Review of The Southern Ocean Freshwater release model experiments Initiative (SOFIA): Scientific objectives and experimental design by Swart et al.

The Southern Ocean Freshwater release model experiments InitiAtive (SOFIA) provides a set of climate model experimental protocols for quantifying the response to Antarctic meltwater. The protocols are general, which allows them to be applied to both high- and intermediate-complexity climate models.

The manuscript reviews the theory behind ice mass budgets, current representation of meltwater forcing in climate models, and historical trends and projections for ice sheet mass. Additionally, the manuscript covers how uncertainties in model architecture, internal variability, and the location of meltwater addition may impact the climate response. A thorough description of the experimental setup is located in the appendix.

The manuscript presents an interesting set of experiments to explore the role of freshwater forcing in the Southern Ocean, which is both highly uncertain and poorly represented in current climate model setups. The experimental setup includes three broad experiment types within tiers 1 and 2:

• (tier 1) Pre-industrial climate conditions forced with constant freshwater forcing,
• (tier 2) Historical climate conditions forced with linear freshwater forcing near the end of the historical runs, and
• (tier 2) Future projections with transient freshwater forcing calculated from ice models forced with RCP climate forcing (although I am unsure whether the ice models are forced with surface temperature changes or radiative forcing from GHGs + land use + aerosols)

There is a third tier of experiments that tests the sensitivity to the horizontal distribution of meltwater addition and to the heat fluxes involved in the phase change.

As I understand it, this paper aims to 1. provide experimental context for future papers which are already in the pipeline, and 2. advertise the SOFIA experiments to other climate modeling groups and encourage them to provide additional runs.  Regarding aim 1, I think the paper is successful. Appendix A provides useful suggestions for creating ensembles, detailed descriptions for how forcings were calculated (particularly for the future projections), and a discussion of how ocean-ice models can contribute if SSS restoring is included. The standardization of experiments through SOFIA will help increase the transparency of future model-based studies and bring new, much-needed knowledge on Southern Ocean dynamics.

Regarding aim 2, I think the manuscript would benefit from some restructuring before submission. Signposted argumentation across the introduction, review, and ending discussion would be helpful in guiding the reader along the manuscript. I had difficulties understanding exactly how specific experimental setups would be used to reduce the various uncertainties and goals described in Sections 2 and 3. As a non-expert, I think the manuscript would benefit from a (sub)section in which the experiment aims are explicitly stated, rather than requiring the reader to infer some of these aims. I also think that connections to other model intercomparison projects (e.g., FAFMIP) should be emphasized, as these connections could attract modeling groups that already participate in these other projects.

Ultimately, I think the manuscript is a useful and timely work that merits submission. My suggestions below are to help readers from other fields within climate modeling appreciate the need to understand meltwater impacts and, in the long term, work towards creating coupled climate models that include interactive land and sea ice.

Signposting suggestions:

As it stands, each main section jumps directly into a subsection. A few sentences at the beginning of each section could provide extra motivation and emphasize the connections between the review material and the proposed experiments.

While the experimental descriptions are in the appendix, it would be useful to the reader to have a brief outline of the experiments and overarching themes of the project in the introduction. Perhaps in the introduction, the aims/motivating questions can even be listed with bullet points. That way, the reader can reference either the introduction or the appendix as they read the rest of the manuscript, depending on their level of involvement.

References to the SOFIA experiments are sparse and general (see line 128 for an example), which hinders readability. I recommend that the references to the experiments be more specific if the authors decide against adding an “experimental setup” section to the main text.

It would be nice to see more discussion, both within the main text and in the descriptions of the experiments in Appendix A, about why these specific experiments were chosen. For instance, regarding the preindustrial antwater experiment, how will the results be used in coordination with the other experiments (if at all), considering the differences in magnitudes? Will the authors test the linearity of the response using the idealized historical experiments? What are the advantages of using SSP-style forcings for future projections over the 1% CO₂ experiments commonly used in other model intercomparison projects such as FAFMIP?

I would be interested to know if there are additional benefits to these runs outside quantifying uncertainty from the Southern Ocean. For instance, is there the potential to use some of these results to improve global or regional projections of sea level rise? The connection to biogeochemistry is mentioned briefly, but I would also be interested to read more about any (even speculative) impacts.

