The formation of dense Antarctic Bottom Water (AABW) and its export northward from the Antarctic continent is one of the key components of the global ocean overturning circulation, and plays a critical role in regulating Earth's climate on multi-decadal-to-millennial time scales. Recent studies of the global ocean overturning circulation have increasingly emphasized its three-dimensional structure: AABW is produced in a handful of distinct sites around the Antarctic continent, and there is a pronounced asymmetry in the allocation of AABW transports into the Atlantic, Indian and Pacific basins. The connectivity of AABW between the Antarctic continental shelf and the northern basins is mediated by the Antarctic Circumpolar Current (ACC), a circumpolar eastward flow that also serves as the primary route for inter-basin exchange.

The mapping from different shelf AABW sources to the northern basins dictates the response of the global MOC to localized variability or shifts in the state of the Antarctic shelf, for example due to major glacier calving events or modified inputs of freshwater from the Antarctic ice sheet. Prior to our project, this mapping was not well constrained, with conflicting conclusions drawn in previous studies: at one extreme the ACC had been suggested to be a ``conduit'' that simply allows each variety of AABW to transit directly northward; at the other extreme, it had been suggested that the ACC ``blended'' all shelf AABW sources together before they reach the northern basins. Such conflicts arose, in part, due to a significant gap in understanding of the physics that determine AABW's pathways across the ACC.

Intellectual Merit: This collaborative project drew on three complementary analytical tools: process-oriented modeling of AABW export across the ACC, a high-resolution global ocean model, and an observationally-constrained estimate of the global circulation. Major findings include: process-oriented simulation experiments identified the elevation of deep sea ridges, such as Drake Passage, in controlling the pathways of AABW across the ACC; deployment of passive ("dye-like") tracers in a high-resolution global ocean/sea ice model revealed two distinct pathways connecting AABW sourced from the Weddell Sea and Prydz Bay to the Atlantic/Indian basins, and connecting AABW sourced from the Ross Sea and Adelie Coast to the Pacific basin; further realistic tracer deployment experiments revealed that lightening and rising of AABW in the Southern Ocean makes a major contribution to "ventilating" mid-depth waters in this region, i.e. supplying them with dissolved gases like oxygen.

Broader Impacts: This project reconciles conflicting previous findings regarding the connectivity between different Antarctic sources of AABW and the ocean's major basins; in this regard the project has transformed the oceanographic and climate science communities' understanding of deep circulation pathways in the Southern Ocean. The consequences of there being two distinct AABW supply pathways in the global overturning circulation remain to be explored. This project also partially supported UCLA's Zodiac educational program, which takes more than 200 undergraduate students on educational research cruises each year. 

This project was led by Principal Investigators Andrew Stewart (UCLA), Andy Thompson (Caltech) and Sarah Purkey (Scripps Institution of Oceanography), in collaboration with Andy Hogg (Australian National University). This work was supported by the National Science Foundation, grant number ANT-2023244.

Antarctic Bottom Water formation and spread in a high-resolution global ocean/sea ice simulation (ACCESS-OM2-01). Green shading indicates the concentration of a simulated dye tracer injected where Antarctic Bottom Water is formed on the continental shelf, after 10 years of spread; darker green colors indicate a higher concentration of Antarctic Bottom Water.  Red shading indicates the rate at which Antarctic Bottom Water is being formed around the Antarctic coast. Black arrows qualitatively indicate the inferred pathways of Antarctic export from the continent. Image credit: Ruth Moorman, Adele Morrison and Andy Hogg, Australian National University. See Moorman et al. (2020), Journal of Climate for further information about this simulation.