Off-bottom turbulence expansions of unbounded flow over a deep-ocean ridge

Internal wave breaking upon sloping seafloors is a potential source of turbulent mixing in the deep-ocean, but we lack details on off-bottom breaking. Turbulence processes are relevant for the dispersal away from the seafloor of suspended materials like those emanating from thermal vents and artificial mining activities. For the present study, high-resolution temperature sensors have been moored up to 406 m above a slope of a 2000 m deep crest of the Mid-Atlantic Ridge. In addition to familiar observations of on-slope propagating highly nonlinear bores dominating turbulent mixing near and in contact with a sloping seafloor, the present observations show occasionally larger than 100 m tall turbulence expansions between 100 and 250 m above the seafloor at the transition from on- to off-slope flow. The details of such turbulence expansion are reminiscent of an asymmetric quasi-mode-2 internal hydraulic jump, with some specific differences compared with near-surface hydraulic jumps. As the expansion is generated at the transition from weak to steep edge of a saw-tooth internal tidal wave, it leads turbulence and stratification in two directions: One down to the seafloor in the direction of tidal phase propagation; the other more horizontally and slightly upward associated with near-homogeneous overturning remaining well away from the seafloor while preceding and sharpening near-bottom frontal bores. The mean turbulence dissipation rates O(10−8–10−7 m2s−3) associated with these expansions are half an order of magnitude less than those of on-slope propagating near-bottom bores, while about equal in duration. As for bores, their appearance, intensity and timing vary every tidal cycle.