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Atlantic origin of observed and modelled freshwater anomalies in the Nordic Seas

Abstract

Between 1965 and 1990, the waters of the Nordic Seas and the subpolar basins of the North Atlantic Ocean freshened substantially1. The Arctic Ocean also became less saline over this time, as a consequence of increasing runoff1,2,3,4, but it is not clear whether flow from the Arctic Ocean was the main source of the Nordic Seas salinity anomaly. As a region of deep-water formation, the Nordic Seas are central to the Atlantic meridional overturning circulation, but this process is inhibited if the surface salinity is too low2. Here we use the instrumental record of Nordic Seas hydrography, along with a global ocean–sea-ice model hindcast simulation, to identify the sources and magnitude of freshwater that has accumulated in the Nordic Seas since 1950. We find that the freshwater anomalies within the Nordic Seas can mostly be explained by less salt entering the southern part of the basin with the relatively saline Atlantic inflow, with seemingly little contribution from the Arctic Ocean. We conclude that hydrographic changes in the Nordic Seas are primarily related to changes in the Atlantic Ocean. We infer that if the Atlantic inflow and Nordic Seas both freshen similarly, this would render the Atlantic meridional overturning circulation relatively insensitive to Nordic Seas freshwater content.

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Figure 1: Climatology of the Nordic Seas.
Figure 2: Observed freshwater (FW) storage of the Nordic Seas.
Figure 3: Observed hydrographic distribution of freshwater anomalies.
Figure 4: Nordic Seas freshwater variance and modelled freshwater budget.

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Acknowledgements

The observational data was provided by the Marine Research Institute; Institute of Marine Research; the Faroese Fisheries Laboratory; and the Geophysical Institute, University of Bergen, through the NISE project.

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Contributions

M.S.G. did the analyses, produced the figures and wrote the manuscript. T.E. supplied the initial idea for this study, produced figures and contributed to writing. J.E.Ø.N. provided the observational data and support in using the data and produced figures. E.B. provided the model simulation. All authors contributed ideas, discussed the results and clarified the implications throughout the study.

Corresponding author

Correspondence to Mirjam Sophia Glessmer.

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The authors declare no competing financial interests.

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Glessmer, M., Eldevik, T., Våge, K. et al. Atlantic origin of observed and modelled freshwater anomalies in the Nordic Seas. Nature Geosci 7, 801–805 (2014). https://doi.org/10.1038/ngeo2259

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