The Southern Ocean Freshwater release model experiments Initiative (SOFIA): Scientific objectives and experimental design
Abstract. As the climate warms, the grounded ice sheet and floating ice shelves surrounding Antarctica are losing mass at an increasing rate and injecting the resulting meltwater into the Southern Ocean. This freshwater input could feed back onto climate change, particularly since the Southern Ocean is a key contributor to global heat and carbon uptake. Nonetheless, almost all existing coupled climate models have fixed ice sheets, and lack the physics required to represent the dominant sources of Antarctic melt. These missing ice dynamics represent a key uncertainty that is unaccounted for in current global climate change projections. Previous studies have inserted additional Antarctic meltwater into models, demonstrating that it can alter Southern Ocean stratification, circulation, and sea ice, as well as influence remote atmospheric circulation, tropical precipitation, and global temperature. However, these previous studies have used widely varying rates of freshwater forcing, been conducted using different climate models and configurations, and have reached differing conclusions on the magnitude of meltwater-climate feedbacks. The Southern Ocean Freshwater release model experiments InitiAtive (SOFIA) brings together a team of scientists to quantify the climate system response to Antarctic meltwater input. In this paper, we summarize the state of knowledge on meltwater discharge from the Antarctic ice sheet and ice shelves to the Southern Ocean and explain the scientific objectives of our initiative. We propose a series of coupled and ocean/sea-ice model experiments, including idealized meltwater experiments, historical experiments with observationally consistent meltwater input, and future scenarios driven by meltwater inputs derived from stand-alone ice sheet models. Through coordinating a multi-model ensemble of simulations, with data housed in a common archive, SOFIA will be able to produce a consistent estimate of the climate system response to Antarctic meltwater, as well as the uncertainty of this response.
Neil Swart et al.
Status: final response (author comments only)
- CC1: 'Comment on egusphere-2023-198', Katherine Turner, 24 Mar 2023
- RC1: 'Comment on egusphere-2023-198', Anonymous Referee #1, 05 Apr 2023
- RC2: 'Comment on egusphere-2023-198', Nicholas Golledge, 19 Apr 2023
- CC2: 'Comment on egusphere-2023-198', Karen J. Heywood, 28 Apr 2023
- CC3: 'Comment on egusphere-2023-198', Paul Holland, 10 May 2023
Neil Swart et al.
Neil Swart et al.
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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:
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.
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.