the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Sep 2023
A comprehensive Earth System Model (AWI-ESM2.1) with interactive icebergs: Effects on surface and deep ocean characteristics
Abstract. The explicit representation of cryospheric components in Earth System models has become more and more important over the last years. However, there are only few advanced coupled Earth System models that employ interactive icebergs, and most iceberg model studies focus on iceberg trajectories or ocean surface conditions.
Here, we present multi-centennial simulations with a fully coupled Earth System model including interactive icebergs to assess the effects of heat and freshwater fluxes by iceberg melting on deep ocean characteristics. The icebergs are modeled as Lagrangian point particles and exchange heat and freshwater fluxes with the ocean. They are seeded in the Southern Ocean, following a realistic present-day size distribution. Total calving fluxes and the locations of discharge are derived from an ice sheet model output which allows for implementation in coupled climate-ice sheet models.
The simulations show a cooling of deep ocean water masses in all ocean basins that propagates from the southern high latitudes northward. We also find enhanced deep water formation in the continental shelf area of the Ross Sea, a process commonly underestimated by current climate models. The vertical stratification is weakened by enhanced sea-ice formation and duration due to the cooling effect of iceberg melting. The deep water formation in this region is increased by up to 10 %. By assessing the effects of heat and freshwater fluxes individually, we find latent heat flux to be the main driver of these water mass changes. The altered freshwater distribution by freshwater fluxes and synergetic effects play only a minor role. Our results emphasize the importance of realistically representing both heat and freshwater fluxes in the high southern latitudes.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(4889 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-2061', Anonymous Referee #1, 10 Oct 2023
Please see the review of Ackermann et al. (2023) in the attached pdf.
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AC2: 'Reply on RC1', Lars Ackermann, 20 Nov 2023
We would like to thank you for the careful reading and the timely and very constructive feedback.
Indeed, ice shelf basal melting and iceberg discharge are not considered explicitly in the default AWI-ESM version. All freshwater entering the ocean is considered surface runoff and treated by the hydrological scheme of the land-surface component. This implies that no latent heat fluxes due to iceberg melt are considered in the default model version. Hence, the two model versions, with and without interactive icebergs, are not identical with respect to the heat fluxes. While the individual patterns of freshwater release are different in both model approaches, the sum of the freshwater fluxes is identical, as the surface runoff from Antarctica (that implicitly includes basal shelf melt fluxes and iceberg calving) is reduced by the amount of freshwater input due to calving in the model version with interactive icebergs. The model icebergs melt, eventually, and release this fresh water into the ocean. We will point this out in more detail in the revised manuscript.
With regard to the realism of the simulation, we will include a validation of sea ice extent as well as temperature and salinity fields from our simulations in the revised manuscript. Certain model biases will be addressed, and the effect of interactive icebergs on these biases will be pointed out. Furthermore, as suggested, some climatic indices will be included for all performed simulations. Thank you for pointing this out.
We appreciate the helpful comments and will address all remarks in the revised manuscript.Citation: https://doi.org/10.5194/egusphere-2023-2061-AC2
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AC2: 'Reply on RC1', Lars Ackermann, 20 Nov 2023
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CEC1: 'Comment on egusphere-2023-2061', Juan Antonio Añel, 11 Oct 2023
Dear authors,Unfortunately, after checking your manuscript, it has come to our attention that it does not comply with our "Code and Data Policy".https://www.geoscientific-model-development.net/policies/code_and_data_policy.htmlYou have archived your code and software on GitHub. However, GitHub is not a suitable repository for scientific publication. GitHub itself instructs authors to use other alternatives for long-term archival and publishing, such as Zenodo. Therefore, please, publish your code in one of the appropriate repositories, and reply to this comment with the relevant information (link and DOI) as soon as possible, as it should be available before the Discussions stage.Please, note that if you do not fix this problem, we will have to reject your manuscript for publication in our journal. I should note that, actually, your manuscript should not have been accepted in Discussions due to this lack of compliance with our policy. Therefore, the current situation with your manuscript is irregular.Also, you must include in a potentially reviewed version of your manuscript the modified 'Code and Data Availability' section, the DOI of the code.Juan A. AñelGeosci. Model Dev. Executive EditorCitation: https://doi.org/
10.5194/egusphere-2023-2061-CEC1 -
AC1: 'Reply on CEC1', Lars Ackermann, 19 Oct 2023
Dear Juan A. Añel,
Thank you for your comment and sorry for any inconvenience. All codes and data are now archived on Zenodo and the 'Code and Data Availability' section of the reviewed manuscript will be modified accordingly.
