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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Sam C. Jones, Neil J. Fraser, Stuart A. Cunningham, Alan D. Fox, and Mark E. Inall
Ocean Sci., 19, 169–192, https://doi.org/10.5194/os-19-169-2023, https://doi.org/10.5194/os-19-169-2023, 2023
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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.
Alan D. Fox, Patricia Handmann, Christina Schmidt, Neil Fraser, Siren Rühs, Alejandra Sanchez-Franks, Torge Martin, Marilena Oltmanns, Clare Johnson, Willi Rath, N. Penny Holliday, Arne Biastoch, Stuart A. Cunningham, and Igor Yashayaev
Ocean Sci., 18, 1507–1533, https://doi.org/10.5194/os-18-1507-2022, https://doi.org/10.5194/os-18-1507-2022, 2022
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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.
Kristin Burmeister, Neil James Fraser, Sam C. Jones, Stuart A. Cunningham, Lewis A. Drysdale, Mark E. Inall, Tiago S. Dotto, and N. Penny Holliday
EGUsphere, https://doi.org/10.5194/egusphere-2025-3167, https://doi.org/10.5194/egusphere-2025-3167, 2025
This preprint is open for discussion and under review for Ocean Science (OS).
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The Rockall Trough carries key ocean currents that affect Europe’s climate and seas. Researchers combined data from underwater sensors and robotic gliders to track water, heat, and freshwater flow from 2014 to 2022. They created a new method to merge this data, producing the first long-term record of one important current. This improves ocean monitoring and helps us better understand climate-related changes.
Donald Alexander Slater, Eleanor Johnstone, Martim Mas e Braga, Neil Fraser, Tom Cowton, and Mark Inall
EGUsphere, https://doi.org/10.5194/egusphere-2024-3934, https://doi.org/10.5194/egusphere-2024-3934, 2025
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Glacial fjords connect ice sheets to the ocean, controlling heat delivery to glaciers, which impacts ice sheet melt, and freshwater discharge to the ocean, affecting ocean circulation. However, their dynamics are not captured in large-scale climate models. We designed a simplified, computationally efficient model – FjordRPM – which accurately captures key fjord processes. It has direct applications for improving projections of ice melt, ocean circulation and sea-level rise.
Sam C. Jones, Neil J. Fraser, Stuart A. Cunningham, Alan D. Fox, and Mark E. Inall
Ocean Sci., 19, 169–192, https://doi.org/10.5194/os-19-169-2023, https://doi.org/10.5194/os-19-169-2023, 2023
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Short summary
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.
Alan D. Fox, Patricia Handmann, Christina Schmidt, Neil Fraser, Siren Rühs, Alejandra Sanchez-Franks, Torge Martin, Marilena Oltmanns, Clare Johnson, Willi Rath, N. Penny Holliday, Arne Biastoch, Stuart A. Cunningham, and Igor Yashayaev
Ocean Sci., 18, 1507–1533, https://doi.org/10.5194/os-18-1507-2022, https://doi.org/10.5194/os-18-1507-2022, 2022
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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.
Tillys Petit, M. Susan Lozier, Simon A. Josey, and Stuart A. Cunningham
Ocean Sci., 17, 1353–1365, https://doi.org/10.5194/os-17-1353-2021, https://doi.org/10.5194/os-17-1353-2021, 2021
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Recent work has highlighted the dominant role of the Irminger and Iceland basins in the production of North Atlantic Deep Water. From our analysis, we find that air–sea fluxes and the ocean surface density field are both key determinants of the buoyancy-driven transformation in the Iceland Basin. However, the spatial distribution of the subpolar mode water (SPMW) transformation is most sensitive to surface density changes as opposed to the direct influence of the air–sea fluxes.
Svein Østerhus, Rebecca Woodgate, Héðinn Valdimarsson, Bill Turrell, Laura de Steur, Detlef Quadfasel, Steffen M. Olsen, Martin Moritz, Craig M. Lee, Karin Margretha H. Larsen, Steingrímur Jónsson, Clare Johnson, Kerstin Jochumsen, Bogi Hansen, Beth Curry, Stuart Cunningham, and Barbara Berx
Ocean Sci., 15, 379–399, https://doi.org/10.5194/os-15-379-2019, https://doi.org/10.5194/os-15-379-2019, 2019
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Two decades of observations of the Arctic Mediterranean (AM) exchanges show that the exchanges have been stable in terms of volume transport during a period when many other components of the global climate system have changed. The total AM import is found to be 9.1 Sv and has a seasonal variation in amplitude close to 1 Sv, and maximum import in October. Roughly one-third of the imported water leaves the AM as surface outflow.
