Ventilation of the abyss in the Atlantic sector of the Southern Ocean

Camille Akhoudas , Jean-Baptiste Sallée, Alexander Haumann, Michael Meredith, Alberto Naveira-Garabato, Gilles Reverdin, Loïc Jullion, Giovanni Aloisi, Marion Benetti, Melanie Leng, Carol Arrowsmith

The Atlantic sector of the Southern Ocean is the world’s main production site of Antarctic Bottom Water, a water-mass that is ventilated at the ocean surface before sinking and entraining older water-masses – ultimately replenishing the abyssal global ocean. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in this region have led to a strikingly broad range of results, with water transport-based calculations (8.4-9.7 Sv) yielding larger rates than tracer-based estimates (3.7-4.9 Sv). Here, we reconcile these conflicting views by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly quantified. We show that the layer of Antarctic Bottom Water denser than 28.35 kg m−3 γn is exported northward at a rate of 8.7 ± 0.9 Sv, composed of 4.8 ± 1.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 1.1 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.5 ± 2.7 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region are well ventilated thus participating to heat and carbon uptake in the deep ocean, and in the Weddell sector, entrainment of older waters occurs at a ratio of 2.1 ± 0.8. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector, and define a baseline, process-discerning target for its realistic representation in climate models.