|Second review of " On the drivers of regime shifts in the Antarctic marginal seas, exemplified by the Weddell Sea" 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 authors have considered most of my major concerns in the revised manuscript, which is improved significantly. The storytelling works better, and it is much easier to follow the structure. However, I would still encourage the authors to look into some of the concerns raised in major issues 1 and 2 from the previous review round. My remaining major concerns are, therefore, closely linked to these.
1) There exist many studies on a possible regime shift in the Weddell Sea. Your main result appears to be that the regime shift strongly depends on the density differences between the off-shelf warm water and the on-shelf dense water, similar to several previous studies. Can you make a stronger introduction to show more clearly how this study differs from the others and what we can learn from it?
2) The description of the experiments has improved from the revised manuscript. However, it would be helpful to include explanatory text to the acronyms and the similarities/differences of each experiment. For instance, at the beginning of chapter 2.3.3, you could say something like: Three experiments look into seasonal alterations, denoted by SA in their acronym/name, and two experiments focus on extending the summer season (and making the winter shorter and milder), denoted by SUMMER in their acronym/name. Could you, perhaps, also make hypotheses you hope to answer based on sub-sets of the experiments to show the purpose of each experiment more clearly?
3) The forcing data needs some more explanation. I appreciate the figure in the supporting information, although the caption needs some more explanation (see comment in annotated pdf). I would like to see an explanation of why you added the difference between the seasonal signal in ERA Interrim and HadCM3 21C-A1B. I guess that the regular model forcing already contains the seasonal signal in ERA Interrim and that you subtract this from the HadCM3 21C-A1B to make sure it is not doubling up – i.e., you only add a seasonal variation that represents the increase from ERA Interrim. But this is not described in plain language.
4) In the letter to reviewers, you explain that, in the SUMMER simulations, you are also providing shortwave radiation for summer conditions in this extended summer. This information should be provided clearly in the manuscript. Exaggerating a season response (e.g., wind/temperature) is one thing, but changing solar radiation beyond what is natural is not something I would recommend. I understand if this choice made it easier to design the experiments, but it needs to be mentioned so that the reader understands what the experiment represents - and a discussion of the caveats should be included in the manuscript.
A series of minor issues are included in the annotated manuscript pdf