Robert Marsh, Ivan D. Haigh, Stuart A. Cunningham, Mark E. Inall, Marie Porter, and Ben I. Moat
Ocean Sci., 13, 315–335, https://doi.org/10.5194/os-13-315-2017, https://doi.org/10.5194/os-13-315-2017, 2017
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To the west of Britain and Ireland, a strong ocean current follows the steep slope that separates the deep Atlantic and the continental shelf. This “Slope Current” exerts an Atlantic influence on the North Sea and its ecosystems. Using a combination of computer modelling and archived data, we find that the Slope Current weakened over 1988–2007, reducing Atlantic influence on the North Sea, due to a combination of warming of the subpolar North Atlantic and weakening winds to the west of Scotland.
D. A. Smeed, G. D. McCarthy, S. A. Cunningham, E. Frajka-Williams, D. Rayner, W. E. Johns, C. S. Meinen, M. O. Baringer, B. I. Moat, A. Duchez, and H. L. Bryden
Ocean Sci., 10, 29–38, https://doi.org/10.5194/os-10-29-2014, https://doi.org/10.5194/os-10-29-2014, 2014
Related subject area
Approach: In situ Observations | Properties and processes: Overturning circulation, gyres and water masses
On mode water formation and erosion in the Arabian Sea: forcing mechanisms, regionality, and seasonality
Hydrographic section along 55° E in the Indian and Southern oceans
Circulation of Baffin Bay and Hudson Bay waters on the Labrador shelf and into the subpolar North Atlantic
Mechanisms of the Overturning Circulation in the Northern Red Sea, more than Convective Mixing
Continued warming of deep waters in the Fram Strait
Observed change and the extent of coherence in the Gulf Stream system
Anomalous North Pacific subtropical mode water volume and density decrease in a recent stable Kuroshio Extension period from Argo observations
New insights into the eastern subpolar North Atlantic meridional overturning circulation from OVIDE
The Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength
Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26° N
Water properties and bottom water patterns in hadal trench environments
Long-term eddy modulation affects the meridional asymmetry of the halocline in the Beaufort Gyre
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The Iceland–Faroe warm-water flow towards the Arctic estimated from satellite altimetry and in situ observations
Estel Font, Sebastiaan Swart, Puthenveettil Narayana Vinayachandran, and Bastien Y. Queste
Ocean Sci., 21, 1349–1368, https://doi.org/10.5194/os-21-1349-2025, https://doi.org/10.5194/os-21-1349-2025, 2025
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Mode water is formed annually and sits between the warm surface water and deeper older waters. In the Arabian Sea, it plays a crucial role in regulating ocean heat and oxygen variability by acting as a doorway between the surface and deeper waters. Using observations and models, we show that its formation is primarily driven by atmospheric forcing, though ocean currents, eddies, and biological heating also influence its life cycle. This water mass contributes up to 40 % of the region's oxygen content.
Katsuro Katsumata, Shigeru Aoki, Kay I. Ohshima, and Michiyo Yamamoto-Kawai
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Ship-based observations provide data of seawater properties like temperature, salinity, nutrients, and various gases, but some important world oceans have still not been covered. A voyage in 2019/20 in the southwest Indian Ocean along approximately 55° E from 30° S to Antarctica attempted to fill one such data-sparse region. The measured cross section of the Antarctic Circumpolar Current and accompanying eddies demonstrates various oceanic behaviours including fronts and eddy mixing.
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Salar Karam, Céline Heuzé, Mario Hoppmann, and Laura de Steur
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The Gulf Stream system is essential for northward ocean heat transport. Here, we use observations along the path of the extended Gulf Stream system and an observationally constrained ocean model to investigate variability in the Gulf Stream system since the 1990s. We find regional differences in the variability between the subtropical, subpolar, and Nordic Seas regions, which warrants caution in using observational records at a single latitude to infer large-scale circulation change.
Jing Sheng, Cong Liu, Yanzhen Gu, Peiliang Li, Fangguo Zhai, and Ning Zhou
Ocean Sci., 20, 817–834, https://doi.org/10.5194/os-20-817-2024, https://doi.org/10.5194/os-20-817-2024, 2024
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The homogeneous water column, named mode water, retains atmosphere conditions and biogeochemical elements from the deep winter mixed layer and became weaker and warmer in the North Pacific subtropical ocean in 2018–2021 even though the Kuroshio Extension was stable. Locally anomalous east wind transporting warm water to the north and enhanced near-surface stratification hinder the deepening of the winter mixed layer. This study has broad implications for climate change and biogeochemical cycles.
