the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Jan 2026
Dynamically Downscaled Future Projections of the Northwest Atlantic Ocean Across Low to High Emissions Scenarios
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.
Competing interests: Dr. Charles A. Stock (one of co-authors) serves as editor for the special issue to which this paper belongs.
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.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6449', Anonymous Referee #1, 13 Feb 2026
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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.
DetailsLine 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
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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:
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).