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Changing Arctic Ocean freshwater pathways

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Abstract

Freshening in the Canada basin of the Arctic Ocean began in the 1990s1,2 and continued3 to at least the end of 2008. By then, the Arctic Ocean might have gained four times as much fresh water as comprised the Great Salinity Anomaly4,5of the 1970s, raising the spectre of slowing global ocean circulation6. Freshening has been attributed to increased sea ice melting1 and contributions from runoff7, but a leading explanation has been a strengthening of the Beaufort High—a characteristic peak in sea level atmospheric pressure2,8—which tends to accelerate an anticyclonic (clockwise) wind pattern causing convergence of fresh surface water. Limited observations have made this explanation difficult to verify, and observations of increasing freshwater content under a weakened Beaufort High suggest that other factors2 must be affecting freshwater content. Here we use observations to show that during a time of record reductions in ice extent from 2005 to 2008, the dominant freshwater content changes were an increase in the Canada basin balanced by a decrease in the Eurasian basin. Observations are drawn from satellite data (sea surface height and ocean-bottom pressure) and in situ data. The freshwater changes were due to a cyclonic (anticlockwise) shift in the ocean pathway of Eurasian runoff forced by strengthening of the west-to-east Northern Hemisphere atmospheric circulation characterized by an increased Arctic Oscillation9 index. Our results confirm that runoff is an important influence on the Arctic Ocean and establish that the spatial and temporal manifestations of the runoff pathways are modulated by the Arctic Oscillation, rather than the strength of the wind-driven Beaufort Gyre circulation.

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Figure 1: 2008 Arctic Ocean salinity anomaly and geostrophic velocity at 50–60 m depth.
Figure 2: Rates of change between 2005 and 2008 of DOT, SPA and freshwater content.
Figure 3: 2006–2008 anomalies relative to 2004–2005 averages of SLP, DOT and SPA.
Figure 4: Schematic views of the idealized Arctic Ocean circulation patterns under low and high AO anomalies.

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Acknowledgements

This work was supported chiefly by NSF grants OPP 0352754, ARC-0634226, ARC-0856330 and NASA grant NNX08AH62G. R.K. was supported at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. GRACE ocean data were processed by D. P. Chambers, supported by the NASA MEASURES Program. We thank the NASA ICESat and GRACE programmes, K. Falkner, R. Collier, M. McPhee, W. Ermold, L. de Steur, A. Proshutinsky and the Beaufort Gyre Exploration Project, J. Toole and R. Krishfield and the Ice Tethered Profiler project at WHOI, and W. Smethie of the Switchyard project for the observations that made this work possible.

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Contributions

The main idea was developed by J.M. and R.K. J.M. wrote most of the text and with R.A. and C.P.-F. drew most of the figures. R.K. developed the DOT records. The SLP and OBP anomaly plots were originally developed by C.P.-F. The 2008 hydrography observations were made by J.M., M.A., R.A. and M.S. The AO spatial pattern data, figures and insight were provided by I.R. The hydrographic data processing was done by R.A. and the chemistry analysis was done by M.A. Switchyard data and freshwater insight was provided by M.S. All authors discussed the results and commented on the manuscript.

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Correspondence to James Morison.

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The authors declare no competing financial interests.

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This file contains Supplementary Text and Data, Supplementary References and Supplementary Figures 1-13 with legends. (PDF 13357 kb)

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Morison, J., Kwok, R., Peralta-Ferriz, C. et al. Changing Arctic Ocean freshwater pathways. Nature 481, 66–70 (2012). https://doi.org/10.1038/nature10705

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