Rethinking the abyssal ocean circulation: bottom-intensified mixing on slopes
The densest water in the world ocean is formed around Antarctica, where surface cooling and sea ice formation extract buoyancy. This causes the water to sink to the bottom, from where it spreads into the deepest parts of the ocean basins. To balance the sinking around Antarctica, there must be a return flow of bottom water back up to the surface, but the mechanics of this return flow are poorly constrained. The flow crucially depends on small-scale turbulence, which furnishes the buoyancy gain required for the upwelling. Traditionally, small-scale turbulence has been assumed to invariably cause upwelling, but observations and theory suggest that the turbulence is strongly bottom-intensified. This has important consequences for the circulation of bottom water, because it causes a buoyancy loss and thus sinking in part of the water column, and it confines the buoyancy gain and thus upwelling to withing a few tens of meters above the sea floor. In this talk, I will discuss how this produces a net upwelling, why it is crucial that the ocean bottom is sloping, and what large-scale circulation the pattern of up- and downwelling produces. In addition, I will argue that the small-scale dynamics of the turbulent layers on slopes are likely richer than previously appreciated and that these dynamics directly affect the abyssal overturning.