the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Apr 2025
Changing European Hydroclimate under a Collapsed AMOC in the Community Earth System Model
Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is expected to weaken or even collapse under anthropogenic climate change. Given the importance of the AMOC in the present-day climate, this would potentially lead to substantial changes in the future projections of the impacts of climate change on regional weather, which is highly relevant for society. Precipitation rates over Europe are expected to decrease under an AMOC collapse, potentially affecting the European hydroclimate. Here, we analyse the impacts of different AMOC collapse and climate change scenarios on the European hydroclimate in a unique set of AMOC experiments executed with the fully-coupled Community Earth System Model (CESM). In general, drier hydroclimatic conditions are expected under an AMOC collapse. The dominant drivers of this change depends on the specific combination of AMOC strength and radiative forcing. In AMOC collapse scenarios under pre-industrial conditions the dominant driver are reduced precipitation rates over the entire European continent. AMOC collapse in combination with increased radiative forcing (RCP4.5, RCP8.5) also leads to higher potential evapotranspiration rates, which further exacerbates the noted shifts to increased seasonal drought (extremes). Here, AMOC collapse enhances well-documented shifts to a drier summer climate in Europe in 'standard' projections of future climate change. In summary, these results indicate a considerable influence of the AMOC on future European hydroclimate. It is therefore vital that climate change projections of European hydroclimate for the (far) future consider the possibility of AMOC changes, and the exacerbated effects this would have on projected regional hydrological changes and consequences for ecosystems and society.
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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
(65076 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
(65076 KB) - Metadata XML
- BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
standardhydroclimate projections. Our results indicate a considerable influence of the AMOC on the European hydroclimate.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-1440', Anonymous Referee #1, 23 May 2025
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AC1: 'Reply on RC1', René van Westen, 28 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1440/egusphere-2025-1440-AC1-supplement.pdf
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AC1: 'Reply on RC1', René van Westen, 28 Jul 2025
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RC2: 'Comment on egusphere-2025-1440', Anonymous Referee #2, 02 Jul 2025
Summary:
This study investigates the hydroclimatic response over Europe under eight climate scenarios featuring different AMOC (Atlantic Meridional Overturning Circulation) strengths, including AMOC collapse cases, across pre-industrial (PI), RCP4.5, and RCP8.5 conditions. The authors use the CESM model, which has sufficiently high resolution for this kind of regional hydroclimatic analysis. They focus on daily water balance analysis by examining daily averaged precipitation and potential evapotranspiration (PET), from which they derive the potential precipitation deficit (PPD). The results indicate that while precipitation dominates the PPD, it does not fully explain the spatial variations, and PET also plays a role—particularly as PET rates increase with warming. The study shows that AMOC collapse leads to drier conditions and increased drought extremes across all climate scenarios, with the drying effects becoming more severe under RCP4.5 and RCP8.5. Particularly noteworthy is the comparison between RCP4.5 scenarios with and without AMOC collapse, which illustrates how radiative forcing combined with an AMOC collapse significantly intensifies drying by extending the dry season.
Overall, this is a compelling and timely contribution, especially given the increasing interest in AMOC-related tipping points. The authors have done an excellent job in designing and executing a set of carefully structured experiments. The clarity of figures, particularly those showing the combined impacts of AMOC and warming scenarios, is commendable. With minor revisions, I believe this study is well-suited for publication in HESS.
General comments:It is intriguing that an AMOC collapse occurs under PI radiative forcing with a freshwater flux of 0.18 Sv, but not under RCP4.5 with the same flux. Further elaboration on this outcome would enhance the reader’s understanding, as it suggests interesting non-linear behaviour and sensitivity to background climate states.
The distinction between AMOC “ON” and “OFF” states needs clarification. Figures and text indicate that both states include freshwater fluxes of 0.18 Sv and 0.45 Sv, raising questions about how the AMOC is still “ON” under such forcing. More detail on how the AMOC states are defined would be useful.
Much of Section 3.3 reads more like methodological description and could be moved to the Methods/Data section. Additionally, justification for the use of the k-means clustering approach should be strengthened. How do the results differ from a traditional analysis of MSLP changes, and why is clustering more appropriate or insightful in this case?
The Discussion section includes very little comparison with other studies that examine hydroclimate or European climate changes in response to AMOC shutdown. It would also be helpful to discuss how the results might change if CESM model biases were accounted for or removed
Specific Comments:
Line 17–18: Replace vague or journalistic terms like "hot topic" or "major tipping point" with more scientific language.
Line 30: Specify how the seasonal cycle shifts.
Lines26–32: It might be worth adding Saini et al., 2025 (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024PA004967) here, as they examine the impact of AMOC shutdown on Australian hydroclimate. They also use a comparable higher-resolution model, and this could be another interesting region where AMOC impacts are transmitted via planetary-scale dynamics.
