Articles | Volume 21, issue 4
https://doi.org/10.5194/os-21-1735-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-21-1735-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Research article
| 
11 Aug 2025
Research article |
| 11 Aug 2025
| 11 Aug 2025
Seasonality of meridional overturning in the subpolar North Atlantic: density flux as a metric for understanding the Atlantic meridional overturning circulation
Scottish Association for Marine Science, Oban, UK
Neil J. Fraser
Scottish Association for Marine Science, Oban, UK
Stuart A. Cunningham
Scottish Association for Marine Science, Oban, UK
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Warm water is transported from the tropical Atlantic towards western Europe and the Arctic. It loses heat to the atmosphere on the way, which strongly influences the climate. We construct a dataset encircling the North Atlantic basin north of 47° N. We calculate how and where heat enters and leaves the basin and how much cooling must happen in the interior. We find that cooling in the north-eastern Atlantic is a crucial step in controlling the conversion of water to higher densities.
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Observations of the eastern subpolar North Atlantic in the 2010s show exceptional freshening and cooling of the upper ocean, peaking in 2016 with the lowest salinities recorded for 120 years. Using results from a high-resolution ocean model, supported by observations, we propose that the leading cause is reduced surface cooling over the preceding decade in the Labrador Sea, leading to increased outflow of less dense water and so to freshening and cooling of the eastern subpolar North Atlantic.
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Cited articles
Baker, J. A., Bell, M. J., Jackson, L. C., Renshaw, R., Vallis, G. K., Watson, A. J., and Wood, R. A.: Overturning pathways control AMOC weakening in CMIP6 models, Geophys. Res. Lett., 50, e2023GL103381, https://doi.org/10.1029/2023gl103381, 2023. a
Baker, J. A., Bell, M. J., Jackson, L. C., Vallis, G. K., Watson, A. J., and Wood, R. A.: Continued Atlantic overturning circulation even under climate extremes, Nature, 638, 987–994, https://doi.org/10.1038/s41586-024-08544-0, 2025. a, b
Berry, D. I. and Kent, E. C.: A new air–sea interaction gridded dataset from ICOADS with uncertainty estimates, B. Am. Meteorol. Soc., 90, 645–656, https://doi.org/10.1175/2008BAMS2639.1, 2009. a
Biastoch, A., Schwarzkopf, F. U., Getzlaff, K., Rühs, S., Martin, T., Scheinert, M., Schulzki, T., Handmann, P., Hummels, R., and Böning, C. W.: Regional imprints of changes in the Atlantic Meridional Overturning Circulation in the eddy-rich ocean model VIKING20X, Ocean Sci., 17, 1177–1211, https://doi.org/10.5194/os-17-1177-2021, 2021. a, b, c
Bryden, H. L. and Hall, M. M.: Heat transport by currents across 25° N latitude in the Atlantic Ocean, Science, 207, 884–886, https://doi.org/10.1126/science.207.4433.884, 1980. a
Böning, C. W., Behrens, E., Biastoch, A., Getzlaff, K., and Bamber, J. L.: Emerging impact of Greenland meltwater on deepwater formation in the North Atlantic Ocean, Nat. Geosci., 9, 523–527, https://doi.org/10.1038/ngeo2740, 2016. a
Chafik, L. and Lozier, M. S.: When simplification leads to ambiguity: a look at two ocean metrics for the subpolar North Atlantic, Geophys. Res. Lett., 52, e2024GL112496, https://doi.org/10.1029/2024GL112496, 2025. a, b
Chidichimo, M. P., Kanzow, T., Cunningham, S. A., Johns, W. E., and Marotzke, J.: The contribution of eastern-boundary density variations to the Atlantic meridional overturning circulation at 26.5° N, Ocean Sci., 6, 475–490, https://doi.org/10.5194/os-6-475-2010, 2010. a
Cunningham, S. A., Kanzow, T., Rayner, D., Baringer, M. O., Johns, W. E., Marotzke, J., Longworth, H. R., Grant, E. M., Hirschi, J. J., Beal, L. M., Meinen, C. S., and Bryden, H. L.: Temporal variability of the Atlantic meridional overturning circulation at 26.5°N, Science, 317, 935–938, https://doi.org/10.1126/science.1141304, 2007. a
