Diapycnal mixing associated with an overflow in a deep submarine canyon

In order to close the global overturning circulation the buoyancy loss to the atmosphere associated with the formation of deep and bottom water at high latitudes must be balanced by buoyancy gain elsewhere. In case of water that is not in contact with the atmosphere except in its formation region the required buoyancy gain is accomplished primarily by diapycnal mixing. Recent observations suggest that a significant portion of the diapycnal buoyancy fluxes in the abyss is associated with overflows in submarine valleys or canyons, especially on the flanks of slow-spreading mid-ocean ridges. In the present study, hydrographic and velocity data from an overflow across a sill rising 1000 m above the floor of the rift valley of the Mid-Atlantic Ridge near 36°N were used to infer the associated diapycnal mixing. The cross-sill density gradients are characterized by a vertical dipole, with an upper layer of streamwise density increase overlying a lower layer where the density decreases along the flow. This is qualitatively consistent with the effects of diapycnal mixing transferring buoyancy from the upper into the lower layer. Hydrographic and velocity profiles provide evidence for strong mixing associated with the overflow across the sill: in addition to thick layers that are susceptible to shear instability, there are numerous density overturns indicating a spatially averaged diffusivity of order 10-2m2s-1. Inversions of the density equation reveal that the cross-sill gradients can be accounted for either by adiabatic vertical advection or by vertical eddy diffusion, although the adiabatic solutions are considered energetically implausible because they require upwelling in the layer of cross-sill density increase. Eddy-diffusive solutions, both with and without adiabatic downwelling, are characterized by diffusivity profiles attaining local maxima of order 10-2m2s-1≈250m above the sill depth. The vertical structure of the inversion-derived diffusivities is consistent with the Thorpe-scale based estimates and with observations from overflows in major ocean passages, which also are often characterized by local diffusivity maxima of order 10-2m2s-1 a few 100 m above sill depth. Combining the diffusivity profiles near the sill with budget-derived bulk estimates for 150 km of rift valley implies that ≈50% of the mixing in the rift valley is associated with overflows, while the remainder is caused by other processes, including the breaking of tidally forced internal waves.