Dynamically Downscaled Future Projections of the Northwest Atlantic Ocean Across Low to High Emissions Scenarios

Kim, Dongmin; Ross, Andrew C.; Shin, Sang-Ik; Gomez, Fabian A.; John, Jasmin G.; Volkov, Denis L.; Lee, Sang-Ki; Alexander, Michael A.; Stock, Charles A.

doi:10.5194/egusphere-2025-6449

Preprints

https://doi.org/10.5194/egusphere-2025-6449

Preprints

06 Jan 2026

| 06 Jan 2026

Dynamically Downscaled Future Projections of the Northwest Atlantic Ocean Across Low to High Emissions Scenarios

Dongmin Kim, Andrew C. Ross, Sang-Ik Shin, Fabian A. Gomez, Jasmin G. John, Denis L. Volkov, Sang-Ki Lee, Michael A. Alexander, and Charles A. Stock

Abstract. We used a high-resolution (1/12°) Modular Ocean Model version 6 implementation for the the Northwest Atlantic Ocean (MOM6-NWA12) to dynamically downscale Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1) projections for the 21st century. Simulations were conducted under four different Coupled Model Intercomparison Project Phase 6 emission scenarios. MOM6-NWA12 accurately simulates the spatial patterns of sea surface temperature, salinity, and dynamic sea surface height (SSH) during the historical period. In particular, the Gulf Stream's strength, position, recirculation, and separation from the U.S. East Coast are significantly improved in MOM6-NWA12 compared to the coarse-resolution GFDL-ESM4.1. Projected end-of-century warming varied strongly between scenarios, from ~ 4 °C under prior "worst case" emissions scenarios (SSP-585), 2~3 °C under intermediate scenarios (SSP-245, SSP-370) more consistent with current trajectories, to ~ 1 °C under aggressive mitigation (SSP-126). Consistent with a significant weakening of the Atlantic Meridional Overturning Circulation projected by GFDL-ESM4.1, MOM6-NWA12 shows a substantial volume transport reduction in the Western Boundary Current (WBC) system (i.e., Yucatan Current, Florida Current, Antilles Current, and the Deep Western Boundary Current) toward the late 21st century (between 23 and 38 %, varying by scenario). This projected weakening of the WBC system and the associated reduction in the coastal upwelling of cold, fresh subsurface waters lead to a significant increase in ocean temperature, salinity, and dynamic SSH along the U.S. southeast and northeast Coasts, particularly in the South Atlantic Bight. These localized changes have significant implications for future sea level rise, marine ecosystems, and fish populations in these highly vulnerable regions.

Received: 23 Dec 2025 – Discussion started: 06 Jan 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Ocean Science. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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26 Jun 2026

Dynamically downscaled future projections of the Northwest Atlantic Ocean across low to high emissions scenarios

Dongmin Kim, Andrew C. Ross, Sang-Ik Shin, Fabian A. Gomez, Jasmin G. John, Denis L. Volkov, Sang-Ki Lee, Michael A. Alexander, and Charles A. Stock

Ocean Sci., 22, 1987–2009, https://doi.org/10.5194/os-22-1987-2026,https://doi.org/10.5194/os-22-1987-2026, 2026

Short summary

Dongmin Kim, Andrew C. Ross, Sang-Ik Shin, Fabian A. Gomez, Jasmin G. John, Denis L. Volkov, Sang-Ki Lee, Michael A. Alexander, and Charles A. Stock

