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1. Introduction
The abyssal meridional overturning circulation is fed by waters that become dense enough to sink into the ocean abyss at high latitudes and return to the surface through convoluted three-dimensional pathways. While it is well established that most of the sinking is confined to the Labrador and Greenland Seas in the North Atlantic and the Weddell and Ross Seas around Antarctica, the return pathways are less understood (Talley et al. 2011). The consensus is that widespread small-scale mixing generated by breaking internal waves provides most of the energy to lift the North Atlantic Deep Waters (NADWs) and the Antarctic Bottom Waters (AABWs) up to a depth of approximately 2000 m; internal waves break within a few hundred meters of abyssal ridges, mountains, and rises, which are most ubiquitous below 2000 m (Munk 1966; Ferrari 2014). Once the waters rise above 2000 m, they flow southward along density surfaces to the Southern Ocean, where the Roaring Forties lift them to the surface to be transformed into intermediate waters that flow back toward Antarctica and the North Atlantic, thereby closing the overturning loop [see the review by Marshall and Speer (2012)]. There would appear to be a major inconsistency in this scenario: strong internal wave–driven mixing is typically bottom intensified and thus drives sinking, not rising, of waters through density surfaces (e.g., St. Laurent et al. 2001; Klocker and McDougall 2010). In this paper, the conundrum is resolved by arguing that abyssal waters come to the surface along turbulent bottom boundary layers, rather than in the stratified ocean interior.
We begin by establishing a few well-documented observations about the ocean. First, the ocean is stably stratified on time scales longer than a few hours (e.g., Talley et al. 2011). Second, estimates of the zonally averaged global overturning circulation based on inverse calculations from ocean observations (Ganachaud and Wunsch 2000; Lumpkin and Speer 2007) and numerical models constrained to observations (Wunsch and Heimbach 2009) show that waters sink to the ocean bottom at high latitudes and then slowly rise throughout the rest of the oceans crossing density surfaces at least up to 2000 m (Marshall and Speer 2012; Talley 2013). Third, there is growing evidence from in situ measurements that the turbulent kinetic energy generated by...