What Sets the Bulk Stratification in the Abyss?

Abstract Turbulent mixing exerts a major control on ocean properties and, in particular, on the meridional overturning circulation. Thin and mostly unmapped layers at the bottom of the ocean are believed to host most of the mixing-driven upwelling of bottom water in the abyss, yet the magnitude of the upwelling in these thin boundary layers and the corresponding downwelling in the ocean interior remain unknown. Furthermore, it is not well understood what determines the strength of the abyssal stratification. The abyssal ocean is studied by assuming the known values of (i) the volume transport of the dense Antarctic Bottom Water plumes at the top of the abyss and (ii) the area-integrated diffusive turbulent buoyancy flux through this upper buoyancy surface. While the ratio of these quantities provides a natural buoyancy scale, it does not determine the bulk stratification in the abyss. Rather, this study shows that the bulk stratification in the abyss depends sensitively on the rate at which fluid is detrained into the ocean interior from the dense sinking plumes of Antarctic Bottom Water. Surprisingly, it is shown that the vertical variation in the turbulent diffusive flux of buoyancy in the ocean interior does not adequately determine the bulk abyssal stratification. With bottom-intensified mixing, we show that observations of the transport-weighted average density of the sinking Antarctic Bottom Water plumes in just a few places, along with the steady-state assumption, directly yield the magnitude of the globally integrated diapycnal upwelling in the thin bottom boundary layers and the globally integrated diapycnal downwelling in the ocean interior.