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-6449', Anonymous Referee #1, 13 Feb 2026
This manuscript analyses results from a high-resolution (1/12°) regional ocean model of the NW Atlantic forced by fluxes and boundary conditions from a lower resolution fully-coupled earth system model. Specifically, four CMIP scenarios for different future warming levels are integrated out to 2100, and the work focuses on changes to currents, surface temperature, salinity, and sea-surface height (SSH). While others have already studied the resulting future reduction to the current systems (Atlantic Meridional Overturning Circulation (AMOC) and Western Boundary Current (WBC)) for the highest warming scenario (RCP8.5 and SSP585) in fully-coupled climate models (for CMIP5, ~1° ocean models, Beadling et al., 2018; for CMIP6, ~1/4 – 1/12° ocean models, Roberts et al., 2020), the present investigation focuses on changes in the societally important near-coastal and shelf regions (Gulf of Mexico/America, West of Florida, South Atlantic Bight, Mid Atlantic Bight and Gulf of Maine), and investigates the range of possible conditions for the four different warming scenarios. These regions are found to become warmer and saltier in the future (changes which could affect the marine ecosystems) and also to have higher sea-levels. This near-coastal/ shelf focus examined across multiple future warming scenarios has not been studied before to my knowledge, and could provide useful information about future conditions for marine planners and societal uses.
However, there are two main drawbacks to the work which should be addressed before publication could be considered, as follows:
Northern Inputs. The manuscript proposes (e.g. ls 41-42, 499-503) several mechanisms which may cause the changes (warming and salinification) of the coastal and shelf waters along the US South and East coasts, such as the reduced upwelling of colder and fresher waters due to the reduced flows in the Loop Current and Gulf Stream (affecting the Gulf of America/Mexico), and a northward and shoreward shift of the Gulf Stream following its separation (affecting the Mid Atlantic Bight (MAB) and Gulf of Maine (GOM)). However, it should also acknowledge the possibility of changes in the current systems which bring waters to the MAB/GOM shelf and slope region. The coastal/shelf waters here are largely derived from the inshore Labrador Current which exits the Hudson Strait, passes over the Grand Banks and then forms the Nova Scotia Current to provide cold, fresh waters to this area. In addition the (offshore) Labrador Current (with origins in the Davis Strait and West Greenland Current) flows around the Tail of the Grand Banks, and after mixing with North Atlantic Central Water, forms Labrador Slope Water which flows southwards along the shelf slopes as far as Cape Hatteras, with a core at depths of 400-600m (see New et al., 2021, and the references therein). The manuscript should acknowledge the possibility of changes in the temperature and salinity of these northern inputs as a cause of the warming and salinification here: the changes in the coastal/shelf waters imply changes in the inshore LC, and in particular the warming and salinification observed on the shelf slopes in the MAB (figure 14) at 400-600m are strongly indicative of changes the in LSLW. This possibility is briefly acknowledged in ls. 333-334 but without any evidence: more discussion of this mechanism is needed either here or, better, in the discussion section: e.g. which of the various mechanisms is actually responsible for the changes in the MAB and GOM? Also, inclusion of a figure showing changes in the near-surface properties of the Labrador Sea (which provide the head waters for the MAB and GOM areas) would be very useful.

Sea-surface height. While the results for the coastal/shelf Sea-Surface Temperature (SST) and Salinity (SSS) are interesting, the results for Sea-Surface Height (SSH) are of limited value because they only consider changes to “dynamic height” i.e. changes caused by changes in the currents (as the model has the Boussinesq approximation and cannot accommodate changes in the SSH due to warming of the water column). As the currents reduce, so does the SSH gradient across them (as expected since the SSH gradients relate to the surface geostrophic flow) meaning that the SSH to the north of the currents (on the shelf regions) generally increases and that to the southward/ oceanward side of the currents decreases. Steric sea-level changes due to the warming (and salinity change) of the water column need to be more fully discussed rather than the brief statement (ls. 473-478) about general future increases: e.g. we would need to know about the spatial structure (presumably not constant everywhere) of these changes which add to the dynamic changes presented, so that a full picture of future SSH can be formed. Perhaps a description of such spatial variability could be included based on the references in ls. 477-478. Furthermore, this lack of steric sea-level change seems to be related to their statement in ls. 222-223 that the basin-averaged SSH anomalies in the NWA12 model do not vary in time, but this should be explained more clearly.

