Abyssal Mixing: Where It Is Not

Abstract

A parameterization based on internal wave/wave interaction theory, which infers turbulence production from finescale internal wave shear, is applied to 114 full-water-depth velocity profiles in the Sargasso Sea. An average eddy diffusivity of 0.1 ? 10?4 m2 s?1, independent of depth, is inferred. This value is consistent with full-water-depth microstructure measurements from abyssal basins in the eastern North Atlantic and eastern North Pacific. It is an order of magnitude smaller than the values inferred from a simple vertical advection-diffusion balance or bulk budgets. Thus, the mixing needed to close deep global water-mass budgets does not appear to occur over midlatitude abyssal plains. This suggests that ocean mixing is either (i) confined to boundary layers as in ideal thermocline theory or (ii) localized to hotspots, such as over rough topography or restrictive passages. Abyssal diffusivities do not display any dependence on bottom slope for slopes less than 7 ? 10?2 based on 5?10 km bathymetry, but are higher over convex than concave topography and higher in stronger bottom currents.