Modelling the evolution of Arctic multiyear sea ice over 2000–2018
Abstract. Multiyear sea ice (MYI) cover in the Arctic has been monitored for decades using increasingly sophisticated remote sensing techniques, and these have documented a significant decline in MYI over time. However, such techniques are unable to differentiate between the processes affecting the evolution of the MYI. Further, estimating the thickness, and thus the volume of MYI remains challenging. In this study we employ a sea ice-ocean model to investigate the changes to MYI over the period 2000–2018. We exploit the Lagrangian framework of the sea ice model to introduce a new method of tracking MYI area and volume, which is based on identifying MYI during freeze onset each autumn. The model is found to successfully reproduce the spatial distribution and evolution of observed MYI extent. We discuss the balance of the processes (melt, ridging, export, and replenishment) linked to the general decline in MYI cover. The model suggests that rather than one process dominating the losses, there is an episodic imbalance between the different sources and sinks of MYI. We identify those key to the significant observed declines of 2007 and 2012; while melt and replenishment are important in 2012, sea ice dynamics play a significant role in 2007. Notably, the model suggests that convergence of the ice, through ridging, can result in large reductions of MYI area without a corresponding loss of MYI volume. This highlights the benefit of using models alongside satellite observations to aid interpretation of the observed MYI evolution in the Arctic.
Heather Christine Regan et al.
Status: final response (author comments only)
RC1: 'Comment on tc-2022-211', Anonymous Referee #1, 24 Nov 2022
- AC1: 'Reply on RC1', Heather Regan, 09 Feb 2023
RC2: 'Comment on tc-2022-211', Anonymous Referee #2, 13 Dec 2022
- AC2: 'Reply on RC2', Heather Regan, 09 Feb 2023
Heather Christine Regan et al.
Heather Christine Regan et al.
Viewed (geographical distribution)
The manuscript "Modeling the evolution of Arctic MYI cover over 2000 - 2018" by Heather Regan uses a sea ice-ocean model to examine how MYI area and volume change in the Arctic. The advantage of the presented approach is that, compared to other studies based on satellite data, budgeting of different sink and source terms such as ridging, melt and replenishment can be done and linked to the MYI retreat. The authors conclude that rather than one process dominating the observed losses. Furthermore, they take a closer look at the processes controlling MYI area and volume changes in anomalous years like in 2007 and 2012.
The study is definitely worth publishing. It is a great addition to existing (remote sensing) studies, excellently written and very well and clearly structured. The model validation and comparison with observations is extensive and convincing. The budgeting of the individual processes contributing to the decline of MYI in the Arctic is comprehensible. The focus on the two extreme years in 2007 and 2012 is also reasonable.
I believe that the paper will be widely cited and can make an important contribution to a better understanding of the changes in the last ice areas.
My comments are mainly "minor". I do, however, encourage a somewhat deeper investigation or at least better explanation of the post-2007 decline in ridging (see my comments below). If possible, please provide a Figure like Fig. 6 for the individual key regions (i.e. in the appendix).
Line 24: Isn’t the statement that “MYI area anomalies being closely linked to anoamlies in Arctic ice volume” contradicting to what is said in the abstract? Line 11: “…can result in large reduction of MYI area without a corresponding loss of MYI volume”? Maybe you can emphasize this contradiction in the abstract since it points to the importance of this study.
Line 43: “ice type classification fails in summer”: I don't think this can be generalized, because of course different methods and sensors are used. Some of them are more robust in summer.
Line 46: A little bit outdated reference: There are more recent studies on reliability of motion products in summer available. Also the products get better and better. E.g. Hiroshi Sumata (Tromsoe) did quite some work on product intercomparison.
Line 50: “…challenging”. May be refer to “von Albedyll 2021, Linking sea ice deformation to cie thickness redistribution using HR satellite and airborne obs.