Minor Points
l. 148. What forcing does the ESM model provide to the NWA12 model: just air-sea luxes or lateral boundary conditions (BCs) as well? And how are the lateral BCs passed across given that the ESM uses hybrid ALE layers and the NWA uses the z* coordinate (so some interpolation should be required)?
l. 157. What is the resolution of the ocean model (the atmosphere model is 1°)
l. 179: this is section 2.4 not 2.5
Fig. 2 caption: l. 918 insert (b) and (c) for the SSS and current speed panels.
Fig. 5 caption. l.s 954-955: (a) is for Yucatan, (b) for the Florida Current: the caption has these the wrong way around.
l. 325: “scenarios” not “scenario” better.
l.s 335-337: insert latitudes and longitudes of the centres of the regions of minimum surface warming to make things clearer.
l.s 338-340: could the region of minimum SST increase South of Gulf Stream in fig 6e be due to the reduced GS strength (fig 7j, i.e. bringing less heat transport) as well as to its northward shift?
l.s 359-360: could refer to Beadling et al. 2018 and Roberts et al. 2019 here in connection with reductions in the WBC system.
l. 1001: fig 8 caption: remove “The vertical dotted lines” at the end.
l. 383 etc. There are several references to the slowdown of the AMOC but a figure of this only appears in the Supplementary material. Please insert this figure (S6) in the main manuscript somewhere near here. This is important because the AMOC is NOT the same as the WBC system, e.g. McCarthy et al., 2015. This also means that in l. 426, refer to the new AMOC figure rather than to figure S6.
Fig 10 caption: say the years are e.g. model years - 2000.
l. 423: after “begins to emerge” insert “after 2070”.
Fig. 12: it would be good to include density changes here as well as the dynamically important quantity: lighter water on the western side of the channel would be consistent with lower (geostrophic) flows through the Florida Straits.
l.s. 514-516: “Further analysis indicates that the weakening of the Florida Current accompanies a substantial reduction of upwelling of cold and fresh subsurface water to the continental slope and shelf region.” – where is this further analysis??
References
Beadling, R. L., Russell, J. L., Stouffer, R. J. & Goodman, P. J. Evaluation of subtropical North Atlantic Ocean circulation in CMIP5 models against the observational array at 26.5°N and its changes under continued warming. J. Clim. 31, 9697-9718, https://doi.org/10.1175/jcli-d-17-0845.1 (2018).
McCarthy, G. D., et al. Measuring the Atlantic Meridional Overturning Circulation at 26°N. Prog. Oceanogr. 130, 91-111, http://dx.doi.org/10.1016/j.pocean.2014.10.006 (2015).
New, A. L. et al. Labrador Slope Water connects the subarctic with the Gulf Stream. Environ. Res. Lett. 16, 084019, https://doi.org/10.1088/1748-9326/ac1293 (2021).
Roberts, M. J. et al. Sensitivity of the Atlantic Meridional Overturning Circulation to model resolution in CMIP6 HighResMIP simulations and implications for future changes. J. Adv. Model. Earth Sy. 12, e2019MS002014, https://doi.org/10.1029/2019MS002014 (2020).
Citation: https://doi.org/10.5194/egusphere-2025-6449-RC1
- AC1: 'Reply on RC1', Dongmin Kim, 11 Apr 2026
  
  The reply on RC1 is attached.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6449-AC1
RC2:
'Comment on egusphere-2025-6449', Anonymous Referee #2, 02 Mar 2026

The manuscript presents results from downscaling four projections under SSP scenarios computed by the 1 degree GFDL earth system model to a 1/12 degree MOM6 regional model of the western North Atlantic. The authors find significant scenario dependence for SST and SSS but much less dependence for currents and sea level change. They attribute the lower sensitivity to a delayed response. Current and sea level changes are connected with the AMOC decline simulated by the parent 1 degree GFDL model. The presented findings are consistent with Saba et al. (2026) but the present study offers results for different realistic scenarios.

The inter comparison between low and high-resolution model results of the historical mean state is quite extensive given that NAW12 is not a new configuration and has been evaluated before by Ross et al. (2023/24). One could easily save some figures particularly since most of the outcome is expected, well documented progress when moving from 1 degree to 1/12 degree. Instead, the dependence of the changes on the resolution should be more the focus of the paper, to discuss in what way enhanced resolution leads to different projections. Therefore I suggest to move the evaluation into the supplement and the results from global climate model out of the supplement. In that process the discussion of the biases can be condensed and the differences of the projections enhanced.

