamic controls to find a reasonable explanation for these prominent values.

These notable numbers of extreme high and low values were initially identified and tested against a multitude of static and dynamic controls, however no correlation between these extreme years was found with any of our variables tested, therefore we make the point of stating further dynamic variables may need to be tested within these areas.

Please see section 4.1 L413-415: Our data have opened exciting new avenues of research for dynamic controls to be tested with a focus on memory signals and the 5 – 8 year cycling of MLIE.

Please see section 4.2. L423-426: The interannual variability and lack of a discernible trend in MLIE we have recorded causes growing concern for increased vulnerability, particularly across the Inuvialuit Settlement Region (ISR), as we are yet to discover a clear dynamic control.

 

• In Figure 4B and 5, the label for Y-axis should be added.

Thanks for picking up on these errors within the formatting of Figure 4B and 5. These have now been edited and corrected.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

now it can be accepted.