The ocean model FESOM2 source code is available on Zenodo at https://doi.org/10.5281/zenodo.10000291. The atmosphere model ECHAM6 is a property of the Max Planck Institute for Meteorology. Its model code is available at https://code.mpimet.mpg.de/login?back_url=https%3A%2F%2Fcode.mpimet.mpg.de%2Fprojects%2Fmpi-esm-users%2Ffiles after registration at this site https://code.mpimet.mpg.de/projects/mpi-esm-license. The relevant changes that were used in this study are available on Zenodo at https://doi.org/10.5281/zenodo.10009096. The esm_tools version used in this study is available at https://doi.org/10.5281/zenodo.10018102.
All model output required to reproduce the figures can be found at https://doi.org/10.5281/zenodo.10017868, https://doi.org/10.5281/zenodo.10017840, https://doi.org/10.5281/zenodo.10018019, https://doi.org/10.5281/zenodo.10018131, and https://doi.org/10.5281/zenodo.10018425. Scripts and input files required to recreate the simulations, as well as scripts for plotting, are available at https://doi.org/10.5281/zenodo.10012787.
Kind regards,
Lars AckermannCitation: https://doi.org/10.5194/egusphere-2023-2061-AC1
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AC1: 'Reply on CEC1', Lars Ackermann, 19 Oct 2023
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CC1: 'Comment on egusphere-2023-2061', R. Marsh, 17 Oct 2023
Publisher’s note: the content of this comment was removed on 19 October 2023 since the comment was posted by mistake.
Citation: https://doi.org/10.5194/egusphere-2023-2061-CC1 -
RC2: 'Comment on egusphere-2023-2061', R. Marsh, 19 Oct 2023
General Comments
The authors present a comprehensive evaluation of interactive icebergs in a set of experiments with the AWI-ESM2.1 Earth System Model, demonstrating an advance in modelling capability that is within the scope of GMD. Multi-centennial experiments are necessary to establish the impacts of icebergs on deep and bottom waters that circulate on these long timescales. To my knowledge, these are the longest ESM experiments undertaken with interactive icebergs, while maintaining suitably high resolution around Antarctica. The separate impacts of iceberg-altered freshwater and heat fluxes are central to understanding the overall impacts of icebergs on the ocean, the latter impact (cooling) being regrettably a late admission to iceberg-ocean coupling in NEMO-ICB. Prospects for coupling with an interactive ice sheet are further promising. Overall, this study provides an advance on previous studies that reported the first implementations and subsequent developments of interactive iceberg modelling, several published in GMD. Methods are clearly outlined, with scope for minor clarification (see specific comments below), and results are sufficient to support conclusions. Following Code Availability, all results should be reproducible with sufficient computing resource. The title clearly reflects the contents of the paper, identifying the ESM by name.
A minor issue is the focus on Antarctica, with Greenland briefly mentioned in the Introduction. For a more complete representation, interactive icebergs can presumably also be seeded around Greenland (and other smaller northern ice caps). I appreciate that Antarctica is the dominant source of icebergs at global scale, but dense water formation in the subpolar North Atlantic may be particularly sensitive to the same processes (perturbing T and S) that are identified around Antarctica. It would be useful for the authors to comment further in Introduction and Discussion on the global impact of icebergs, in particular the net results of perturbations to deep and bottom water formation in both hemispheres. The Abstract provides a clear and concise summary of findings, although more quantitative detail could be provided (see specific comments). Overall presentation is well-structured throughout, and the manuscript is well written, with appropriate level of Supplementary Material.