Minor points:

•  Some sentences are difficult to read and have slight grammatical mistakes – for instance lines 19-21 has a misplaced modifier, and the clauses in 156-157 do not share the same subject. Overall, the manuscript has quite complicated sentence structures where the meaning can be unclear (e.g., line 163-164). I would recommend the longer sentences throughout the manuscript be broken up to improve clarity.
• Line 45: is there a contrasting study to Beadling et al., 2022 missing?
• Lines 97-98: What are the estimates from the Shepard et al. and Green et al. papers?
• Line 195: Behrens et al., 2015 (doi: 10.1002/2015JC011286) investigates this with the CMIP5 models – perhaps good to include
• Lines 288-289: A reference to FAFMIP in the main paper would also be a useful comparison as it is an established model intercomparison that includes freshwater forcing.

This is a fantastic initiative and a most interesting paper.   I wondered if you were aware of the paper by Richardson et al. In 2005, which was, I believe, the first to undertake an experiment using a coupled climate model (HadCM3) to investigate the impact of a meltwater addition around Antarctica, demonstrating an increase in Antarctic sea ice and subsurface ocean warming.

Short-term climate response to a freshwater pulse in the Southern Ocean

Glen Richardson, Martin R. Wadley, Karen J. Heywood, David P. Stevens, Helene T. Banks

Hi Folks

Many congratulations on this initiative, which is very timely and will be a very important addition to this field.  The range of experiments out there at present does complicate matters, and SOFIA should clear that up very nicely. I have a few comments on the experimental design.  Apologies for the unsolicited review! 

Clearly the over-riding goal of SOFIA is to provide a consistent set of experiments that are practically achievable to the modelling centres, and that will necessitate some compromise on the physics.  I am fully sympathetic to that. Therefore I am really expressing the below in order to raise a few possible caveats to the results that emerge, and to prompt future discussion, rather than to suggest any change in experimental design.

• The climate models will still have their ‘runoff’ (as shown in Figure 1b) from Antarctica during these experiments, but that will provide an uncontrolled additional Antarctic freshwater source to the ocean that may complicate the intercomparison of models. For example, there is a large increase in precipitation onto Antarctica during SSP5, which varies between models, and that will flow straight into the ocean. This precipitation increase can be the same size as changes in ice discharge during the 21^st century so I think it is important (e.g. Seroussi et al 2020). The best solution could be to turn off runoff during these experiments and just implicitly accumulate the mass on Antarctica.  Next best could be requesting further control experiments where needed (e.g. SSP585 without added FW) so that the effect of adding FW can be isolated from the model runoff changes. Simplest would be just analysing the runoff alongside the other results.
• It is a very uncertain choice taking the future parameterised melt from ISMIP6 models and multiplying by ~2 to account for calving. Calving could not change at all (larger, thinner ice shelves are implicit in the ISMIP6 melt rates) or could increase massively (ice shelves collapse as the grounding line retreats). Even if calving did increase, melting should be reduced accordingly, so it is not clear whether x2 is needed. One solution could be to take the change in ice sheet discharge across the grounding line from the ISMIP6 models, because that circumvents this point.
• Inputting freshwater into the surface, uniformly around Antarctica, with no latent heat extraction, produces a clear, tractable experiment but it is an unrealistic choice. In reality almost all of the additional mass flux has appeared in the Amundsen Sea, at depth, with a latent heat extraction. (Admittedly some ice shelf collapses have occurred on the peninsula - though those bergs were quickly exported - and the Ross and FRIS cavities could melt by 2100 under SSP5.) As with all of my points, I am not concerned about the model experimental design, just its interpretation. For me the stated design creates three issues:  1) Hosing the ocean with freshwater at the surface and no strong cooling produces a strong stabilising effect, which is not representative of real glacial melting. I assume this will cause excessive shutdown of dense water formation in ‘cold’ shelf regions, and excessive warm-water feedbacks in ‘warm’ shelf regions; 2) The uniform near-coastal distribution hoses the ocean directly in the ‘cold’ regions that form dense AABW, unlike in reality, thus over-stating the potential effects on AABW; 3) the 60S distribution hoses the ocean beneath the sea ice, over-stating the potential effects on sea ice and Southern Ocean SSTs. Issue 1 is compounded in issues 2 and 3, of course. So I think the current experiments are all extreme in over-stating the role of FW.  For me the best solution would be to add the opposite extreme experiment with the all FW added in the Amundsen Sea only, distributed over depth, and with corresponding latent heat removed.

As I said above, all of these are just thoughts for discussion. SOFIA is fine as-is, and the authors should feel free to ignore these comments or take whatever action they feel sensible. I hope these thoughts help more than they hinder.  Best of luck with SOFIA.