Line 52: I guess there are more recent publications assessing the accuracy of altimetry missions. E.g. the Nature Paper by Landy (2022). Avoid terms like “relative uncertain”
Paragraph 42 - 55: In this paragraph, the author focuses strongly on the drawbacks of the satellite-based methods. I think that this is not necessary, because the advantages of the models are obvious. Its just a comment, no need to change anything J
Line 89/70: MYI
Line 125: Just out of curiosity: How well do the results on temporal and spatial variability agree with satellite-based methods? Satellite-based methods probably rely on changes in surface properties, while temperature differences and heat fluxes come into play in models? However, in general, model and satellite data on freeze up should be comparable?
Line 143: “MYI is both thicker and stronger…”: This assumption probably holds one of the largest uncertainties: In particular, in the marignal ice zones and throughout the Transpolar Drift, FYI and SYI (i.e., MYI in this study) are likely to have similar thicknesses and are otherwise difficult to distinguish. When is ridging in the model considered to be completed in an ice age class? Or in other words: At what point has enough FYI been deformed for MYI to proceed?
Fig 2: All very interesting!
Line 191: May be you can get in touch with the producer and ask what the reason may be?
Line 201: “…conditions in certain years” and areas. See my comment to Line 143: I believe that in the marginal ice zones the assumption may not be correct
Line 209: CDR data.Using
Line 2014: “The model and satellite-based data?
Line 250: Just a general comment: I find the evaluation and comparison with the CDR and NSIDC data highly interesting and well done. Thanks, I appreciate reading
Fig. 6e) Hard to read. Can you make it bigger or refer to Fig. 8
Line 280: I get what you saying and I find it interesting. May be you can clarify statement a bit better
Fig 8: Can you add notations of Fig. 6e? (A,B,C,etc)
Line 291: The CE is the largest contributor because this is where most of the FYI turns into SYI?
Line 294/295: I am sceptical about the conclusion. May be move it to the discussion.
Line 296: I guess you refer to Fig. 8 in this statement, but are you sure there is no trend if looking at the Arctic wide sources and sink terms? To me it seems that ridging is reduced, although it has this staircase appearance.
Comment to export: How well does the modelled (Fram Strait) export compare with exports from others? Ricker, Smedsrud, etc? Does it capture the seasonal and interannual variability correctly?
Chapter 4.2.1: Great chapter. However, it took me a while to understand it all. Maybe you can refer to Figures more often in one or the other place? Same for 4.2.2
Line 332: I guess the replenishment rate is directly related to the FYI area available at the end of the summer. May be just state that more FYI was melted then usual, such that replenishment rate was reduced? I hope I did not get this wrong though. Note that according to this study (https://doi.org/10.1038/s41598-019-41456-y) there is a generally reduced survival rate of FYI, and hence a generally reduced replenishment rate (at least in the TP Drift)? Can you confirm this?
Line 354-356: I guess this sentence is not needed since it is well described in 4.2.1?
Line 378: I think the Smedsrud study is not solely based on observations, but observations were used to establish a relationship between pressure gradients across Fram Strait and export?
Chapter 5.2: This is all very interesting! However, I have a few questions related to the chapter
Line 401: Not sure if I got this correct: You mean, as the ice cover shrinks, less FYI survive the summer and hence, there is less replenishment taking place in the Central West region (and others)? Again, this would support the Krumpen story of a reduced survival rate and the timing of the drop-down in ridging and FYI survival rate (Krumpen) is about the same.
And Line 400: “mostly unaffected” I would expect that ridging in the Laptev See and other Russian shelf seas went to almost zero, since MYI production zones are shifted elsewhere (north)? May be it would be a good idea to provide a Fig like Fig 6 for each section (A,B,C,D… ) in the appendix. This would also be a valuable information for other studies
Line 401: You mean: As the MYI cover shrinks….
Line 402 – 405: Still this does not explain the stepwise decrease in ridging after 2008 (or may be I just did not get it). If this would be solely related to a shrinking MYI cover and shifting replenishment zones, it would be a gradual change, right?
Conclusion: Line 447: “no one process stands out”… I think that the stepwise reduction of ridging somewhat stands out…
Line 457: “This change in behaviour related to the general reduction…”. I don't think this fact has actually been explored deeply enough in the discussion to make that statement.