the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Apr 2025
Contribution of meridional overturning circulation and sea ice changes to large-scale temperature asymmetries in CMIP6 overshoot scenarios
Abstract. Analysis of overshoot scenarios, characterized by a peak in radiative forcing levels followed by a decline, show that changes during the CO2 increasing phase are not necessarily compensated during the CO2 decreasing phase, particularly at regional level. Even if the global mean temperature may recover after the overshoot, at the regional level the situation post-overshoot may differ from the situation pre-overshoot, with spatial patterns characterized by large-scale temperature asymmetries. These asymmetries, found between Northern (NH) and Southern Hemisphere (SH), between high and mid-latitudes of the NH, and between western and eastern areas of the Southern Ocean, alter atmospheric dynamics and through it the hydroclimate. Changes in the sea ice, changes in the ocean circulation and heat transport, and thermal inertia of the ocean have been identified as potential sources of hysteresis, highlighting the impact of oceanic changes in the behavior of atmospheric variables in case of overshoot. This work analyzes SSP5-3.4OS and SSP1-1.9 overshoot experiments from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to assess how well these mechanisms can explain the large-scale temperature asymmetries that characterize the difference between pre-overshoot and post-overshoot states. These analyses show that the relative contribution of each mechanism strongly depends on the model. Certain models like MRI-ESM2-0 are mainly impacted by changes in the Atlantic Meridional Overturning Circulation (AMOC), others like CNRM-ESM2-1 show a relevant impact of sea ice changes in high-latitude areas, and others like IPSL-CM6A-LR show also a relevant impact of changes in the Southern Meridional Overturning Circulation (SMOC). Inter-model differences in the contributions of the meridional overturning can be associated with different climatologies of Mixed Layer Depth (MLD) in the northern North Atlantic (NNA) and in certain areas of the Southern Ocean. Despite these differences across models, all the mechanisms contribute to shape the regional temperatures after overshoot, with the temperature asymmetries between NH and SH mainly explained by changes in the AMOC, those between high and mid-latitudes of the NH by sea ice changes, and those between western and eastern areas of the Southern Ocean by the SMOC. These results highlight the importance of model intercomparison and analysis of ocean dynamics to understand the regional impacts of an overshoot, and more generally the responses to forcing changes.
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Status: closed
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RC1: 'Comment on egusphere-2025-1784', Anonymous Referee #1, 09 Jun 2025
- AC1: 'Reply on RC1', Pedro Roldán, 17 Jul 2025
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RC2: 'Comment on egusphere-2025-1784', Anonymous Referee #2, 16 Jun 2025
Review of the manuscript: “Contribution of meridional overturning circulation and sea ice changes to large-scale temperature asymmetries in CMIP6 overshoot scenarios”
The manuscript, "Contribution of meridional overturning circulation and sea ice changes to large-scale temperature asymmetries in CMIP6 overshoot scenarios" by Pedro José Roldán-Gómez et al., presents an insightful investigation into the interplay between the Atlantic Meridional Overturning Circulation (AMOC), sea ice dynamics, and temperature patterns under overshoot climate scenarios. By analyzing a variety of CMIP6 models and focusing on different ensemble and temporal configurations, the study provides a nuanced understanding of the mechanisms shaping global and regional temperature asymmetries. I find the study extremely relevant for advancing our understanding of climate system responses to transient forcing. The use of multiple models and ensemble approaches enhances the robustness of the findings. The study is very valuable to the climate modeling and policy-making communities. However, I recommend a major revision, primarily due to issues with length, and figure overload that currently hinder the readability and accessibility of the results. I believe it requires significant revisions to enhance clarity, organization, and conciseness. Below, I outline both general and specific suggestions for improving the manuscript. I believe that, with a more concise and structured presentation, the manuscript could make a strong contribution to the literature on overshoot scenarios and climate feedback mechanisms.
General Comments:
The “Results” section could benefit from clarification and reorganization. It is hard to read, as the authors compare multiple dimensions simultaneously: scenarios (SSP5-3.4OS and SSP1-1.9), pre- and post-overshoot periods, global averages and regional differences, EXT and ALL ensembles. It contains 13 figures, made of several plots and maps, which are overloading the manuscript. I suggest selecting the most relevant figures, increase their quality and illustrate the inter model results description in a more efficient and concise way.
