the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Nov 2024
Defining Antarctic polynyas in satellite observations and climate model output to support ecological climate change research
Abstract. Antarctic polynyas are key components of Antarctic marine ecosystems, influencing light and nutrient availability and open water access for marine predators. Thus, changes in the physical characteristics of polynyas can influence how these ecosystems respond to a changing climate. Here, we explore how to identify polynyas using satellite and Earth System Model data, and we assess the impacts of using different polynya-identification metrics (sea ice concentration or thickness). Our results show optimal metrics for polynya definition will depend on the temporal and spatial resolution of the data, as well as the season and region of interest. These results highlight the importance of identifying polynyas on grids of the same type and resolution when comparing polynyas from different data products. We find that sea ice thickness is more suitable for identifying polynyas in model data in winter months in contrast to spring months when both sea ice thickness and concentration may be suitable metrics. We then use the Community Earth System Model Version 2 (CESM2) to investigate ecosystem function within polynyas and find that there is enhanced phytoplankton productivity in modeled polynya features in both hindcast and fully coupled simulations, with springtime polynyas remaining an important control on Antarctic productivity under future climate change.
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RC1: 'Comment on egusphere-2024-3490', Anonymous Referee #1, 16 Dec 2024
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I see four main results in this paper:
- Sensitivity analysis of the various choices involved in polynya detection using observations (thresholds, concentration vs thickness, grid type, daily vs monthly, CDR vs NASA Team);
- Assessment of the representation of polynyas in CESM2-LE and the effect of internal variability;
- Comparison of the representation of polynyas between the fully-coupled CESM2 and the forced JRA-CESM, i.e. detangling the contributions of ocean/ice and atmosphere;
- Polynyas as a hotspot for ecosystem productivity.
All these four are interesting and important areas of research.
Unfortunately, the first three are mixed together in a story that I struggled to follow. In particular, the representation of polynyas in the models is presented as a detection issue; the fact that ”there may be model biases” is only finally acknowledged line 618. Yes, one needs to know how to detect polynyas in models before being able to evaluate them, but that’s why you are doing all these tests on the CDR where you degraded the resolution and/or changed the low values to reproduce known model/observation differences: You have a reference to compare your tests to. I would recommend you set the detection method, using the one most adapted to the models' detection based on the many tests you did on the observations, and only then look at the models, using this on method only. The model comparison of the two temporal resolutions would remain useful though, since different communities use the monthly and daily output.
It is also confusing that for most of the analyses you present only the results of JRA-CESM, when 1. Figure 1 suggests that CESM-LE has a more accurate sea ice and 2. The NPP analysis at the end is done on CESM-LE. That is, the case for using JRA-CESM is not well motivated. As I said above, the comparison between JRA-CESM and CESM-LE would allow you to discuss the effect of forcing vs full-coupling, but you do not discuss this for now. Removing JRA-CESM from this paper could allow you to keep this discussion for a dedicated study. You could even consider using only the subset of ensemble members that are the most accurate for your NPP analysis; according to Figure 8b, bottom panels, some are ok-enough.
Despite how this may sound, I suspect that all the work is there already, and that it is only the text that needs re-arranging for your argument to be convincing. The rewriting would be substantial though, so I do not provide minor comments that could become irrelevant this time.
The figures need adjusting as well to increase readability:
- Figure 1, black and dark blue are hard to distinguish (grey instead of black?);
- Figure 2, the dark blue asterisks are hard to see against the dark blue low ice concentration (have them e.g. orange, and switch red to magenta?);
- Figure 3 onwards, the four different shades of blue are barely distinguishable;
- Figures 4-6, please have bigger fonts in the legend
Citation: https://doi.org/10.5194/egusphere-2024-3490-RC1 -
RC2: 'Comment on egusphere-2024-3490', Tarkan Bilge, 29 Dec 2024
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The study under review presents an analysis of the sensitivity of polynya occurrence and area to polynya definition thresholds in satellite observations and climate model output. The authors subsequently use this analysis to construct methodological recommendations for future studies on polynyas. In my opinion, these two aspects represent the main two contributions of the study; methodological advice for polynya studies, and numerical evidence for the way in which resolution/data-type/season influences polynya statistics. This submission is original, being the first such study to quantitatively demonstrate the influence of resolution and data type (e.g. model/observational) on polynya number and area. The main significance of the study lies in the methodological recommendations and accompanying numerical justifications which can be used by future studies to consistently define polynyas without the requirement for authors to invent their own definitions (e.g. Mohrmann, 2021). In my opinion, the scientific content of the article is clearly articulated and well presented and only requires minor corrections prior to publication.
