the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Improving Antarctic Bottom Water precursors in NEMO for climate applications
Katherine Hutchinson
Julie Deshayes
Christian Éthé
Clément Rousset
Casimir Lavergne
Martin Vancoppenolle
Nicolas C. Jourdain
Pierre Mathiot
Abstract. The world’s largest ice shelves are found in the Antarctic Weddell and Ross Seas where complex interactions between the atmosphere, sea ice, ice shelves and ocean transform shelf waters into High Salinity Shelf Water (HSSW) and Ice Shelf Water (ISW), the parent waters of Antarctic Bottom Water (AABW). This process feeds the lower limb of the global overturning circulation as AABW, the world’s densest and deepest water-mass, spreads outwards from Antarctica. None of the coupled climate models contributing to CMIP6 directly simulated ocean-ice shelf interactions, thereby omitting a potentially critical piece of the climate puzzle. As a first step towards better representing these processes in a global ocean model, we run a 1° resolution forced configuration of NEMO (eORCA1) to explicitly simulate circulation beneath Filchner-Ronne (FRIS), Larsen C (LCIS), and Ross (RIS) ice shelves. These locations are thought to supply the majority of the source waters for AABW and so melt in all other cavities is provisionally prescribed. Results show that the grid resolution of 1° is sufficient to produce melt rate patterns and net melt rates of FRIS (117 ± 21 Gt/yr), LCIS (36 + 7 Gt/yr) and RIS (112 + 22 Gt/yr) that agree well with both high resolution models and satellite measurements. Most notably, allowing sub-ice shelf circulation reduces salinity biases (0.1 psu), produces the previously unresolved water mass ISW, and re-organises the shelf circulation to bring the regional model hydrography closer to observations. A change in AABW within the Weddell and Ross Seas towards colder, fresher values is identified but the magnitude is limited by the absence of a realistic overflow. This study presents a NEMO configuration that can be used for climate applications with improved realism of the Antarctic continental shelf circulation and a better representation of the precursors of AABW.
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Katherine Hutchinson et al.
Status: final response (author comments only)
- RC1: 'Review of egusphere-2023-99', Xylar Asay-Davis, 07 Mar 2023
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RC2: 'Reviewer comment on egusphere-2023-99', Anonymous Referee #2, 11 Mar 2023
The study “Improving Antarctic Bottom Water precursors in NEMO for climate applications” by Katherine Hutchinson and colleagues is an exciting demonstration on feasibility and importance of including ice shelf–ocean interaction in coarse resolution (1˚), global ocean and climate models. Over the past two decades a growing body of literature has documented the processes of ice melt in ice shelf cavities around Antarctica, their implications on water mass transformation and modelling approaches thereof. It is an ongoing debate, how this is best implemented in coarse resolution models used for multi-centennial climate simulations. Hutchinson et al. show that even on a 1˚ ocean model grid one can include and simulate the three largest Antarctic ice-shelf cavities interactively using complex physic.
The manuscript is well written and composed, the arguments supported by mostly clear and matching figure. It would be desirable though if the authors could provide a little more background in the introduction on modelling approaches for ice shelf melt and ice cavity circulation, reference early studies such as Hellmer (2004, doi:10.1029/2004GL019506) and other approaches like the PICO model (Reese et al., 2018, doi:10.5194/tc-12-1969-2018), just to name a couple of examples. By the way, Hellmer 2004 is listed as reference but not cited in the main text. Introduction and discussion are sufficient to introduce the subject and to validate the results but do not really reach beyond. The authors make a great effort to properly validate their simulations, clearly state weaknesses of their model (and its strengths) but also where observations are sparse and thus differences may not necessarily mean a model bias.
The authors provide bathymetry and initial conditions used for the ice shelf cavities along with some simulation output, which is very much appreciated. Information missing but likely of interest to the community would be the additional computational effort caused by including, “opening” the cavities.
While the manuscript overall is strong and certainly merits publication with GMD being a very suitable journal, I see three weak spots that could be revised prior to publication:
1) Arguments are typically well made and supported throughout the text. Only towards the end of subsection 4.4 and in subsection 4.5 the text makes the impression that the authors want to address the given topics but the text is rather short, referring to several supplementary figures (e.g., lines 494-501) without really being able to pinpoint the interaction with open ocean deep convection and sea ice and the role of the ASC.
A particular issue is that in the “Open” case one can see an offshore displacement of the deep convection region (Fig. S2e) but not really an overall weakening/shoaling, which seems to contradict studies simulating ice-shelf melt impacts by freshwater release (Beadling et al., 2022, doi:10.1029/2021JC017608; Bronselaer et al., 2018, doi:10.1038/s41586-018-0712-z). The authors show that the ASC is weak in their simulation, which would rather facilitate a freshening of the interior Weddell Gyre and hence decrease MLD. Please discuss.
