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Tide-mediated warming of Arctic halocline by Atlantic heat fluxes over rough topography

Abstract

The largest oceanic heat input to the Arctic Ocean results from inflowing Atlantic water. This inflowing water is warmer than it has been in the past 2,000 years1,2. Yet the fate of this heat remains uncertain3, partly because the water is relatively saline, and thus dense: it therefore enters the Arctic Ocean at intermediate depths and is separated from surface waters by stratification. Vertical mixing is generally weak within the Arctic Ocean basins, with very modest heat fluxes (0.05–0.3 W m−2) arising largely from double diffusion4,5,6,7,8. However, geographically limited observations have indicated substantially enhanced turbulent mixing rates over rough topography9,10,11,12,13,14. Here we present pan-Arctic microstructure measurements of turbulent kinetic energy dissipation. Our measurements further demonstrate that the enhanced continental slope dissipation rate, and by implication vertical mixing, varies significantly with both topographic steepness and longitude. Furthermore, our observations show that dissipation is insensitive to sea-ice conditions. We identify tides as the main energy source that supports the enhanced dissipation, which generates vertical heat fluxes of more than 50 W m−2. We suggest that the increased transfer of momentum from the atmosphere to the ocean as Arctic sea ice declines is likely to lead to an expansion of mixing hotspots in the future Arctic Ocean.

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Figure 1: Map of the Arctic Ocean showing the bathymetry and the location of the microstructure profiler measurements.
Figure 2: Variation of profile-average dissipation with bathymetry and topographic slope.
Figure 3: Transect-mean integrated AW dissipation, εAW, against longitude.

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Acknowledgements

Data collection and analysis by the Bangor team was funded through the UK Natural Environmental Research Council ASBO and TEA-COSI Consortia (PI SB). The Norwegian Polar Institute data collection was funded by the NPI Centre for Ice, Climate and Ecosystems (ICE). Y-D.L. was financially supported by a NERC Fellowship. VMP technical support was ably provided by B. Powell. The authors wish to thank the captain, crew, principal scientists and participating scientists of the Viktor Buynitsky, I/B Kapitan Dranitsyn, CCGS Louis S St-Laurent, RRS James Clark Ross and the RV Lance for their cooperation in the data collection. The authors gratefully acknowledge cruise funding from NERC, the US NABOS programme (http://nabos.iarc.uaf.edu), and the joint US-Canadian Beaufort Gyre Exploration Project (http://www.whoi.edu/beaufortgyre).

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S.B., T.P.R., Y-D.L., B.J.L. and A.S. planned and directed the measurements presented in this paper. The microstructure data collection and analysis was undertaken by B.J.L., Y-D.L., A.S. and T.P.R. The inverse tidal modelling was undertaken by J.A.M.G. T.P.R. wrote the first draft of the paper with all the authors contributing to its revision.

Corresponding author

Correspondence to Tom P. Rippeth.

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The authors declare no competing financial interests.

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Rippeth, T., Lincoln, B., Lenn, YD. et al. Tide-mediated warming of Arctic halocline by Atlantic heat fluxes over rough topography. Nature Geosci 8, 191–194 (2015). https://doi.org/10.1038/ngeo2350

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