On the drivers of regime shifts in the Antarctic marginal seas
Abstract. Recent studies found evidence for a potential future tipping point when the density of Antarctic continental shelf waters, specifically in the southern Weddell Sea, allows the onshore flow of warm waters of open ocean origin. A cold-to-warm shift in the adjacent ice shelf cavities entails a multiplication of ice shelf basal melt rates and can possibly trigger instabilities in the ice sheet. From a suite of numerical experiments, aimed to force such a regime shift on the continental shelf, we identified the density balance between the shelf waters formed by sea ice production and the warmer water at the shelf break as the deciding element for a tipping into a warm state. In our experiments, this process is reversible but with evidence for hysteresis behaviour. Using HadCM3 20th-century output as atmospheric forcing, the resulting state of the Filchner-Ronne cavity depends on the initial state. In contrast, ERA Interim forcing pushes even a warm initialisation into a cold state, i.e., the system back to the cold side of the reversal threshold. However, it turns out that for forcing data perturbations of a realistic magnitude, a unique and universal recipe for triggering a regime shift in Antarctic marginal seas does not exist. Whether or not any given forcing or perturbation yields a density imbalance and thus allows for the inflow of warm water depends on the interplay between bottom topography, mean ocean state, sea ice processes, and atmospheric conditions.
Verena Haid et al.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2022-1044', Anonymous Referee #1, 06 Dec 2022
AC1: 'Reply on RC1', Verena Haid, 16 Feb 2023
- EC2: 'Reply on AC1', Karen J. Heywood, 22 Feb 2023
- AC1: 'Reply on RC1', Verena Haid, 16 Feb 2023
RC2: 'Comment on egusphere-2022-1044', Anonymous Referee #2, 07 Dec 2022
- AC2: 'Reply on RC2', Verena Haid, 16 Feb 2023
- EC1: 'Comment on egusphere-2022-1044', Karen J. Heywood, 23 Jan 2023
Verena Haid et al.
Verena Haid et al.
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Review of "On the drivers of regime shifts in the Antarctic marginal seas" by Verena Haid, Ralph Timmermann, Özgür Gürses, and Hartmut H. Hellmer
The manuscript presents a set of model experiments used to explore a potential regime shift in the Antarctic marginal seas, with a strong focus on the Weddell Sea. The authors use various modifications of atmospheric forcing data to study the development of the HSSW and WDW density and find that the density difference between these water masses is a deciding element for shifting into a warm regime. The results agree with other model studies using various forcing combinations, but in this study, the regime shift is reversible, provided that the forcing returns to present-day conditions.
The manuscript builds on a series of model experiments that contribute to understanding a possible regime shift in the Weddell Sea. However, it is quite hard to follow the story as a reader, and I would recommend the authors revise the manuscript, considering the following major issues.
In addition, here are som minor recommendations to pay attention to:
Figure 1: Is there a reference you can add to the caption? Is this adapted from other work?
Figure 3: It is very hard to decipher the different lines. It would be helpful with varying colors and line styles to make it possible for colorblind people to interpret the results and bigger panels or different vertical ranges to see the results better. Also, is there a reason you would analyze the Ross data in the SA_W scenarios? It would be good to explain why.
Figure 4: What is the purpose of this figure? The datasets are very similar. If you want to show these figures, it would be helpful with a third set of panels showing the difference between the datasets to enhance the understanding of the differences.
Figure 5: I suggest plotting REF as it is, and plotting the others as anomalies (subtracting REF). This would make it ieasier to compare differences. The divergent colormap also makes temperature variation near 1.5 degC appear stronger than they are. Is there a reason why you split the warm and cold colors at 1.5 deg C? If not, I would suggest to use a non-diverging color scheme.
Figure 7: I do not think you need to include this figure.
Paragraph 45: I am unsure if it is appropriate to flag specific projects and funding sources here. I would suggest putting this information in the acknowledgment and changing the sentence to target your contribution in general.
Paragraph 75: Is there a reference for the HadCM3 model data and the empirical relationships you are using?
Paragraph 95: It would be helpful to present a graphic that shows the REF and the SA_G, and possibly the seasonal means for the datasets for at least one of the atmospheric variables.
Paragraph 105: How does it affect the forcing data when July and August are eliminated? Are there discontinuities that affect the results? What happens in Feb-August in the transition zone at 50 deg S?
Paragraph 125: Is the mean temperature an integral over the entire volume or only the deeper part (how deep)? The figure caption states that it is below 200 m, but you should mention this here too.
Paragraph 125: The sentence starting with “The coincidence of…” needs to be revised for clarity. It would also benefit from more elaboration as to the causality question.
Paragraph 125: All other references to figures use “Fig. X”. I suggest you do this for Figure 5 here too.
Paragraph 135: Please refer to a figure where you find the results.
Paragraph 145: Some of the points here would be good to mention in the introduction to help motivate your selection of experiments.
Paragraph 175: How is the ACC altered in the SUMMER_S? Is warm water available at the depths where it can access the shelf, and do you see a relaxation of the front?
Paragraph 180: Could you provide some motivation for why you compare the density in front of Ronne, and not the density in the Filchner Trough, where the warm water enters the shelf?
Paragraph 195: Please elaborate on what happens when the on-shelf density becomes lighter than the off-shelf waters. Is there any delay in response or any seasonal variation of interest?
Paragraph 325: This paragraph appears to be more appropriate for the discussion section
Paragraph 330: The final paragraph is irrelevant to the conclusion. I am unsure if it ok to advertise other parts of a specific project in this manner.