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Intense mixing of lower thermocline water on the crest of the Mid-Atlantic Ridge

Abstract

Buoyancy exchange between the deep and the upper ocean, which is essential for maintaining global ocean circulation, mainly occurs through turbulent mixing1,2. This mixing is thought to result primarily from instability of the oceanic internal wave field3, but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence4 and the resulting distribution of turbulent mixing remains unknown. Another, more direct, mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography, but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements. Here we present observations from the crest of the Mid-Atlantic Ridge in the subtropical North Atlantic Ocean that suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Our measurements show that diffusivities as large as 0.03 m2 s-1 characterize the mixing downstream of a sill in a well-stratified boundary layer, with mixing levels remaining of the order of 10-4 m2 s-1 at the base of the main thermocline. These mixing rates are significantly higher than the diffusivities of the order of 10-5 m2 s-1 that characterize much of the global thermocline and the abyssal ocean5. Our estimates suggest that overflows associated with narrow passages on the Mid-Atlantic Ridge in the North Atlantic Ocean produce as much buoyancy flux as has previously been estimated for the entire Romanche fracture zone6,7, a large strait in the Mid-Atlantic Ridge that connects the North and South Atlantic basins. This flux is equivalent to the interior mixing that occurs in the entire North Atlantic basin at the depth of the passages, suggesting that turbulence generated in narrow passages on mid-ocean ridges may be important for buoyancy flux at the global scale.

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Figure 1: Regional setting (box) and topography (inset) of the Lucky Strike segment.
Figure 2: Velocity and neutral-density contours along the deep passage east of the Lucky Strike volcano.
Figure 3: Turbulence dissipation-rate profiles along the eastern passage at the Lucky Strike site.
Figure 4: Turbulent diffusivity estimates for station groups at the upstream, sill and downstream passage locations.

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Acknowledgements

We thank V. Ballu for inviting us to join the GRAVILUCK cruise. We also thank P. Bouruet-Aubertot, G. Reverdin, and the scientific staff and crew of the N/O Atalante for their assistance during the field programme. Technical support of the DMP by R. Lueck, F. Wolk, and P. Stern of Rockland Scientific, and by E. Howarth of FSU, was invaluable. E. Kunze provided comments on the early draft of the manuscript. The FSU turbulence instrumentation programme is supported by the US Office of Naval Research. The LDEO lowered ADCP programme, and our participation in GRAVILUCK, was sponsored by the US National Science Foundation.

Author Contributions L.C.StL. and A.M.T. contributed equally to this work. L.C.StL. led the microstructure sampling programme, and analysed the turbulence data. A.M.T. led the lowered ADCP measurement programme, and analysed the velocity data.

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Correspondence to Louis C. St Laurent.

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St Laurent, L., Thurnherr, A. Intense mixing of lower thermocline water on the crest of the Mid-Atlantic Ridge. Nature 448, 680–683 (2007). https://doi.org/10.1038/nature06043

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