Forget, G., Campin, J.-M., Heimbach, P., Hill, C. N., Ponte, R. M., and Wunsch, C.: ECCO version 4: an integrated framework for non-linear inverse modeling and global ocean state estimation, Geosci. Model Dev., 8, 3071–3104, https://doi.org/10.5194/gmd-8-3071-2015, 2015. a
Fox, A. D.: Python code and data for producing the results described in: Seasonality of meridional overturning in the subpolar North Atlantic: implications for relying on the streamfunction maximum as a metric of AMOC slowdown, Zenodo [code], https://doi.org/10.5281/zenodo.15350863, 2025. a
Fraser, N. J. and Cunningham, S. A.: 120 years of AMOC variability reconstructed from observations using the Bernoulli inverse, Geophys. Res. Lett., 48, e2021GL093893, https://doi.org/10.1029/2021GL093893, 2021. a
Fraser, N. J., Fox, A. D., Cunningham, S. A., Rath, W., Schwarzkopf, F. U., and Biastoch, A.: Vertical velocity dynamics in the North Atlantic and implications for AMOC, J. Phys. Oceanogr., 54, 2011–2024, https://doi.org/10.1175/JPO-D-23-0229.1, 2024. a, b
Fraser, N. J., Fox, A. D., and Cunningham, S. A.: Impact of Ekman pumping on the meridional coherence of the AMOC, Geophys. Res. Lett., 52, e2024GL108846, https://doi.org/10.1029/2024GL108846, 2025. a
Fu, Y., Lozier, M. S., Biló, T. C., Bower, A. S., Cunningham, S. A., Cyr, F., de Jong, M. F., deYoung, B., Drysdale, L., Fraser, N., Fried, N., Furey, H. H., Han, G., Handmann, P., Holliday, N. P., Holte, J., Inall, M. E., Johns, W. E., Jones, S., Karstensen, J., Li, F., Pacini, A., Pickart, R. S., Rayner, D., Straneo, F., and Yashayaev, I.: Seasonality of the Meridional Overturning Circulation in the subpolar North Atlantic, Communications Earth and Environment, 4, 1–13, https://doi.org/10.1038/s43247-023-00848-9, 2023a. a, b, c, d, e, f, g, h, i, j
Fu, Y., Lozier, M. S., Biló, T. C., Bower, A. S., Cunningham, S. A., Cyr, F., de Jong, M. F., deYoung, B., Drysdale, L., Fraser, N., Fried, N., Furey, H. H., Han, G., Handmann, P., Holliday, N. P., Holte, J., Inall, M. E., Johns, W. E., Jones, S., Karstensen, J., Li, F., Pacini, A., Pickart, R. S., Rayner, D., Straneo, F., and Yashayaev, I.: Meridional Overturning Circulation Observed by the Overturning in the Subpolar North Atlantic Program (OSNAP) Array from August 2014 to June 2020, GT library [data set], https://doi.org/10.35090/gatech/70342, 2023b. a, b
Gary, S. F., Cunningham, S. A., Johnson, C., Houpert, L., Holliday, N. P., Behrens, E., Biastoch, A., and Böning, C. W.: Seasonal cycles of oceanic transports in the eastern subpolar North Atlantic, J. Geophys. Res.-Oceans, 123, 1471–1484, https://doi.org/10.1002/2017JC013350, 2018. a, b
Getzlaff, K. and Schwarzkopf, F. U.: VIKING20X-JRA-short: daily to multi-decadal ocean dynamics under JRA55-do atmospheric forcing, World Data Center for Climate (WDCC) at DKRZ [data set], https://doi.org/10.26050/WDCC/VIKING20XJRAshort, 2024. a, b
Han, L.: Exploring the AMOC connectivity between the RAPID and OSNAP lines with a model-based data set, Geophys. Res. Lett., 50, e2023GL105225, https://doi.org/10.1029/2023GL105225, 2023a. a
Han, L.: Mechanism on the short-term variability of the Atlantic meridional overturning circulation in the subtropical and tropical regions, J. Phys. Oceanogr., 53, 2231–2244, https://doi.org/10.1175/JPO-D-23-0027.1, 2023b. a
Hersbach, H., Bell, B., Berrisford, P., Hirahara, S., Horányi, A., Muñoz-Sabater, J., Nicolas, J., Peubey, C., Radu, R., Schepers, D., Simmons, A., Soci, C., Abdalla, S., Abellan, X., Balsamo, G., Bechtold, P., Biavati, G., Bidlot, J., Bonavita, M., De Chiara, G., Dahlgren, P., Dee, D., Diamantakis, M., Dragani, R., Flemming, J., Forbes, R., Fuentes, M., Geer, A., Haimberger, L., Healy, S., Hogan, R. J., Hólm, E., Janisková, M., Keeley, S., Laloyaux, P., Lopez, P., Lupu, C., Radnoti, G., de Rosnay, P., Rozum, I., Vamborg, F., Villaume, S., and Thépaut, J. N.: The ERA5 global reanalysis, Q. J. Roy. Meteor. Soc., 146, 1999–2049, https://doi.org/10.1002/QJ.3803, 2020. a
