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

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

doi:10.1038/s41598-021-86043-2

Abstract

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 28.36 kg m^{-3}gamma _{n} is exported northward at a rate of 8.4 ± 0.7 Sv, composed of 4.5 ± 0.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 0.5 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.4 ± 0.6 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region is well ventilated and thus draws down heat and carbon into the deep ocean. 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.

Highlights

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 watermasses— replenishing the abyssal global ocean
Ventilation rate of Antarctic Bottom Water (AABW) formation is the critical term that needs to be known to understand the exchange rate of carbon-dioxide between the atmosphere and the abyssal ocean. This complex coastal AABW formation is difficult to represent in global climate models that instead mainly create AABW through a third mechanism; open-ocean deep convection where well-ventilated near surface waters such as Winter Water (WW) sink under strong atmospheric forcing conditions to create AABW by cooling
In the southwestern Weddell Sea, which is the focus of this study, the circulation of the gyre corresponds to a preferential conversion dense WSDW (28.35 kg of Circumpolar Deep Water (CDW) and light WSDW ( 28.27 kg m−3 m− 3≤γn < 28.4 kg m−3 ) and W SBW25,26

Summary

Introduction

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 watermasses— replenishing the abyssal global ocean. DSW sinks down the continental slope, entraining warmer Circumpolar Deep Water (CDW), to form AABW3,11,27 While these two mechanisms effectively contribute to the formation of AABW, only the first one actively ventilates the abyssal ocean, with changes in the rates of the two mechanisms dependent on different forcings[3]. Ventilation rate of AABW formation is the critical term that needs to be known to understand the exchange rate of carbon-dioxide between the atmosphere and the abyssal ocean This complex coastal AABW formation is difficult to represent in global climate models that instead mainly create AABW through a third mechanism; open-ocean deep convection where well-ventilated near surface waters such as Winter Water (WW) sink under strong atmospheric forcing conditions to create AABW by cooling.

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Results

Conclusion