Transformation and upwelling of bottom water in fracture zone valleys Thurnherr, Andreas M. Clément, Louis St. Laurent, Louis C. Ferrari, Raffaele Ijichi, Takashi 2020-04-28T16:09:06Z 2020-04-28T16:09:06Z 2020-03-03
dc.description 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. en_US
dc.description.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. en_US
dc.description.sponsorship The data used in this study were collected in the context of several projects funded by the U.S. National Science Foundation (NSF), in particular BBTRE (OCE-9415589 and OCE-9415598) and DoMORE (OCE-1235094). Funding for the analysis was provided as part of the NSF DoMORE and DECIMAL (OCE-1735618) projects. Author Ijichi is a Japan Society for the Promotion of Science (JSPS) Overseas Research Fellow. Comments on an early draft of this paper by Jim Ledwell and Bryan Kaiser, as well as topical discussions with Jörn Callies and Trevor McDougall, are gratefully acknowledged. The paper was greatly improved during the review process, in particular because of the critical comments from one of the two anonymous reviewers. en_US
dc.identifier.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. en_US
dc.identifier.doi 10.1175/JPO-D-19-0021.1
dc.publisher American Meteorological Society en_US
dc.subject Diapycnal mixing en_US
dc.subject Topographic effects en_US
dc.subject Turbulence en_US
dc.subject Upwelling/downwelling en_US
dc.subject Bottom currents/bottom water en_US
dc.title Transformation and upwelling of bottom water in fracture zone valleys en_US
dc.type Article en_US
dspace.entity.type Publication
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