Hersbach, H., Bell, B., Berrisford, P., Biavati, G., Horányi, A., Muñoz-Sabater, J., Nicolas, J., Peubey, C., Radu, R., Rozum, I., Schepers, D., Simmons, A., Soci, C., Dee, D., and Thépaut, J.-N.: ERA5 monthly averaged data on single levels from 1940 to present, Copernicus Climate Change Service (C3S) Climate Data Store (CDS) [data set], https://doi.org/10.24381/cds.f17050d7, 2023. a, b
Hirschi, J. J., Barnier, B., Böning, C., Biastoch, A., Blaker, A. T., Coward, A., Danilov, S., Drijfhout, S., Getzlaff, K., Griffies, S. M., Hasumi, H., Hewitt, H., Iovino, D., Kawasaki, T., Kiss, A. E., Koldunov, N., Marzocchi, A., Mecking, J. V., Moat, B., Molines, J. M., Myers, P. G., Penduff, T., Roberts, M., Treguier, A. M., Sein, D. V., Sidorenko, D., Small, J., Spence, P., Thompson, L. A., Weijer, W., and Xu, X.: The Atlantic meridional overturning circulation in high-resolution models, J. Geophys. Res.-Oceans, 125, e2019JC015522, https://doi.org/10.1029/2019JC015522, 2020. a
Johnson, H. L., Cessi, P., Marshall, D. P., Schloesser, F., and Spall, M. A.: Recent contributions of theory to our understanding of the Atlantic Meridional Overturning Circulation, J. Geophys. Res.-Oceans, 124, 5376–5399, https://doi.org/10.1029/2019JC015330, 2019. a
Kanzow, T., Cunningham, S. A., Rayner, D., Hirschi, J. J., Johns, W. E., Baringer, M. O., Bryden, H. L., Beal, L. M., Meinen, C. S., and Marotzke, J.: Observed flow compensation associated with the MOC at 26.5°N in the Atlantic, Science, 317, 938–941, https://doi.org/10.1126/science.1141293, 2007. a
Killworth, P. D.: A simple linear model of the depth dependence of the wind-driven variability of the Meridional Overturning Circulation, J. Phys. Oceanogr., 38, 492–502, https://doi.org/10.1175/2007JPO3811.1, 2008. a
Koman, G., Bower, A. S., Holliday, N. P., Furey, H. H., Fu, Y., and Biló, T. C.: Observed decrease in Deep Western Boundary Current transport in subpolar North Atlantic, Nat. Geosci. 2024 17:11, 17, 1148–1153, https://doi.org/10.1038/s41561-024-01555-6, 2024. a
Le Bras, I. A. A., Straneo, F., Holte, J., and Holliday, N. P.: Seasonality of freshwater in the East Greenland current system from 2014 to 2016, J. Geophys. Res.-Oceans, 123, 8828–8848, https://doi.org/10.1029/2018JC014511, 2018. a, b
Le Bras, I. A., Straneo, F., Holte, J., de Jong, M. F., and Holliday, N. P.: Rapid export of waters formed by convection near the Irminger Sea's Western Boundary, Geophys. Res. Lett., 47, e2019GL085989, https://doi.org/10.1029/2019GL085989, 2020. a, b
Lee, T. and Marotzke, J.: Seasonal cycles of Meridional Overturning and heat transport of the Indian Ocean, J. Phys. Oceanogr., https://journals.ametsoc.org/view/journals/phoc/28/5/1520-0485_1998_028_0923_scomoa_2.0.co_2.xml, 1998. a
Lozier, M. S., Li, F., Bacon, S., Bahr, F., Bower, A. S., Cunningham, S. A., de Jong, M. F., de Steur, L., DeYoung, B., Fischer, J., Gary, S. F., Greenan, B. J. W., Holliday, N. P., Houk, A., Houpert, L., Inall, M. E., Johns, W. E., Johnson, H. L., Johnson, C., Karstensen, J., Koman, G., Le Bras, I. A., Lin, X., Mackay, N., Marshall, D. P., Mercier, H., Oltmanns, M., Pickart, R. S., Ramsey, A. L., Rayner, D., Straneo, F., Thierry, V., Torres, D. J., Williams, R. G., Wilson, C., Yang, J., Yashayaev, I., and Zhao, J.: A sea change in our view of overturning in the subpolar North Atlantic, Science, 363, 516–521, https://doi.org/10.1126/science.aau6592, 2019. a, b, c, d, e
Madan, G., Gjermundsen, A., Iversen, S. C., and LaCasce, J. H.: The weakening AMOC under extreme climate change, Clim. Dynam., 62, 1291–1309, https://doi.org/10.1007/s00382-023-06957-7, 2023. a
McCarthy, G. D., Smeed, D. A., Johns, W. E., Frajka-Williams, E., Moat, B. I., Rayner, D., Baringer, M. O., Meinen, C. S., Collins, J., and Bryden, H. L.: Measuring the Atlantic Meridional Overturning Circulation at 26°N, Prog. Oceanogr., 130, 91–111, https://doi.org/10.1016/j.pocean.2014.10.006, 2015. a
