Transformation and Upwelling of Bottom Water in Fracture Zone Valleys

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.

A Parameterization of Local and Remote Tidal Mixing

“…Microstructure‐derived dissipations also have intrinsic uncertainty, thought to be about a factor of 2, attributable in part to sensor calibration and to the translation of microscale shear spectra into frictional dissipation rates (Gregg, ; Toole et al, ). We note that a recent reinterpretation of the DoMORE raw data (see Appendix in Thurnherr et al, ) produces dissipation rates (used here) that are larger than those initially estimated (Clément et al, ), leading to better agreement with BBTRE and with the present parameterization. An extra source of uncertainty lies in the assumed mixing efficiency R f =1/6.…”

“…Our compilation of microstructure data (Figure a) includes field campaigns described and analyzed by Waterhouse et al () as well as data from eight additional projects: INDOMIX (Bouruet‐Aubertot et al, ); OUTPACE (Bouruet‐Aubertot et al, ); three cruises over the Izu‐Ogasawara ridge, hereafter referred to as IZU (Hibiya et al, ); DoMORE (Thurnherr et al, ); RidgeMix (Vic et al, ); OVIDE (Ferron et al, ); RREX (Petit et al, ); and PROVOLO (Fer et al, ). The compilation encompasses a total of 19 campaigns cumulating 1,171 microstructure profiles.…”

A parameterization of local and remote tidal mixing

“…Microstructure-derived dissipations also have intrinsic uncertainty, thought to be about a factor of 2, attributable in part to sensor calibration and to the translation of microscale shear spectra into frictional dissipation rates (Gregg, 1999;Toole et al, 1994). We note that a recent reinterpretation of the DoMORE raw data (see Appendix in Thurnherr et al, 2020) produces dissipation rates (used here) that are larger than those initially estimated (Clément et al, 2017), leading to better agreement with BBTRE and with the present parameterization. An extra source of uncertainty lies in the assumed mixing efficiency R f = 1∕6.…”

“…Our compilation of microstructure data (Figure 7a) includes field campaigns described and analyzed by Waterhouse et al (2014) as well as data from eight additional projects: INDOMIX (Bouruet-Aubertot et al, 2018a); OUTPACE (Bouruet-Aubertot et al, 2018b); three cruises over the Izu-Ogasawara ridge, hereafter referred to as IZU (Hibiya et al, 2012); DoMORE (Thurnherr et al, 2020); RidgeMix (Vic et al, 2018); OVIDE (Ferron et al, 2014); RREX (Petit et al, 2018); and PROVOLO (Fer et al, 2019). The compilation encompasses a total of 19 campaigns cumulating 1,171 microstructure profiles.…”

Control of the abyssal ocean overturning circulation by mixing-driven bottom boundary layers

“…For example, any of the observations of bottom-enhanced mixing, stratification, and diabatic upwelling from the region are confined to (500 m)-deep fracture zone canyons which cut across the ridge (Polzin et al, 1997;Ledwell et al, 2000;St. Laurent et al, 2001;Thurnherr et al, 2020).…”