Details
Line 44-45 Are these implication part of the investigations of the manuscript? I did not find much about it that would deserve being mentioned in the abstract.
Line 148-150 I assume ESM4 is run under the same 4 scenarios as NWA12. Where are those runs described and how does ESM4 compare to other climate models in climate sensitivity. A littel bit more information should be provided here.
Line 196-200 Not clear what the difference between the method here and the previous method is. For instance Liu et al. 2012 also corrected with the difference between the climatologies. Alexander et al. however used the differences between the mean. I don't understand how this would assume that the climate variability remains the same as in the historical period. Correcting with mean or climatology does not constrain the climate variability.
Line 204-206 This argument is not convincing. Jackson and Wood do not really assess runoff uncertainty and Giuntoli argued that hydrological models add uncertainty in addition to the contribution of GCMs. Following just the latter argument you may also disregard the GCM contribution. If there is not meaningful signal and only uncertainty is added, projected runoff could be neglected. But I doubt that there is not meaningful runoff signal, since sea level projections nowadays include contributions from glacial and ice sheet melt and a clear positive tend in river discharge is visible already now (https://doi.org/10.5194/hess-28-2179-2024). So, neglecting the runoff contribution is likely to reduce realism rather than uncertainty.
Line 216-223 You may want to mention other components that are not included in the sea level projections, particularly since you refer to this information in the conclusions.
Line 241-242 I think it is mostly broader, I am not sure if I see the shift away from the coast.
Line 315-319 What are "these regions" over which the temperature change was calculated?
Line 360-363 These two statements seem to contradict each other. Fig.7 is from the end of the century and I believe to see gradual differences. I suggest to point to the gradual differences visible in Fig.7 but then also to the surprisingly similar development until 2070 seen in Fig. 8. Additionaly mention here when the scenario forcings start to differ, because later you use an argument of delayed response that requires the reader to know until when scenarios were still similar.
Line 373-366 Is the reduction really significant given the error bars? Given the error bar it is probably more appropriate to say that the Antilles Current disappears (no significant mean transport).
Line 383 Point to Fig. S6 already here
Line 368-388 Do the lateral boundary conditions make NWA12 entirely a slave of the ESM4.1 for AMOC? You may want say something about this.
Line 398-399 not shown?
Line 405-406 How much is AMOC vs gyre weakening? Although ultimately as Yin et al. (2010) suggested the AMOC may be the origin of the changes, it would be useful to mention changes in the barotropic circulation of the subtropical gyre (stream function). because it seems that the transport change directly associated with AMOC weakening can account only for a fraction (<50% from the numbers reported on page 18 and the AMOC decline Fig.S6) of the reduced transports.
Line 454-455 How does geostrophic adjustment weakens upwelling. Isn't it more just the flattening of the isopynals.

Citation: https://doi.org/10.5194/egusphere-2025-6449-RC2
- AC2: 'Reply on RC2', Dongmin Kim, 11 Apr 2026
  
  The reply on RC2 is attached.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6449-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-6449', Anonymous Referee #1, 13 Feb 2026
This manuscript analyses results from a high-resolution (1/12°) regional ocean model of the NW Atlantic forced by fluxes and boundary conditions from a lower resolution fully-coupled earth system model. Specifically, four CMIP scenarios for different future warming levels are integrated out to 2100, and the work focuses on changes to currents, surface temperature, salinity, and sea-surface height (SSH). While others have already studied the resulting future reduction to the current systems (Atlantic Meridional Overturning Circulation (AMOC) and Western Boundary Current (WBC)) for the highest warming scenario (RCP8.5 and SSP585) in fully-coupled climate models (for CMIP5, ~1° ocean models, Beadling et al., 2018; for CMIP6, ~1/4 – 1/12° ocean models, Roberts et al., 2020), the present investigation focuses on changes in the societally important near-coastal and shelf regions (Gulf of Mexico/America, West of Florida, South Atlantic Bight, Mid Atlantic Bight and Gulf of Maine), and investigates the range of possible conditions for the four different warming scenarios. These regions are found to become warmer and saltier in the future (changes which could affect the marine ecosystems) and also to have higher sea-levels. This near-coastal/ shelf focus examined across multiple future warming scenarios has not been studied before to my knowledge, and could provide useful information about future conditions for marine planners and societal uses.
However, there are two main drawbacks to the work which should be addressed before publication could be considered, as follows:
Northern Inputs. The manuscript proposes (e.g. ls 41-42, 499-503) several mechanisms which may cause the changes (warming and salinification) of the coastal and shelf waters along the US South and East coasts, such as the reduced upwelling of colder and fresher waters due to the reduced flows in the Loop Current and Gulf Stream (affecting the Gulf of America/Mexico), and a northward and shoreward shift of the Gulf Stream following its separation (affecting the Mid Atlantic Bight (MAB) and Gulf of Maine (GOM)). However, it should also acknowledge the possibility of changes in the current systems which bring waters to the MAB/GOM shelf and slope region. The coastal/shelf waters here are largely derived from the inshore Labrador Current which exits the Hudson Strait, passes over the Grand Banks and then forms the Nova Scotia Current to provide cold, fresh waters to this area. In addition the (offshore) Labrador Current (with origins in the Davis Strait and West Greenland Current) flows around the Tail of the Grand Banks, and after mixing with North Atlantic Central Water, forms Labrador Slope Water which flows southwards along the shelf slopes as far as Cape Hatteras, with a core at depths of 400-600m (see New et al., 2021, and the references therein). The manuscript should acknowledge the possibility of changes in the temperature and salinity of these northern inputs as a cause of the warming and salinification here: the changes in the coastal/shelf waters imply changes in the inshore LC, and in particular the warming and salinification observed on the shelf slopes in the MAB (figure 14) at 400-600m are strongly indicative of changes the in LSLW. This possibility is briefly acknowledged in ls. 333-334 but without any evidence: more discussion of this mechanism is needed either here or, better, in the discussion section: e.g. which of the various mechanisms is actually responsible for the changes in the MAB and GOM? Also, inclusion of a figure showing changes in the near-surface properties of the Labrador Sea (which provide the head waters for the MAB and GOM areas) would be very useful.