Specific Comments:Abstract:
Line 7: add a comma after “through it”
Line 8: Remove one instance of “changes in”
Line 10: The sentence beginning with “This work analyzes…” should be the first sentence of the abstract, describing the method before the results. Also, specify how many models are considered.
Line 13-18: Mechanisms are described for only three models; this is too detailed for the abstract. The abstract also needs to be shortened — please remove these lines.Introduction:
Line 1 and 3: The verb “increase” is used twice — consider rephrasing to avoid repetition.
Line 27: “is recovered” please rephrase to clarify
Line 33: Please describe what “OS” in SSP5-3OS stands for. Also introduce “SSP”
Line 61: Add this more recent paper, where transient forcing is used:
https://doi.org/10.1029/2025GL114611Methods:
Line 74 and later: please change “have been” by “is” or “are”, ex: “The analysis have been based” → “The analysis is based”, more simple, direct
Line 84: Ocean outputs have also been regridded to 2.8 degrees? Why, since the coarsest model has a resolution of 1.7 degrees? Please justify.
Table 1: The first line of the caption is too long and redundant. When you mention “available simulations,” do you mean that all available simulations on ESGF nodes were used? Please specify.
Line 93-99: The description of the periods is unclear; please consider adding a figure for illustration.
Figure 1: This figure is key; please make it twice as large. Some suggestions: add land in color, and draw the RS, WS, and NNA rectangles completely (over land as well).
Results:
- The description of the results needs reorganization (see general comments). Inter-model comparison results are discussed in the first subsection, yet another subsection is labeled “Differences across models”.- The number, size, and selection of figures must be improved. Some suggestions are provided for Figure 2, which are expected to be applied to other figures (up to Figure 15) where possible. Consider moving Figures 12 to 15 - which focus on one model - to the appendix.
- Figure 2: The size of the plots and maps is much too small; sea ice concentration is impossible to see, while the caption is unnecessarily long. Increase plot size, change the plot legend to use complete model names (e.g., “CAN” to “CanESM5”), and remove redundant explanations from the caption. Similarly, comparisons between periods need not be described in such detail in the caption. Is the year of maximum SST really relevant? As you are comparing periods, consider moving this to the appendix. The purpose of the vertical brown line is unclear. Are the vertical and horizontal lines relevant for discussion? Please specify how the spread is evaluated.
- Sea ice results should be moved to a specific subsection, rather than being merged with AMOC.
- Figures 8 to 11 are very hard to read and contain 24 different plots. Please compress the information.
Discussion and Conclusions:Discussion
The authors’ interpretation of the AMOC–sea ice linkage would be strengthened by situating the results within key CMIP6 studies (Meccia et al, 2022) and discussing whether the model spread is robust across spatial domains. A more explicit acknowledgment of model limitations—such as the coarse resolution for sea-ice processes and potential biases in AMOC strength—would improve transparency. Structuring the discussion into thematic subsections (‘Sea ice feedbacks’, ‘AMOC sensitivity’, ‘Combined mechanisms’) would greatly enhance readability.
Conclusions
The conclusion should begin by restating the study’s objective and summarizing main key findings. It should briefly emphasize the study’s novelty, explicitly state inherent limitations (e.g., model uncertainties), and suggest two or three clear avenues for future research, such as high-resolution model validation or observational comparison.
Citation: https://doi.org/10.5194/egusphere-2025-1784-RC2 - AC1: 'Reply on RC1', Pedro Roldán, 17 Jul 2025
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EC1: 'Comment on egusphere-2025-1784', Meric Srokosz, 23 Jun 2025
Both reviewers are generally positive but both think revisons, especially in terms of length and clarity, are required:
"my main general comment is that the authors need to make a greater effort to distil the research (and in particularly the number of figure panels) into that which is necessary to reach these conclusions."
"recommend a major revision, primarily due to issues with length, and figure overload that currently hinder the readability and accessibility of the results. I believe it requires significant revisions to enhance clarity, organization, and conciseness. "
These issues need to be addressed in the revision.