Below I have included a small number of specific issues which I would like to bring to the authors' attention.
The authors have explained that this study is focused on coastal polynyas, (line 165), but in the study abstract and in the the conclusions (e.g. conclusions 3, 4, 6) refer to "polynyas" more generally. Given that the mechanisms which cause coastal polynyas and open-water polynyas are different, it is not necessarily possible to extrapolate conclusions from the analysis of one to the other. For example, the authors explain that SIT may be a better metric in winter due to air temperatures causing almost immediate surface freezing - I would expect this effect to disproportionately impact coastal polynya identification, because open-water polynyas feature upwelling of warm water which might less readily freeze. Related to this comment, Mohrmann et al. (2021) found that the "CMIP6 CESM2 models show coastal polynyas, but never OWPs", so extending analysis to open-water polynyas using CESM2 might not be possible. Instead, I would recommend the authors to caveat these potential differences, and clearly state in the abstract, conclusions, and main text that results are relevant for *coastal* polynyas. This may sound like a pedantic point, but since this study aims to make recommendations on polynya definition and thresholding to future studies, it is important that future studies on open-water polynyas recognise that not all of the thresholding analysis has been carried out with open-water polynyas in mind, and therefore the conclusions may not be suitable for application in their studies.
In section 4.6, polynya number and area are analysed in the CESM2-LE, and are found to decrease towards the end of the century. I think the study would benefit from some analysis or informed speculation as to the mechanism which drives this decrease in future polynyas under emissions scenarios. The reduction could be a result of changing mean-wind fields, or more simply due to a reduction in overall Antarctic sea-ice. If the authors compare the ~2090 mean Antarctic July and November sea-ice field with the ~2020 fields, they might find that a mean field difference could explain this reduction. This might also explain why the November polynya statistics drop off more readily than the July ones.
On line 312, the authors say that April to October the polynya area is higher for daily mean than monthly mean data, often by more than one standard deviation. Should this mean that the dark blue line and light red shaded areas should be separate in Figures 3a, S2 (Apr-Oct)? Perhaps the lines are too thick or the plots are too small to appreciate this, could the authors check this statement. The authors go on to conclude than monthly mean and daily mean polynya areas are comparable on a hemispheric basis, so this point does not seem critical for the main argument.
On line 456, the authors mention that the SIC threshold modelled polynya areas are closer to the satellite areas, this seems to be only really true for the monthly data. On this note, there seems to be quite a large difference between daily and monthly averaged JRA-CESM SIC thresholds in November. Conclusion 5) states that polynya areas are comparable between monthly/daily thresholds on a hemispheric basis, but as mentioned in Line 596, and as is visible in Figure 4c, there is a seasonality to this, and while it is true on an annual-average basis, the CDR daily to monthly cross-correlation drops to ~0.65 in December. In general, Conclusion 5) is important result, and I think the authors should consider modifying the phrasing to be '...data are comparable on an annual averaged and hemispheric basis', or similar.
Further technical corrections:
Line 162: reference should be Mohrmann et al. (2021). Please check spelling of Mohrmann elsewhere in the text.
Line 243: could authors double-check that '≥' sign is correct. Will depend on journal precedent.
Line 293-297: repeated point about CDR vs NASA Team polynya comparison.
Line 454: unmatched bracket at end of line.
Line 547: Figure 8 caption; missing space before comma, should use 'a)' and 'b)' in caption text, formatting of following needs to be consistent "(red; 85% SIC)", "(85% SIC; black)", "(0.4m SIT, blue)".
Line 630: long space.
Line 715: Figure A2 caption '(REF)' left in.
Line 735: Figure B1 caption capitilisation of 'Sea Ice Concentration' inconsistent with other figures e.g. Figure B2.
Line 760: Figure C1 caption has no '(d)'.Many thanks to the authors for their submission, which I feel contains a valuable contribution to the study of Antarctic sea ice, and I hope that the comments above prove useful in finalising this work.
Citation: https://doi.org/10.5194/egusphere-2024-3490-RC2
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