Sea-surface height. While the results for the coastal/shelf Sea-Surface Temperature (SST) and Salinity (SSS) are interesting, the results for Sea-Surface Height (SSH) are of limited value because they only consider changes to “dynamic height” i.e. changes caused by changes in the currents (as the model has the Boussinesq approximation and cannot accommodate changes in the SSH due to warming of the water column). As the currents reduce, so does the SSH gradient across them (as expected since the SSH gradients relate to the surface geostrophic flow) meaning that the SSH to the north of the currents (on the shelf regions) generally increases and that to the southward/ oceanward side of the currents decreases. Steric sea-level changes due to the warming (and salinity change) of the water column need to be more fully discussed rather than the brief statement (ls. 473-478) about general future increases: e.g. we would need to know about the spatial structure (presumably not constant everywhere) of these changes which add to the dynamic changes presented, so that a full picture of future SSH can be formed. Perhaps a description of such spatial variability could be included based on the references in ls. 477-478. Furthermore, this lack of steric sea-level change seems to be related to their statement in ls. 222-223 that the basin-averaged SSH anomalies in the NWA12 model do not vary in time, but this should be explained more clearly.

Minor Points
l. 148. What forcing does the ESM model provide to the NWA12 model: just air-sea luxes or lateral boundary conditions (BCs) as well? And how are the lateral BCs passed across given that the ESM uses hybrid ALE layers and the NWA uses the z* coordinate (so some interpolation should be required)?
l. 157. What is the resolution of the ocean model (the atmosphere model is 1°)
l. 179: this is section 2.4 not 2.5
Fig. 2 caption: l. 918 insert (b) and (c) for the SSS and current speed panels.
Fig. 5 caption. l.s 954-955: (a) is for Yucatan, (b) for the Florida Current: the caption has these the wrong way around.
l. 325: “scenarios” not “scenario” better.
l.s 335-337: insert latitudes and longitudes of the centres of the regions of minimum surface warming to make things clearer.
l.s 338-340: could the region of minimum SST increase South of Gulf Stream in fig 6e be due to the reduced GS strength (fig 7j, i.e. bringing less heat transport) as well as to its northward shift?
l.s 359-360: could refer to Beadling et al. 2018 and Roberts et al. 2019 here in connection with reductions in the WBC system.
l. 1001: fig 8 caption: remove “The vertical dotted lines” at the end.
l. 383 etc. There are several references to the slowdown of the AMOC but a figure of this only appears in the Supplementary material. Please insert this figure (S6) in the main manuscript somewhere near here. This is important because the AMOC is NOT the same as the WBC system, e.g. McCarthy et al., 2015. This also means that in l. 426, refer to the new AMOC figure rather than to figure S6.
Fig 10 caption: say the years are e.g. model years - 2000.
l. 423: after “begins to emerge” insert “after 2070”.
Fig. 12: it would be good to include density changes here as well as the dynamically important quantity: lighter water on the western side of the channel would be consistent with lower (geostrophic) flows through the Florida Straits.
l.s. 514-516: “Further analysis indicates that the weakening of the Florida Current accompanies a substantial reduction of upwelling of cold and fresh subsurface water to the continental slope and shelf region.” – where is this further analysis??
References
Beadling, R. L., Russell, J. L., Stouffer, R. J. & Goodman, P. J. Evaluation of subtropical North Atlantic Ocean circulation in CMIP5 models against the observational array at 26.5°N and its changes under continued warming. J. Clim. 31, 9697-9718, https://doi.org/10.1175/jcli-d-17-0845.1 (2018).
McCarthy, G. D., et al. Measuring the Atlantic Meridional Overturning Circulation at 26°N. Prog. Oceanogr. 130, 91-111, http://dx.doi.org/10.1016/j.pocean.2014.10.006 (2015).
New, A. L. et al. Labrador Slope Water connects the subarctic with the Gulf Stream. Environ. Res. Lett. 16, 084019, https://doi.org/10.1088/1748-9326/ac1293 (2021).
Roberts, M. J. et al. Sensitivity of the Atlantic Meridional Overturning Circulation to model resolution in CMIP6 HighResMIP simulations and implications for future changes. J. Adv. Model. Earth Sy. 12, e2019MS002014, https://doi.org/10.1029/2019MS002014 (2020).
Citation: https://doi.org/10.5194/egusphere-2025-6449-RC1
- AC1: 'Reply on RC1', Dongmin Kim, 11 Apr 2026
  
