Is the Abyssal Branch of the Overturning Driven by Breaking Internal Waves?

Processes controlling the strength and structure of the meridional overturning remain poorly identified. Here, we clarify the respective roles of internal tides, lee waves and geothermal heating in supplying buoyancy to the abyss and maintaining the overturning. Using a hydrographic climatology, common GCM-type parameterizations of internal tide and lee wave energy dissipation and two different models for the mixing efficiency, we construct climatologies of buoyancy fluxes and quantify resulting rates of water mass transformation in the ocean interior. We find that upwelling rates induced by bottom-intensified mixing and geothermal heating peak within the γⁿ = ~ 28.11 water mass, which occupies the largest seafloor area and defines the boundary between southward-flowing deep waters and northward-flowing bottom waters. This result is consistent with an overturning characterized by a diabatic, northward abyssal branch contrasting with a predominantly adiabatic, deep southward branch. Nevertheless, we show that topographically-enhanced mixing by breaking internal tides and lee waves cannot account for the full strength of the abyssal branch. Indeed, we estimate that locally-dissipating internal tides contribute only about 4 Sv of bottom water upwelling, mostly north of 30ºS. This is comparable to the estimated ~ 5 Sv of abyssal flow maintained by geothermal heat fluxes. In contrast, breaking lee waves cause significant transformation only in the Southern Ocean, where they decrease the mean density but enhance the northward flow of abyssal waters. The possible role of remotely-dissipating internal tides in complementing bottom or deep water upwelling is explored.