2) Section 5, the discussion mostly reads like a summary and I suggest to actually rename the section accordingly (“Summary and Discussion”). This is okay as the model results are intensively discussed and validated already in Sections 3 and 4, which reach beyond the typical presentation of results. In addition, results of other modelling approaches could be discussed, like with the UKESM (Smith et al., 2021, already cited) or the PICO model mentioned above. This would be particularly helpful as the authors use a forced ocean model but understand their study as a contribution to improving climate models as they state in the introduction.
3) The sea ice production and polynya activity analysis in the supplementary material goes to some length but without clear results. There is no significant difference in sea ice production due to opening the cavities (table S1) except the discussed spatial displacements. The latter are rather simple responses to changes in surface water mass properties. The authors should be careful not to contradict arguments made in the main text and overinterpret the role of sea ice. This seems to be the case in the summary section S1.6, where in particular references to figures of bottom(!) salinity (Figs. 2i, 3i) are more confusing than supportive.
The list of references needs to be properly checked for consistency.
While extensive, all comments are rather minor and should be addressible without requiring an additional round of review.
Minor comments by line:Line 1, the title: I wish “by opening ice shelf cavities” would somehow be included. The formulation “AABW precursors” appears kind of indirect and less appealing to me -- though less technical, I admit.
l.33: add references like Fröhlicher et al. (2015, doi:10.1175/JCLI-D-14-00117.1) and maybe Bourgeois et al. (2022, doi:10.1038/s41467-022-27979-5) and
l.35-38: introductions of both HSSW and ISW could use a reference each where the interested reader could find more detail on formation processes
l.175-177 details of sea ice advection scheme etc. is provided here. Please provide similar information on ocean model in prev. paragraph. Focus is the ocean model.
l.187, 199, etc: term “cold-core ice shelves” or “cold ice shelves” needs introduction or at least a reference; maybe in addition refer to “dense shelf” as defined in Thompson et al. (2018, doi:10.1029/2018RG000624)
Section 2.4 The entire approach of attaining the initial conditions is wonderful, very thoughtful and a great example. I would be curious though, how much the model state still drifts in the ice shelf cavities after starting the main 100+ year long run. Could be added to the supplementary.
l.241 and 251: Figure S3 is addressed before S2. Switch figures in supplementary.
l.303: add “ice shelf” in “The average ice shelf melt rate pattern …”
l.313: careful with the resolution statement. I assume the earlier studies use NEMO3.x and potentially also different settings complicating a direct comparison.
Table 1: this is an awesome overview, excellent!
l.325f add information on figure number in Rignot et al and Haussmann et al.
l.348 & 370: drop subsection header; subsections consist of 1 paragraph each only.
l.362 remind the reader here that eORCA1, i.e. 1 degree, in fact means 22km actual resolution
l.403f & 416f: please provide an average difference estimate for “fresher, slightly cooler” and “cooler fresher values”
l.437: MCDW, abbreviation is not introduced
l.498: Figures 8 and 9 are only properly introduced in line 525; referring to their panels (d) is more confusing than helpful here
l.528, “little indication of a coherent cascading”: cascading is a rapid process, is it possible that the time mean over 10 years masks such process?
l.551: please also reference Colombo et al. (2020, doi:10.5194/gmd-13-3347-2020) on overlow in z-coordinates in NEMO
SUPPLEMENTARY TEXTl.929: “An evaluation of sea ice production …” (not to confuse ice shelves and sea ice)
l.933ff: rewrite: “Sea ice growth, melt and transport exert an influence on water mass properties. Polynyas are large areas of open water or very thin ice forced by local winds or heat from the ocean. Near the Ronne and Ross ice shelf margins, large polynyas source dense saline waters to the surface ocean during the cold season.”
l.948f: rewrite: “… concentration outputs, of which the 2003-2009 average in the is displayed in Figs. S3b and S4b for the “Closed” simulation.”
l.1018 & 1020: you refer to temperature and salinity, which are shown in panels (h) & (i) in Figures 2 and 3 (not only panel (i)).
FIGURESFigure 1: switch panels (a) and (b) as Weddell Sea (currently panel b) is addressed first in the text. Also, please increase line thickness of the circulation arrows for visibility.
Figure 2 (and all other contour/pcolor plots): use discrete colors and fewer color levels (max. 20 or even only 10) often help to improve readability of the graphic.
Further, please increase line thickness of the gray dashed line of freezing point (panels a,d,g).
Caption: add “of WOA and “Closed” after “…bottom temperature and salinity” in line 272. And add last “Panels (a) and (g) exclude ice shelf cavity data matching the “Closed” configuration of panel (d).” (as stated later in line 388)Figure 4: panels should have some location (lat/lon) or distance labelling along frame.
Figure 5a: velocity arrows are way too small, reduce number and increase length and thickness; also red arrows on green shading is not color-blind friendly, black arrows would do
Figure S1, caption: mention that scatter dots are placed in T-S space according to “Closed” and coloring shows “Open”-“Closed”; also this should be Figure 3 according to the sequence in which supplementary figures are addressed in the main text
Citation: https://doi.org/10.5194/egusphere-2023-99-RC2
Katherine Hutchinson et al.
Katherine Hutchinson et al.
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