In summary, the authors provide a compelling case for a relatively straightforward model improvement, of consequence for climate simulation and projection, with the intriguing possibility that neglected appreciation of iceberg influences could explain recent observed variability around. Antarctica (lines 266-269). As a development and technical paper, the manuscript should be suitable for publication in GMD, subject to minor revisions as suggested above and specified below.
Specific Comments
- Abstract: Provide additional quantitative support for statements, e.g., how much cooling, extent of (%) weakening in stratification.
- Introduction, from line 25: Given the focus here on a new model capability in AWI-ESM2.1, and earlier comment in the Abstract that ‘only few advanced coupled Earth System models that employ interactive icebergs’, it would be appropriate to more comprehensively summarize these models, perhaps tabulating resolutions tested, details of coupling (with/without heat flux coupling), seeding scheme, duration of simulation, reference/s. This would clearly establish the novelty and leading position this the present model development and study.
- Introduction, from line 49. Provide more justification for focus on Antarctica and the Southern Ocean in a global model, given the potential importance of Greenland icebergs for dense water formation and properties in the North Atlantic.
- 5, line 120: clarify what you mean by ‘continuous’, through time, or over space?
- 5, lines 122-123, and in Fig. 1 caption: I do not quite follow ‘integration of the calving flux over each basin to get total amount of ice discharge’, and Fig. 1a; can this be further explained?
- 11, Fig. 7: There is considerable multidecadal-to-centennial variability in ocean temperature and AABW indices; while this is not a focus of the study, it is perhaps worthy of more comment and even some explanation.
- 12, line 208: correct as ‘shelf-region’
- 13, line 219: do you mean ‘pushed onshore due to Ekman dynamics’? (geostrophy sets up long-shore drift
- 14, lines 250-251: Remarking that ‘realistic representation of AABW formation along continental shelves is not feasible in out model setup’, can this be further explained; it is a consequence of missing ice shelf cavities, or limited resolution, or both?
- 15, line 265: In discussing the delay of Southern Ocean greenhouse warming in climate projections due to cooling and freshening, do the authors imply the consequences of increasing iceberg calving? Perhaps be more explicit here.
Citation: https://doi.org/10.5194/egusphere-2023-2061-RC2 -
AC3: 'Reply on RC2', Lars Ackermann, 20 Nov 2023
We would like to thank you for the interest in ourr study and the timely and very constructive comments. While the main focus of this study is an enhanced process understanding of the long-term effects of interactive icebergs, we acknowledge the importance of Northern Hemisphere icebergs on deep-water formation in the North Atlantic and, ultimately, on the AMOC and the climate state. We will point this out in more detail in the revised manuscript and will address all further remarks.
Citation: https://doi.org/10.5194/egusphere-2023-2061-AC3
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2061', Anonymous Referee #1, 10 Oct 2023
Please see the review of Ackermann et al. (2023) in the attached pdf.
-
AC2: 'Reply on RC1', Lars Ackermann, 20 Nov 2023
We would like to thank you for the careful reading and the timely and very constructive feedback.
Indeed, ice shelf basal melting and iceberg discharge are not considered explicitly in the default AWI-ESM version. All freshwater entering the ocean is considered surface runoff and treated by the hydrological scheme of the land-surface component. This implies that no latent heat fluxes due to iceberg melt are considered in the default model version. Hence, the two model versions, with and without interactive icebergs, are not identical with respect to the heat fluxes. While the individual patterns of freshwater release are different in both model approaches, the sum of the freshwater fluxes is identical, as the surface runoff from Antarctica (that implicitly includes basal shelf melt fluxes and iceberg calving) is reduced by the amount of freshwater input due to calving in the model version with interactive icebergs. The model icebergs melt, eventually, and release this fresh water into the ocean. We will point this out in more detail in the revised manuscript.
With regard to the realism of the simulation, we will include a validation of sea ice extent as well as temperature and salinity fields from our simulations in the revised manuscript. Certain model biases will be addressed, and the effect of interactive icebergs on these biases will be pointed out. Furthermore, as suggested, some climatic indices will be included for all performed simulations. Thank you for pointing this out.