  The reply on RC1 is attached.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6449-AC1
RC2:
'Comment on egusphere-2025-6449', Anonymous Referee #2, 02 Mar 2026

The manuscript presents results from downscaling four projections under SSP scenarios computed by the 1 degree GFDL earth system model to a 1/12 degree MOM6 regional model of the western North Atlantic. The authors find significant scenario dependence for SST and SSS but much less dependence for currents and sea level change. They attribute the lower sensitivity to a delayed response. Current and sea level changes are connected with the AMOC decline simulated by the parent 1 degree GFDL model. The presented findings are consistent with Saba et al. (2026) but the present study offers results for different realistic scenarios.

The inter comparison between low and high-resolution model results of the historical mean state is quite extensive given that NAW12 is not a new configuration and has been evaluated before by Ross et al. (2023/24). One could easily save some figures particularly since most of the outcome is expected, well documented progress when moving from 1 degree to 1/12 degree. Instead, the dependence of the changes on the resolution should be more the focus of the paper, to discuss in what way enhanced resolution leads to different projections. Therefore I suggest to move the evaluation into the supplement and the results from global climate model out of the supplement. In that process the discussion of the biases can be condensed and the differences of the projections enhanced.

Details
Line 44-45 Are these implication part of the investigations of the manuscript? I did not find much about it that would deserve being mentioned in the abstract.
Line 148-150 I assume ESM4 is run under the same 4 scenarios as NWA12. Where are those runs described and how does ESM4 compare to other climate models in climate sensitivity. A littel bit more information should be provided here.
Line 196-200 Not clear what the difference between the method here and the previous method is. For instance Liu et al. 2012 also corrected with the difference between the climatologies. Alexander et al. however used the differences between the mean. I don't understand how this would assume that the climate variability remains the same as in the historical period. Correcting with mean or climatology does not constrain the climate variability.
Line 204-206 This argument is not convincing. Jackson and Wood do not really assess runoff uncertainty and Giuntoli argued that hydrological models add uncertainty in addition to the contribution of GCMs. Following just the latter argument you may also disregard the GCM contribution. If there is not meaningful signal and only uncertainty is added, projected runoff could be neglected. But I doubt that there is not meaningful runoff signal, since sea level projections nowadays include contributions from glacial and ice sheet melt and a clear positive tend in river discharge is visible already now (https://doi.org/10.5194/hess-28-2179-2024). So, neglecting the runoff contribution is likely to reduce realism rather than uncertainty.
Line 216-223 You may want to mention other components that are not included in the sea level projections, particularly since you refer to this information in the conclusions.
Line 241-242 I think it is mostly broader, I am not sure if I see the shift away from the coast.
Line 315-319 What are "these regions" over which the temperature change was calculated?
Line 360-363 These two statements seem to contradict each other. Fig.7 is from the end of the century and I believe to see gradual differences. I suggest to point to the gradual differences visible in Fig.7 but then also to the surprisingly similar development until 2070 seen in Fig. 8. Additionaly mention here when the scenario forcings start to differ, because later you use an argument of delayed response that requires the reader to know until when scenarios were still similar.
Line 373-366 Is the reduction really significant given the error bars? Given the error bar it is probably more appropriate to say that the Antilles Current disappears (no significant mean transport).
Line 383 Point to Fig. S6 already here
Line 368-388 Do the lateral boundary conditions make NWA12 entirely a slave of the ESM4.1 for AMOC? You may want say something about this.
Line 398-399 not shown?
Line 405-406 How much is AMOC vs gyre weakening? Although ultimately as Yin et al. (2010) suggested the AMOC may be the origin of the changes, it would be useful to mention changes in the barotropic circulation of the subtropical gyre (stream function). because it seems that the transport change directly associated with AMOC weakening can account only for a fraction (<50% from the numbers reported on page 18 and the AMOC decline Fig.S6) of the reduced transports.
Line 454-455 How does geostrophic adjustment weakens upwelling. Isn't it more just the flattening of the isopynals.