Citation: https://doi.org/10.5194/egusphere-2025-1784-EC1
Status: closed
-
RC1: 'Comment on egusphere-2025-1784', Anonymous Referee #1, 09 Jun 2025
Review of ‘Contribution of meridional overturning circulation and sea -ice changes ot large-scale temperature asymmetries in CMIP6 overshoot scenarios’. By Roldán-Gómez
An analysis is conducted three different overshoot scenarios from 13 different coupled models. The focus is determining the spatial patterns of SST anomalies that emerge in the overshoot period. They find different spatial patterns emerge depending on whether there are large changes in the AMOC or sea-ice. The AMOC influence is greater when there is a larger reduction in NA mixed layer. This happens when the models have a larger climatological MLD to start with. These findings are in the main well-reasoned and the work should make a useful contribution to the literature. It is noted that the paper represents a considerable amount of analysis across multiple models to reach these conclusions. Indeed, aside of minor specific comments, my main general comment is that the authors need to make a greater effort to distil the research (and in particularly the number of figure panels) into that which is necessary to reach these conclusions.
General Comments
A greater effort should be made to distil the research into that which is necessary to convince the reader of their conclusions. In particular, there is an overabundance of figures.
Although only 15 figures, this is made up of over 140 figure panels. I would encourage the authors to aggressively cull this number. There are a number of ways to evaluate if they are all necessary. Some of the plots contain linear regression, many of these could be probably best summarized by a table of the correlation values (and if they are significant or not). Furthermore, the authors may want to consider if they need to include results from all the scenarios. In particularly do the additional insights obtained from SSP1-1.9 warrant its inclusion.
Further Points
Abstract
Line 8: In the abstract. Candidate mechanisms are listed: ‘Changes in the sea ice, changes in the ocean circulation and heat transport, and thermal inertia of the ocean have been identified as potential sources of hysteresis ‘. It would be helpful when listing these to enumerate these and when listing them elsewhere in the manuscript, to use consistent terminology. (For example, later in the manuscript ‘AMOC’ is used rather than ‘ocean circulation and heat transport ‘ ). This will help the reader relate the analysis in the manuscript with the abstract. For example, the term ‘Thermal inertia’ is not mentioned in the manuscript beyond the introduction.
Line 30. A figure illustrating the time series of CO2 concentration in two scenarios, would help the initiated.
Line 65. In light of the range of processes mentioned in the existing literature (line 40-44) why have the authors decided to focus on the “ocean dynamics and sea-ice changes”?
Methods
What is guiding the choice of simulations? Is it just everything available? The authors may consider if the inclusion of SP1-1.9 adds significantly to the work.
Line 93. Could the authors state here how the post peak period is defined.
Line 110. As before, please clarify the reason for the choice of mechanisms investigated.
Figures 2ab, 3, 5, 6, and 7. These figures are quite complicated. Could the authors provide a bit primer, so the readers can clearly identify the key points from them.
Line 121. ‘After the stabilization’. Stabilization of what. Could the authors refer to specific years.
Line 122. ‘Larger than expected’. It is not clear what is expected. Please state the years that are being referred to, what specifically is it larger than (and how much).
Line 150. MRI shows a 2.0 C EN-ES asymmetry at year 2200 for SSP5-4.4OS (Figure 3a). However, for SSP1.-19, the maximum asymmetry is only about 0.6C at about 2040 (Figure 3b)”. Unless I fail to understand the figure.
Line 150 ‘Strong’. Please define ‘strong’ in turns of magnitude and state the time at which this can be seen in the relevant figure.
3.1 Line 150-156. What does the variety in the results reveal?
Line 173. SO higher mean values of OHT are found in SSP5-3.4OS-ALL (middle panel) but not SSP5-3.4OS EXT (left) or SSP1-1.9 ALL (right)
Line 175. ‘Some models like MRI-ES2.0’. The wording of this sentence is a little strange as figure Fig. 5 abcd show that MRI-ES2 is an outlier, i.e. there are no other models like it.
Line 177. ’moderate strengthening of OHT in SO in Fig 5ef’. Could the authors specify the post-overshoot years in which OHT is moderately strengthened.
Line 180. MLD Fig 5e. IPSL is strong outlier in terms of WS MLD. For SSP5-340S, it looks like the change in the other models is not so significant.
