Mixing at the ocean's bottom boundary

Abstract The lower limb of the global overturning circulation represents the transformation of waters previously made dense by cryogenic processes around Antarctica that have found their way to the oceans' bottom. Along the pathways north, these waters are made warmer and lighter. The essential character of the lower limb is defined by the importance of upwelling across density surfaces (isopycnals) rather than pathways that are largely along isopycnals. This crossing of isopycnals is accomplished via turbulent mixing, and our current understanding is that the turbulent transformations predominantly occur near the ocean's bottom boundary. The intent of this chapter is to review the intellectual foundations underpinning the lower limb and to summarise the observational grounding. Both the theory and observations of the upwelling in the abyss are in the early stages of changing paradigms, and the efforts here to connect the two approaches are unsatisfying. Reconciling deep-ocean mixing observations and theories is unfinished business, and should remain an exciting field for another generation of oceanographers.