Citation: https://doi.org/10.5194/egusphere-2025-6449-RC2
- AC2: 'Reply on RC2', Dongmin Kim, 11 Apr 2026
  
  The reply on RC2 is attached.
  
  Citation: https://doi.org/10.5194/egusphere-2025-6449-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Dongmin Kim on behalf of the Authors (13 Apr 2026) Author's response Author's tracked changes

EF by Mario Ebel (14 Apr 2026) Manuscript

ED: Referee Nomination & Report Request started (20 Apr 2026) by Anne Marie Treguier

RR by Anonymous Referee #2 (30 Apr 2026)

RR by Adrian New (15 May 2026)

Suggestions for revision or reasons for rejection

This manuscript analyses results from a high-resolution (1/12°) regional ocean model of the NW Atlantic forced by fluxes and boundary conditions from a lower-resolution fully-coupled earth system model. Specifically, four CMIP scenarios for different future warming levels are integrated out to 2100, and the work focuses on changes to currents, surface temperature, salinity, and sea-surface height (SSH). While others have already studied the resulting future reduction to the current systems (Atlantic Meridional Overturning Circulation (AMOC) and Western Boundary Current (WBC)) for the highest warming scenario (RCP8.5 and SSP585) in fully-coupled climate models (for CMIP5, ~1° ocean models, Beadling et al., 2018; and for CMIP6, ~1/4 – 1/12° ocean models, Roberts et al., 2020), the present investigation focuses on changes in the societally important near-coastal and shelf regions (Gulf of Mexico/America, West of Florida, South Atlantic Bight, Mid Atlantic Bight and Gulf of Maine), and investigates the range of possible conditions for the four different warming scenarios. These regions are found to become warmer and saltier in the future (changes which could affect the marine ecosystems) and also to have higher sea-levels. This near-coastal/ shelf focus examined across multiple future warming scenarios has not been studied before to my knowledge, and could provide useful information about future conditions for marine planners and societal uses.

The revised version of the manuscript is now improved, and would be suitable for publication after further minor revision as described below.

The main issues from my first review concerned the impact of the flows from the north, and questions about the representation of sea-surface height (SSH), and these are addressed as follows:

1. Northern Inputs. The authors have added new text about the impact of the northern inputs in lines 539-554 (not lines 580-595 as they say in their response, which initially confused me). This is: “In addition, as discussed in New et al. (2021), the MAB and the Gulf of Maine are also strongly influenced by the Labrador Current and the Labrador Slope Water (LSLW). The Slope Current in MOM6-NWA12 shows a large bias in its position and the strength. More specifically, it is much weaker compared to that in GLORYS12, and is replaced by northward flow in the upper 400m or so (Supplementary Fig. 6). Another core of southward flow appears immediately shoreward of the Gulf Stream in MOM6-NWA12. Since it is positioned away from the continental slope (near 73°W), it is referred to as the northern recirculation flow of the Gulf Stream. In the future scenarios, both the northern recirculation flow and the Slope Current (below 600m) drastically weaken. The Gulf Stream also weakens and its core shifts shoreward. In the SSP370 and SSP585 scenarios, the Gulf Stream core is positioned along the continental slope. Thus, both the northern recirculation flow and the Slope Current (below 600m or so) completely disappear in those high emission scenarios. Therefore, despite a large bias in the location and strength of the Slope Current in MOM6-NWS12, we can still conclude that the future warming and saltening in the MAB, shown in Fig. 17, are the result of a compounding effect - a weakening and shoreward shift of the Gulf Stream combined with reduced advection of cold, fresh Labrador Sea waters.” So, while it is clear that the model does not reproduce the southward-flowing shelf-break jet in the upper 400 m (which should appear at the top of the shelf slope as part of their “slope current”), and this is a serious limitation to the realism of the model, there is a weakening of the deeper flows (500-1000m), and the role of these changing currents in the warming and salinification of the region is now reasonably discussed as suggested.