We appreciate the helpful comments and will address all remarks in the revised manuscript.Citation: https://doi.org/10.5194/egusphere-2023-2061-AC2
-
AC2: 'Reply on RC1', Lars Ackermann, 20 Nov 2023
-
CEC1: 'Comment on egusphere-2023-2061', Juan Antonio Añel, 11 Oct 2023
Dear authors,Unfortunately, after checking your manuscript, it has come to our attention that it does not comply with our "Code and Data Policy".https://www.geoscientific-model-development.net/policies/code_and_data_policy.htmlYou have archived your code and software on GitHub. However, GitHub is not a suitable repository for scientific publication. GitHub itself instructs authors to use other alternatives for long-term archival and publishing, such as Zenodo. Therefore, please, publish your code in one of the appropriate repositories, and reply to this comment with the relevant information (link and DOI) as soon as possible, as it should be available before the Discussions stage.Please, note that if you do not fix this problem, we will have to reject your manuscript for publication in our journal. I should note that, actually, your manuscript should not have been accepted in Discussions due to this lack of compliance with our policy. Therefore, the current situation with your manuscript is irregular.Also, you must include in a potentially reviewed version of your manuscript the modified 'Code and Data Availability' section, the DOI of the code.Juan A. AñelGeosci. Model Dev. Executive EditorCitation: https://doi.org/
10.5194/egusphere-2023-2061-CEC1 -
AC1: 'Reply on CEC1', Lars Ackermann, 19 Oct 2023
Dear Juan A. Añel,
Thank you for your comment and sorry for any inconvenience. All codes and data are now archived on Zenodo and the 'Code and Data Availability' section of the reviewed manuscript will be modified accordingly.
The ocean model FESOM2 source code is available on Zenodo at https://doi.org/10.5281/zenodo.10000291. The atmosphere model ECHAM6 is a property of the Max Planck Institute for Meteorology. Its model code is available at https://code.mpimet.mpg.de/login?back_url=https%3A%2F%2Fcode.mpimet.mpg.de%2Fprojects%2Fmpi-esm-users%2Ffiles after registration at this site https://code.mpimet.mpg.de/projects/mpi-esm-license. The relevant changes that were used in this study are available on Zenodo at https://doi.org/10.5281/zenodo.10009096. The esm_tools version used in this study is available at https://doi.org/10.5281/zenodo.10018102.
All model output required to reproduce the figures can be found at https://doi.org/10.5281/zenodo.10017868, https://doi.org/10.5281/zenodo.10017840, https://doi.org/10.5281/zenodo.10018019, https://doi.org/10.5281/zenodo.10018131, and https://doi.org/10.5281/zenodo.10018425. Scripts and input files required to recreate the simulations, as well as scripts for plotting, are available at https://doi.org/10.5281/zenodo.10012787.
Kind regards,
Lars AckermannCitation: https://doi.org/10.5194/egusphere-2023-2061-AC1
-
AC1: 'Reply on CEC1', Lars Ackermann, 19 Oct 2023
-
CC1: 'Comment on egusphere-2023-2061', R. Marsh, 17 Oct 2023
Publisher’s note: the content of this comment was removed on 19 October 2023 since the comment was posted by mistake.
Citation: https://doi.org/10.5194/egusphere-2023-2061-CC1 -
RC2: 'Comment on egusphere-2023-2061', R. Marsh, 19 Oct 2023
General Comments
The authors present a comprehensive evaluation of interactive icebergs in a set of experiments with the AWI-ESM2.1 Earth System Model, demonstrating an advance in modelling capability that is within the scope of GMD. Multi-centennial experiments are necessary to establish the impacts of icebergs on deep and bottom waters that circulate on these long timescales. To my knowledge, these are the longest ESM experiments undertaken with interactive icebergs, while maintaining suitably high resolution around Antarctica. The separate impacts of iceberg-altered freshwater and heat fluxes are central to understanding the overall impacts of icebergs on the ocean, the latter impact (cooling) being regrettably a late admission to iceberg-ocean coupling in NEMO-ICB. Prospects for coupling with an interactive ice sheet are further promising. Overall, this study provides an advance on previous studies that reported the first implementations and subsequent developments of interactive iceberg modelling, several published in GMD. Methods are clearly outlined, with scope for minor clarification (see specific comments below), and results are sufficient to support conclusions. Following Code Availability, all results should be reproducible with sufficient computing resource. The title clearly reflects the contents of the paper, identifying the ESM by name.
