Transformation and upwelling of bottom water in fracture zone valleys

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Date
2020-03-03
Authors
Thurnherr, Andreas M.
Clément, Louis
St. Laurent, Louis C.
Ferrari, Raffaele
Ijichi, Takashi
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DOI
10.1175/JPO-D-19-0021.1
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Keywords
Diapycnal mixing
Topographic effects
Turbulence
Upwelling/downwelling
Bottom currents/bottom water
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 midocean ridges, which cover more than one-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.
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Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(3), (2020): 715-726, doi:10.1175/JPO-D-19-0021.1.
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Citation
Thurnherr, A. M., Clement, L., St Laurent, L., Ferrari, R., & Ijichi, T. (2020). Transformation and upwelling of bottom water in fracture zone valleys. Journal of Physical Oceanography, 50(3), 715-726.
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