However, as the new text is quite substantial, I strongly recommend that their new figure (Supplementary figure 6, or R1) in association with this new text, is added as figure 18 to the main text, rather than being “relegated” to the supplementary material.

2. Sea-surface height (SSH). Following Steinberg et al. (2024), it is clear that the model does allow for local or regional changes in SSH due to steric effects (i.e. due to warming of the water column), which answers my question here, and this is now adequately discussed in the revised text, lines 225-235. In addition, it appears that changes in the Global Mean Sea Level (GMSL) cannot be properly represented in this regional model (forced by boundary conditions) so explaining why the model is constrained to maintain its overall sea-level as constant, with the GMSL being added on later to simulate the full effects.

Further minor issues

l. 156: insert the degree symbol, °, after “1/2”.

l. 188: remove the space between “2” and ”4”.

l. 200: “period” not “frequency”.

Fig 4 caption: use (c) not (C).

Formatting error: starting in section 3.1 and continuing throughout the manuscript: please close the gaps in all references to the component panels of the figures (e.g. not Fig. 2d, rather use Fig. 2d, etc). This also occurs in the text as well, e.g lines 352, 355, 394 etc etc.

ls. 342-344: Figure 6 shows the reduction in the loop current and Gulf Stream but where is the evidence that the increased SST and SSS in the SAB and West Florida comes from reduced upwelling. This is just an arbitrary statement which needs to be justified.

ls. 349-350: After “Warming via this mechanism is fortified by commensurate mean reductions of the advection of cold high-latitude waters from the Labrador Sea” add “as described further in the Discussion”.

Figure 6: where is panel f?

l. 359: there is warming in the Gulf of Maine, so change “almost completely absent” to “much reduced”.

ls. 407-408: rather than saying the AMOC is a contributor to the modulation of the WBC system, it’s the other way around i.e. the WBC components contribute to the AMOC.

No panel (f) in fig. 11 or fig. 12?

ls. 436-437: use “MAB” not “north of Cape Hatteras” etc for consistency with the surrounding sentences.

ls. 489-490: presumably the mass redistribution from the open ocean to the coast is in the upper layers, as the opposite would be occurring deeper down?

l. 503: Say that “dynamic” ssh is that arising from changes in the currents.

l. 504: Specify that GMSL is “Global Mean Sea Level”.

l. 506: say “even though GMSL ….”

l. 509: say “projected GMSL …”

l. 551: “NWA12” not “NWS12”.

l. 575: say “drive” not “drives”

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ED: Reconsider after major revisions (17 May 2026) by Anne Marie Treguier

AR by Dongmin Kim on behalf of the Authors (29 May 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (31 May 2026) by Anne Marie Treguier

RR by Anonymous Referee #2 (01 Jun 2026)

RR by Adrian New (04 Jun 2026)

ED: Publish subject to technical corrections (10 Jun 2026) by Anne Marie Treguier

AR by Dongmin Kim on behalf of the Authors (10 Jun 2026) Manuscript

Journal article(s) based on this preprint

26 Jun 2026

Dynamically downscaled future projections of the Northwest Atlantic Ocean across low to high emissions scenarios

Dongmin Kim, Andrew C. Ross, Sang-Ik Shin, Fabian A. Gomez, Jasmin G. John, Denis L. Volkov, Sang-Ki Lee, Michael A. Alexander, and Charles A. Stock

Ocean Sci., 22, 1987–2009, https://doi.org/10.5194/os-22-1987-2026,https://doi.org/10.5194/os-22-1987-2026, 2026

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Dongmin Kim, Andrew C. Ross, Sang-Ik Shin, Fabian A. Gomez, Jasmin G. John, Denis L. Volkov, Sang-Ki Lee, Michael A. Alexander, and Charles A. Stock

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https://doi.org/10.5194/egusphere-2025-6449-supplement

Dongmin Kim, Andrew C. Ross, Sang-Ik Shin, Fabian A. Gomez, Jasmin G. John, Denis L. Volkov, Sang-Ki Lee, Michael A. Alexander, and Charles A. Stock

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Using high-resolution MOM6, we projected Northwest Atlantic changes under four SSP scenarios. Results show a weakening Gulf Stream reduces upwelling, causing significant shelf warming and salinification. This also leads to dynamic sea-level rise along the U.S. East Coast, particularly in the South Atlantic Bight, with critical implications for marine ecosystems and coastal risks.


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