Fig. 8. It is difficult to see difference between dotted and solid line, please thicken the solid line or strengthen the difference in some other way. Probably no need for AMOC vs OHT, the relationship is well established.
Line 199. Suggest ‘Figure 8e and f) shows…’. The description of Figure 8 is not quite correct. EN-ES is only related to OHT. AMOC and MLD are related to OHT not to EN-ES SST. It might be more useful to have EN-ES vs OHT and EN-ES vs MLD.
Fig 8f) What is the physical explanation for the lack of correlation between EN-ES SST and OHT in SSP1.1.9?
There is no reference in text to Figure 9 c through f). I suggest these panels are removed.
Line 215. ‘The contribution of sea ice change seems more important’. Don’t the dashed lines in Figure 10 indicate the contribution of sea-ice change is insignificant? I suggest the references to the influence of sea-ice are removed.
Line 226: ‘Unlike for the ENM-ENH asymmetry, the contribution of sea ice changes to the SSW-SSE asymmetry (Fig. 11i,j) seems marginal’. Or like the ENM-ENH asymmetry it seems insignificant. If the authors do not wish to attach any importance to the significance test, they should probably explain why. The authors might consider if Figs, 10 and 11 are necessary.
Citation: https://doi.org/10.5194/egusphere-2025-1784-RC1 - AC1: 'Reply on RC1', Pedro Roldán, 17 Jul 2025
-
RC2: 'Comment on egusphere-2025-1784', Anonymous Referee #2, 16 Jun 2025
Review of the manuscript: “Contribution of meridional overturning circulation and sea ice changes to large-scale temperature asymmetries in CMIP6 overshoot scenarios”
The manuscript, "Contribution of meridional overturning circulation and sea ice changes to large-scale temperature asymmetries in CMIP6 overshoot scenarios" by Pedro José Roldán-Gómez et al., presents an insightful investigation into the interplay between the Atlantic Meridional Overturning Circulation (AMOC), sea ice dynamics, and temperature patterns under overshoot climate scenarios. By analyzing a variety of CMIP6 models and focusing on different ensemble and temporal configurations, the study provides a nuanced understanding of the mechanisms shaping global and regional temperature asymmetries. I find the study extremely relevant for advancing our understanding of climate system responses to transient forcing. The use of multiple models and ensemble approaches enhances the robustness of the findings. The study is very valuable to the climate modeling and policy-making communities. However, I recommend a major revision, primarily due to issues with length, and figure overload that currently hinder the readability and accessibility of the results. I believe it requires significant revisions to enhance clarity, organization, and conciseness. Below, I outline both general and specific suggestions for improving the manuscript. I believe that, with a more concise and structured presentation, the manuscript could make a strong contribution to the literature on overshoot scenarios and climate feedback mechanisms.
General Comments:
The “Results” section could benefit from clarification and reorganization. It is hard to read, as the authors compare multiple dimensions simultaneously: scenarios (SSP5-3.4OS and SSP1-1.9), pre- and post-overshoot periods, global averages and regional differences, EXT and ALL ensembles. It contains 13 figures, made of several plots and maps, which are overloading the manuscript. I suggest selecting the most relevant figures, increase their quality and illustrate the inter model results description in a more efficient and concise way.
Specific Comments:Abstract:
Line 7: add a comma after “through it”
Line 8: Remove one instance of “changes in”
Line 10: The sentence beginning with “This work analyzes…” should be the first sentence of the abstract, describing the method before the results. Also, specify how many models are considered.
Line 13-18: Mechanisms are described for only three models; this is too detailed for the abstract. The abstract also needs to be shortened — please remove these lines.Introduction:
Line 1 and 3: The verb “increase” is used twice — consider rephrasing to avoid repetition.
Line 27: “is recovered” please rephrase to clarify
Line 33: Please describe what “OS” in SSP5-3OS stands for. Also introduce “SSP”
Line 61: Add this more recent paper, where transient forcing is used:
https://doi.org/10.1029/2025GL114611Methods:
Line 74 and later: please change “have been” by “is” or “are”, ex: “The analysis have been based” → “The analysis is based”, more simple, direct
Line 84: Ocean outputs have also been regridded to 2.8 degrees? Why, since the coarsest model has a resolution of 1.7 degrees? Please justify.
