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Transformation and upwelling of bottom water in fracture zone valleys

Transformation and upwelling of bottom water in fracture zone valleys
Transformation and upwelling of bottom water in fracture zone valleys
Closing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading mid-ocean ridges, which cover more than half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture-zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.
0022-3670
Thurnherr, A.M.
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Clément, L.
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Laurent, L. St.
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Ferrari, R.
d785df74-e6bb-4f6f-bd27-50ca42032e42
Ijichi, T.
0444d9ff-abe8-4a7e-a02e-861e19f1401b
Thurnherr, A.M.
36bc0299-6c80-4e1e-b34a-e457c54deb0e
Clément, L.
bd1aafc4-3240-46e3-88d4-141f2b9cc1cc
Laurent, L. St.
7fb326a0-b0f1-4747-bb93-19d820bcb204
Ferrari, R.
d785df74-e6bb-4f6f-bd27-50ca42032e42
Ijichi, T.
0444d9ff-abe8-4a7e-a02e-861e19f1401b

Thurnherr, A.M., Clément, L., Laurent, L. St., Ferrari, R. and Ijichi, T. (2019) Transformation and upwelling of bottom water in fracture zone valleys. Journal of Physical Oceanography. (doi:10.1175/JPO-D-19-0021.1).

Record type: Article

Abstract

Closing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading mid-ocean ridges, which cover more than half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture-zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.

Text
upwelling - Accepted Manuscript
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e-pub ahead of print date: 27 December 2019

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Local EPrints ID: 437955
URI: http://eprints.soton.ac.uk/id/eprint/437955
ISSN: 0022-3670
PURE UUID: a9f01d5a-4ee9-4f09-b74e-acb8423fe16d

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Date deposited: 24 Feb 2020 17:31
Last modified: 27 Apr 2022 12:37

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Contributors

Author: A.M. Thurnherr
Author: L. Clément
Author: L. St. Laurent
Author: R. Ferrari
Author: T. Ijichi

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