Herlé Mercier, Damien Desbruyères, Pascale Lherminier, Antón Velo, Lidia Carracedo, Marcos Fontela, and Fiz F. Pérez
Ocean Sci., 20, 779–797, https://doi.org/10.5194/os-20-779-2024, https://doi.org/10.5194/os-20-779-2024, 2024
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We study the Atlantic Meridional Overturning Circulation (AMOC) measured between Greenland and Portugal between 1993–2021. We identify changes in AMOC limb volume and velocity as two major drivers of AMOC variability at subpolar latitudes. Volume variations dominate on the seasonal timescale, while velocity variations are more important on the decadal timescale. This decomposition proves useful for understanding the origin of the differences between AMOC time series from different analyses.
Romain Caneill, Fabien Roquet, and Jonas Nycander
Ocean Sci., 20, 601–619, https://doi.org/10.5194/os-20-601-2024, https://doi.org/10.5194/os-20-601-2024, 2024
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In winter, heat loss increases density at the surface of the Southern Ocean. This increase in density creates a mixed layer deeper than 250 m only in a narrow deep mixing band (DMB) located around 50° S. North of the DMB, the stratification is too strong to be eroded, so mixed layers are shallower. The density of cold water is almost not impacted by temperature changes. Thus, heat loss does not significantly increase the density south of the DMB, so no deep mixed layers are produced.
Harry Bryden, Jordi Beunk, Sybren Drijfhout, Wilco Hazeleger, and Jennifer Mecking
Ocean Sci., 20, 589–599, https://doi.org/10.5194/os-20-589-2024, https://doi.org/10.5194/os-20-589-2024, 2024
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There is widespread interest in whether the Gulf Stream will decline under global warming. We analyse 19 coupled climate model projections of the AMOC over the 21st century. The model consensus is that the AMOC will decline by about 40 % due to reductions in northward Gulf Stream transport and southward deep western boundary current transport. Whilst the wind-driven Gulf Stream decreases by 4 Sv, most of the decrease in the Gulf Stream is due to a reduction of 7 Sv in its thermohaline component.
Jessica Kolbusz, Jan Zika, Charitha Pattiaratchi, and Alan Jamieson
Ocean Sci., 20, 123–140, https://doi.org/10.5194/os-20-123-2024, https://doi.org/10.5194/os-20-123-2024, 2024
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We collected observations of the ocean environment at depths over 6000 m in the Southern Ocean, Indian Ocean, and western Pacific using sensor-equipped landers. We found that trench locations impact the water characteristics over these depths. Moving northward, they generally warmed but differed due to their position along bottom water circulation paths. These insights stress the importance of further research in understanding the environment of these deep regions and their importance.
Jinling Lu, Ling Du, and Shuhao Tao
Ocean Sci., 19, 1773–1789, https://doi.org/10.5194/os-19-1773-2023, https://doi.org/10.5194/os-19-1773-2023, 2023
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With the recent developments in observations and reanalysis data in the Beaufort Gyre, we investigate an improved understanding of eddy activity and asymmetrical halocline variability in the upper ocean. The halocline structures on the southern and northern sides of the central gyre have tended to be identical since 2014. The results suggest that enhanced eddy modulation through eddy fluxes influences oceanic stratification, resulting in reduced meridional asymmetry of the halocline.
Enrico P. Metzner and Marc Salzmann
Ocean Sci., 19, 1453–1464, https://doi.org/10.5194/os-19-1453-2023, https://doi.org/10.5194/os-19-1453-2023, 2023
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The Arctic Ocean cold halocline separates the cold surface mixed layer from the underlying warm Atlantic Water, and thus provides a precondition for sea ice formation. Here, we introduce a new method for detecting the halocline base and compare it to two existing methods. We show that the largest differences between the methods are found in the regions that are most prone to a halocline retreat in a warming climate, and we discuss the advantages and disadvantages of the three methods.
Bogi Hansen, Karin M. H. Larsen, Hjálmar Hátún, Steffen M. Olsen, Andrea M. U. Gierisch, Svein Østerhus, and Sólveig R. Ólafsdóttir
Ocean Sci., 19, 1225–1252, https://doi.org/10.5194/os-19-1225-2023, https://doi.org/10.5194/os-19-1225-2023, 2023
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Based on in situ observations combined with sea level anomaly (SLA) data from satellite altimetry, volume as well as heat (relative to 0 °C) transport of the Iceland–Faroe warm-water inflow towards the Arctic (IF inflow) increased from 1993 to 2021. The reprocessed SLA data released in December 2021 represent observed variations accurately. The IF inflow crosses the Iceland–Faroe Ridge in two branches, with retroflection in between. The associated coupling to overflow reduces predictability.
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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...