Table 1: The first line of the caption is too long and redundant. When you mention “available simulations,” do you mean that all available simulations on ESGF nodes were used? Please specify.
Line 93-99: The description of the periods is unclear; please consider adding a figure for illustration.
Figure 1: This figure is key; please make it twice as large. Some suggestions: add land in color, and draw the RS, WS, and NNA rectangles completely (over land as well).
Results:
- The description of the results needs reorganization (see general comments). Inter-model comparison results are discussed in the first subsection, yet another subsection is labeled “Differences across models”.- The number, size, and selection of figures must be improved. Some suggestions are provided for Figure 2, which are expected to be applied to other figures (up to Figure 15) where possible. Consider moving Figures 12 to 15 - which focus on one model - to the appendix.
- Figure 2: The size of the plots and maps is much too small; sea ice concentration is impossible to see, while the caption is unnecessarily long. Increase plot size, change the plot legend to use complete model names (e.g., “CAN” to “CanESM5”), and remove redundant explanations from the caption. Similarly, comparisons between periods need not be described in such detail in the caption. Is the year of maximum SST really relevant? As you are comparing periods, consider moving this to the appendix. The purpose of the vertical brown line is unclear. Are the vertical and horizontal lines relevant for discussion? Please specify how the spread is evaluated.
- Sea ice results should be moved to a specific subsection, rather than being merged with AMOC.
- Figures 8 to 11 are very hard to read and contain 24 different plots. Please compress the information.
Discussion and Conclusions:Discussion
The authors’ interpretation of the AMOC–sea ice linkage would be strengthened by situating the results within key CMIP6 studies (Meccia et al, 2022) and discussing whether the model spread is robust across spatial domains. A more explicit acknowledgment of model limitations—such as the coarse resolution for sea-ice processes and potential biases in AMOC strength—would improve transparency. Structuring the discussion into thematic subsections (‘Sea ice feedbacks’, ‘AMOC sensitivity’, ‘Combined mechanisms’) would greatly enhance readability.
Conclusions
The conclusion should begin by restating the study’s objective and summarizing main key findings. It should briefly emphasize the study’s novelty, explicitly state inherent limitations (e.g., model uncertainties), and suggest two or three clear avenues for future research, such as high-resolution model validation or observational comparison.
Citation: https://doi.org/10.5194/egusphere-2025-1784-RC2 - AC1: 'Reply on RC1', Pedro Roldán, 17 Jul 2025
-
EC1: 'Comment on egusphere-2025-1784', Meric Srokosz, 23 Jun 2025
Both reviewers are generally positive but both think revisons, especially in terms of length and clarity, are required:
"my main general comment is that the authors need to make a greater effort to distil the research (and in particularly the number of figure panels) into that which is necessary to reach these conclusions."
"recommend a major revision, primarily due to issues with length, and figure overload that currently hinder the readability and accessibility of the results. I believe it requires significant revisions to enhance clarity, organization, and conciseness. "
These issues need to be addressed in the revision.
Citation: https://doi.org/10.5194/egusphere-2025-1784-EC1
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- 1
Review of ‘Contribution of meridional overturning circulation and sea -ice changes ot large-scale temperature asymmetries in CMIP6 overshoot scenarios’. By Roldán-Gómez
An analysis is conducted three different overshoot scenarios from 13 different coupled models. The focus is determining the spatial patterns of SST anomalies that emerge in the overshoot period. They find different spatial patterns emerge depending on whether there are large changes in the AMOC or sea-ice. The AMOC influence is greater when there is a larger reduction in NA mixed layer. This happens when the models have a larger climatological MLD to start with. These findings are in the main well-reasoned and the work should make a useful contribution to the literature. It is noted that the paper represents a considerable amount of analysis across multiple models to reach these conclusions. Indeed, aside of minor specific comments, my main general comment is that the authors need to make a greater effort to distil the research (and in particularly the number of figure panels) into that which is necessary to reach these conclusions.
General Comments
A greater effort should be made to distil the research into that which is necessary to convince the reader of their conclusions. In particular, there is an overabundance of figures.