Line 62: Should read “details on how…”
Line 195: Should read “a factor of 3.5.”
Lines 235–237: Grammar needs correction here. Consider rephrasing for clarity and correct sentence structure.
Lines 303–305: Please indicate which scenario (e.g., RCP8.5) is being discussed to ensure clarity.
Lines 332–333: This sentence appears to repeat the introductory paragraph of the section. Consider removing or rephrasing to avoid redundancy.
Citation: https://doi.org/10.5194/egusphere-2025-1440-RC2 -
AC2: 'Reply on RC2', René van Westen, 28 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1440/egusphere-2025-1440-AC2-supplement.pdf
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AC2: 'Reply on RC2', René van Westen, 28 Jul 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-1440', Anonymous Referee #1, 23 May 2025
-
AC1: 'Reply on RC1', René van Westen, 28 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1440/egusphere-2025-1440-AC1-supplement.pdf
-
AC1: 'Reply on RC1', René van Westen, 28 Jul 2025
-
RC2: 'Comment on egusphere-2025-1440', Anonymous Referee #2, 02 Jul 2025
Summary:
This study investigates the hydroclimatic response over Europe under eight climate scenarios featuring different AMOC (Atlantic Meridional Overturning Circulation) strengths, including AMOC collapse cases, across pre-industrial (PI), RCP4.5, and RCP8.5 conditions. The authors use the CESM model, which has sufficiently high resolution for this kind of regional hydroclimatic analysis. They focus on daily water balance analysis by examining daily averaged precipitation and potential evapotranspiration (PET), from which they derive the potential precipitation deficit (PPD). The results indicate that while precipitation dominates the PPD, it does not fully explain the spatial variations, and PET also plays a role—particularly as PET rates increase with warming. The study shows that AMOC collapse leads to drier conditions and increased drought extremes across all climate scenarios, with the drying effects becoming more severe under RCP4.5 and RCP8.5. Particularly noteworthy is the comparison between RCP4.5 scenarios with and without AMOC collapse, which illustrates how radiative forcing combined with an AMOC collapse significantly intensifies drying by extending the dry season.
Overall, this is a compelling and timely contribution, especially given the increasing interest in AMOC-related tipping points. The authors have done an excellent job in designing and executing a set of carefully structured experiments. The clarity of figures, particularly those showing the combined impacts of AMOC and warming scenarios, is commendable. With minor revisions, I believe this study is well-suited for publication in HESS.
General comments:It is intriguing that an AMOC collapse occurs under PI radiative forcing with a freshwater flux of 0.18 Sv, but not under RCP4.5 with the same flux. Further elaboration on this outcome would enhance the reader’s understanding, as it suggests interesting non-linear behaviour and sensitivity to background climate states.
The distinction between AMOC “ON” and “OFF” states needs clarification. Figures and text indicate that both states include freshwater fluxes of 0.18 Sv and 0.45 Sv, raising questions about how the AMOC is still “ON” under such forcing. More detail on how the AMOC states are defined would be useful.
Much of Section 3.3 reads more like methodological description and could be moved to the Methods/Data section. Additionally, justification for the use of the k-means clustering approach should be strengthened. How do the results differ from a traditional analysis of MSLP changes, and why is clustering more appropriate or insightful in this case?
The Discussion section includes very little comparison with other studies that examine hydroclimate or European climate changes in response to AMOC shutdown. It would also be helpful to discuss how the results might change if CESM model biases were accounted for or removed
Specific Comments:
Line 17–18: Replace vague or journalistic terms like "hot topic" or "major tipping point" with more scientific language.
Line 30: Specify how the seasonal cycle shifts.
Lines26–32: It might be worth adding Saini et al., 2025 (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024PA004967) here, as they examine the impact of AMOC shutdown on Australian hydroclimate. They also use a comparable higher-resolution model, and this could be another interesting region where AMOC impacts are transmitted via planetary-scale dynamics.
Line 62: Should read “details on how…”
Line 195: Should read “a factor of 3.5.”
Lines 235–237: Grammar needs correction here. Consider rephrasing for clarity and correct sentence structure.
Lines 303–305: Please indicate which scenario (e.g., RCP8.5) is being discussed to ensure clarity.
Lines 332–333: This sentence appears to repeat the introductory paragraph of the section. Consider removing or rephrasing to avoid redundancy.
Citation: https://doi.org/10.5194/egusphere-2025-1440-RC2 -
AC2: 'Reply on RC2', René van Westen, 28 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1440/egusphere-2025-1440-AC2-supplement.pdf
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AC2: 'Reply on RC2', René van Westen, 28 Jul 2025
Peer review completion
Journal article(s) based on this preprint
standardhydroclimate projections. Our results indicate a considerable influence of the AMOC on the European hydroclimate.
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Karin van der Wiel
Swinda K. J. Falkena
Frank Selten
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(65076 KB) - Metadata XML
The comment is attached in the pdf.