Mercier, H., Desbruyères, D., Lherminier, P., Velo, A., Carracedo, L., Fontela, M., and Pérez, F. F.: New insights into the eastern subpolar North Atlantic meridional overturning circulation from OVIDE, Ocean Sci., 20, 779–797, https://doi.org/10.5194/os-20-779-2024, 2024. a, b, c, d, e, f, g, h, i, j
Nurser, A. J., Marsh, R., and Williams, R. G.: Diagnosing water mass formation from air-sea fluxes and surface mixing, J. Phys. Oceanogr., 29, 1468–1487, https://doi.org/10.1175/1520-0485(1999)029<1468:DWMFFA>2.0.CO;2, 1999. a, b
Petit, T., Lozier, M. S., Josey, S. A., and Cunningham, S. A.: Atlantic deep water formation occurs primarily in the Iceland Basin and Irminger Sea by local buoyancy forcing, Geophys. Res. Lett., 47, e2020GL091028, https://doi.org/10.1029/2020GL091028, 2020. a
Rühs, S., Oliver, E. C., Biastoch, A., Böning, C. W., Dowd, M., Getzlaff, K., Martin, T., and Myers, P. G.: Changing spatial patterns of deep convection in the subpolar North Atlantic, J. Geophys. Res.-Oceans, 126, e2021JC017245, https://doi.org/10.1029/2021JC017245, 2021. a
Speer, K. and Tziperman, E.: Rates of water mass formation in the North Atlantic Ocean, J. Phys. Oceanogr., 22, 93–104, https://doi.org/10.1175/1520-0485(1992)022<0093:rowmfi>2.0.co;2, 1992. a, b
Swingedouw, D., Braconnot, P., Delecluse, P., Guilyardi, E., and Marti, O.: Quantifying the AMOC feedbacks during a 2×CO2 stabilization experiment with land-ice melting, Clim. Dynam., 29, 521–534, https://doi.org/10.1007/s00382-007-0250-0, 2007. a
Tsujino, H., Urakawa, S., Nakano, H., Small, R. J., Kim, W. M., Yeager, S. G., Danabasoglu, G., Suzuki, T., Bamber, J. L., Bentsen, M., Böning, C. W., Bozec, A., Chassignet, E. P., Curchitser, E., Boeira Dias, F., Durack, P. J., Griffies, S. M., Harada, Y., Ilicak, M., Josey, S. A., Kobayashi, C., Kobayashi, S., Komuro, Y., Large, W. G., Le Sommer, J., Marsland, S. J., Masina, S., Scheinert, M., Tomita, H., Valdivieso, M., and Yamazaki, D.: JRA-55 based surface dataset for driving ocean–sea-ice models (JRA55-do), Ocean Model., 130, 79–139, https://doi.org/10.1016/j.ocemod.2018.07.002, 2018. a
Tziperman, E.: On the role of interior mixing and air-sea fluxes in determining the stratification and circulation of the oceans, J. Phys. Oceanogr., 16, 680–693, https://doi.org/10.1175/1520-0485(1986)016<0680:OTROIM>2.0.CO;2, 1986. a, b
Wang, H., Zhao, J., Li, F., and Lin, X.: Seasonal and interannual variability of the Meridional Overturning Circulation in the subpolar North Atlantic diagnosed from a high resolution reanalysis data set, J. Geophys. Res.-Oceans, 126, e2020JC017130, https://doi.org/10.1029/2020JC017130, 2021. a, b, c, d, e
Weijer, W., Cheng, W., Garuba, O. A., Hu, A., and Nadiga, B. T.: CMIP6 models predict significant 21st century decline of the Atlantic Meridional Overturning Circulation, Geophys. Res. Lett., 47, e2019GL086075, https://doi.org/10.1029/2019gl086075, 2020. a
Yang, J.: Local and remote wind stress forcing of the seasonal variability of the Atlantic Meridional Overturning Circulation (AMOC) transport at 26.5°N, J. Geophys. Res.-Oceans, 120, 2488–2503, https://doi.org/10.1002/2014JC010317, 2015. a
Zhao, J. and Johns, W.: Wind-driven seasonal cycle of the atlantic meridional overturning circulation, J. Phys. Oceanogr., 44, 1541–1562, https://doi.org/10.1175/JPO-D-13-0144.1, 2014. a, b
Short summary
Understanding the seasonality of the overturning circulation is important for mitigating the impacts of Atlantic meridional overturning circulation (AMOC) changes on European weather and climate. We examine the seasonal cycle in various common measures of overturning and find each to be dominated by different processes, not necessarily reflective of the processes driving overturning. We advocate for the use of a density flux measure as a valuable addition to understanding AMOC.
Understanding the seasonality of the overturning circulation is important for mitigating the...