Although only 15 figures, this is made up of over 140 figure panels. I would encourage the authors to aggressively cull this number. There are a number of ways to evaluate if they are all necessary. Some of the plots contain linear regression, many of these could be probably best summarized by a table of the correlation values (and if they are significant or not). Furthermore, the authors may want to consider if they need to include results from all the scenarios. In particularly do the additional insights obtained from SSP1-1.9 warrant its inclusion.
Further Points
Abstract
Line 8: In the abstract. Candidate mechanisms are listed: ‘Changes in the sea ice, changes in the ocean circulation and heat transport, and thermal inertia of the ocean have been identified as potential sources of hysteresis ‘. It would be helpful when listing these to enumerate these and when listing them elsewhere in the manuscript, to use consistent terminology. (For example, later in the manuscript ‘AMOC’ is used rather than ‘ocean circulation and heat transport ‘ ). This will help the reader relate the analysis in the manuscript with the abstract. For example, the term ‘Thermal inertia’ is not mentioned in the manuscript beyond the introduction.
Line 30. A figure illustrating the time series of CO2 concentration in two scenarios, would help the initiated.
Line 65. In light of the range of processes mentioned in the existing literature (line 40-44) why have the authors decided to focus on the “ocean dynamics and sea-ice changes”?
Methods
What is guiding the choice of simulations? Is it just everything available? The authors may consider if the inclusion of SP1-1.9 adds significantly to the work.
Line 93. Could the authors state here how the post peak period is defined.
Line 110. As before, please clarify the reason for the choice of mechanisms investigated.
Figures 2ab, 3, 5, 6, and 7. These figures are quite complicated. Could the authors provide a bit primer, so the readers can clearly identify the key points from them.
Line 121. ‘After the stabilization’. Stabilization of what. Could the authors refer to specific years.
Line 122. ‘Larger than expected’. It is not clear what is expected. Please state the years that are being referred to, what specifically is it larger than (and how much).
Line 150. MRI shows a 2.0 C EN-ES asymmetry at year 2200 for SSP5-4.4OS (Figure 3a). However, for SSP1.-19, the maximum asymmetry is only about 0.6C at about 2040 (Figure 3b)”. Unless I fail to understand the figure.
Line 150 ‘Strong’. Please define ‘strong’ in turns of magnitude and state the time at which this can be seen in the relevant figure.
3.1 Line 150-156. What does the variety in the results reveal?
Line 173. SO higher mean values of OHT are found in SSP5-3.4OS-ALL (middle panel) but not SSP5-3.4OS EXT (left) or SSP1-1.9 ALL (right)
Line 175. ‘Some models like MRI-ES2.0’. The wording of this sentence is a little strange as figure Fig. 5 abcd show that MRI-ES2 is an outlier, i.e. there are no other models like it.
Line 177. ’moderate strengthening of OHT in SO in Fig 5ef’. Could the authors specify the post-overshoot years in which OHT is moderately strengthened.
Line 180. MLD Fig 5e. IPSL is strong outlier in terms of WS MLD. For SSP5-340S, it looks like the change in the other models is not so significant.
Fig. 8. It is difficult to see difference between dotted and solid line, please thicken the solid line or strengthen the difference in some other way. Probably no need for AMOC vs OHT, the relationship is well established.
Line 199. Suggest ‘Figure 8e and f) shows…’. The description of Figure 8 is not quite correct. EN-ES is only related to OHT. AMOC and MLD are related to OHT not to EN-ES SST. It might be more useful to have EN-ES vs OHT and EN-ES vs MLD.
Fig 8f) What is the physical explanation for the lack of correlation between EN-ES SST and OHT in SSP1.1.9?
There is no reference in text to Figure 9 c through f). I suggest these panels are removed.
Line 215. ‘The contribution of sea ice change seems more important’. Don’t the dashed lines in Figure 10 indicate the contribution of sea-ice change is insignificant? I suggest the references to the influence of sea-ice are removed.
Line 226: ‘Unlike for the ENM-ENH asymmetry, the contribution of sea ice changes to the SSW-SSE asymmetry (Fig. 11i,j) seems marginal’. Or like the ENM-ENH asymmetry it seems insignificant. If the authors do not wish to attach any importance to the significance test, they should probably explain why. The authors might consider if Figs, 10 and 11 are necessary.