May 27, 2019: FB1 seminar

11:00, Lecture Hall, Düsternbrooker Weg 20

Closing the global overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling to shallower depths. In spite of several decades of research, where and how the buoyancy is gained and the water is transported upward remain topics of debate, not least because the available data generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we present hydrographic observations indicating that bottom waters are transformed to lighter densities and upwelled by mean along-valley flows in many of the fracture zones that corrugate the flanks of slow-spreading mid-ocean ridges, which cover more than half of the seafloor area of major ocean basins. The observations indicate that mean buoyancy-flux convergence inside the fracture zone valleys extends hundreds of meters above ~100-m-thick bottom boundary layers, where height-above-bottom averaged dissipation profiles show downward-decreasing turbulence levels. Together with critical-layer and hydraulic processes, vertical buoyancy flux convergence in these boundary layers accounts for the local transformation to lighter densities. When integrated across a valley, turbulent buoyancy fluxes show maxima near the crests of the sidewalls, consistent with net buoyancy flux convergence below, with little sensitivity of this pattern to the vertical structure of the assumed turbulence profiles, indicating that the observed quasi-steady along-valley velocities and density gradients represent cross-valley averages. We conclude that on large scales, fracture zone topography exerts a controlling influence on the transformation to lighter densities and upwelling of bottom waters in many areas of the global ocean.

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