The Antarctic Circumpolar Current: A Conduit or Blender of Antarctic Bottom Waters?

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. At present this mapping is not well constrained, with conflicting conclusions drawn in previous studies: at one extreme the ACC has been suggested to be a ``conduit'' that simply allows each variety of AABW to transit directly northward; at the other extreme, it has been suggested that the ACC ``blends'' all shelf AABW sources together before they reach the northern basins. Such conflicts arise, in part, because little is understood about the physics that determines AABW's pathways across the ACC.

This collaborative project draws 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. We are using these tools  to not only constrain the three-dimensional circulation of AABW from Antarctica to the northern basins, but also provides a mechanistic understanding of the circulation that can be transferred to past or future climates. These outcomes will transform the oceanographic and climate science communities' understanding of the three-dimensional global overturning circulation in the Southern Ocean. The proposed identification and mechanistic understanding of AABW pathways will help to guide future direct observations or monitoring efforts of AABW export, and thus of the broader Southern Ocean overturning circulation. 

This project is 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 is 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.