A minor issue is the focus on Antarctica, with Greenland briefly mentioned in the Introduction. For a more complete representation, interactive icebergs can presumably also be seeded around Greenland (and other smaller northern ice caps). I appreciate that Antarctica is the dominant source of icebergs at global scale, but dense water formation in the subpolar North Atlantic may be particularly sensitive to the same processes (perturbing T and S) that are identified around Antarctica. It would be useful for the authors to comment further in Introduction and Discussion on the global impact of icebergs, in particular the net results of perturbations to deep and bottom water formation in both hemispheres. The Abstract provides a clear and concise summary of findings, although more quantitative detail could be provided (see specific comments). Overall presentation is well-structured throughout, and the manuscript is well written, with appropriate level of Supplementary Material.
In summary, the authors provide a compelling case for a relatively straightforward model improvement, of consequence for climate simulation and projection, with the intriguing possibility that neglected appreciation of iceberg influences could explain recent observed variability around. Antarctica (lines 266-269). As a development and technical paper, the manuscript should be suitable for publication in GMD, subject to minor revisions as suggested above and specified below.
Specific Comments
- Abstract: Provide additional quantitative support for statements, e.g., how much cooling, extent of (%) weakening in stratification.
- Introduction, from line 25: Given the focus here on a new model capability in AWI-ESM2.1, and earlier comment in the Abstract that ‘only few advanced coupled Earth System models that employ interactive icebergs’, it would be appropriate to more comprehensively summarize these models, perhaps tabulating resolutions tested, details of coupling (with/without heat flux coupling), seeding scheme, duration of simulation, reference/s. This would clearly establish the novelty and leading position this the present model development and study.
- Introduction, from line 49. Provide more justification for focus on Antarctica and the Southern Ocean in a global model, given the potential importance of Greenland icebergs for dense water formation and properties in the North Atlantic.
- 5, line 120: clarify what you mean by ‘continuous’, through time, or over space?
- 5, lines 122-123, and in Fig. 1 caption: I do not quite follow ‘integration of the calving flux over each basin to get total amount of ice discharge’, and Fig. 1a; can this be further explained?
- 11, Fig. 7: There is considerable multidecadal-to-centennial variability in ocean temperature and AABW indices; while this is not a focus of the study, it is perhaps worthy of more comment and even some explanation.
- 12, line 208: correct as ‘shelf-region’
- 13, line 219: do you mean ‘pushed onshore due to Ekman dynamics’? (geostrophy sets up long-shore drift
- 14, lines 250-251: Remarking that ‘realistic representation of AABW formation along continental shelves is not feasible in out model setup’, can this be further explained; it is a consequence of missing ice shelf cavities, or limited resolution, or both?
- 15, line 265: In discussing the delay of Southern Ocean greenhouse warming in climate projections due to cooling and freshening, do the authors imply the consequences of increasing iceberg calving? Perhaps be more explicit here.
Citation: https://doi.org/10.5194/egusphere-2023-2061-RC2 -
AC3: 'Reply on RC2', Lars Ackermann, 20 Nov 2023
We would like to thank you for the interest in ourr study and the timely and very constructive comments. While the main focus of this study is an enhanced process understanding of the long-term effects of interactive icebergs, we acknowledge the importance of Northern Hemisphere icebergs on deep-water formation in the North Atlantic and, ultimately, on the AMOC and the climate state. We will point this out in more detail in the revised manuscript and will address all further remarks.
Citation: https://doi.org/10.5194/egusphere-2023-2061-AC3
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Thomas Rackow
Kai Himstedt
Paul Gierz
Gregor Knorr
Gerrit Lohmann
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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