USAP-DC

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. To close this gap in understanding, 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. The PIs will first identify and quantify the pathways of AABW across the ACC by using these tools to propagate passive tracers that identify each of the four major AABW formation sites. They will then use a suite of process model sensitivity experiments to develop a theory for what controls meridional versus inter-basin transport of AABW in the ACC, and transfer this theory to interpret the AABW pathways simulated in the global model. Finally, they will combine the process model, global model and the observationally-constrained circulation product to map the rates at which AABW is transformed into lighter waters, and relate these transformation rates to the diagnosed pathways of AABW across the ACC. This combination of approaches allow the PIs 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.

Person	Role
Stewart, Andrew	Investigator and contact
Thompson, Andrew	Investigator
Purkey, Sarah	Investigator

Antarctic Ocean and Atmospheric Sciences	Award # 2023303
Antarctic Ocean and Atmospheric Sciences	Award # 2023259
Antarctic Ocean and Atmospheric Sciences	Award # 2023244

USAP-2023244_1

None in the Database

Repository	Title (link)	Format(s)	Status
USAP-DC	Ocean CFC reconstructed data product	Not Provided	exists

1. Si, Y., Stewart, A., & Eisenman, I. (2021). Coupled ocean/sea ice dynamics of the Antarctic Slope Current driven by topographic eddy suppression and sea ice momentum redistribution. (doi:10.1002/essoar.10507558.1)
2. Stewart, A. L., Chi, X., Solodoch, A., & Hogg, A. M. (2021). High‐Frequency Fluctuations in Antarctic Bottom Water Transport Driven by Southern Ocean Winds. Geophysical Research Letters, 48(17). (doi:10.1029/2021gl094569)
3. Wilson, E. A., Thompson, A. F., Stewart, A. L., & Sun, S. (2022). Bathymetric Control of Subpolar Gyres and the Overturning Circulation in the Southern Ocean. Journal of Physical Oceanography, 52(2), 205–223. (doi:10.1175/jpo-d-21-0136.1)
4. Stewart, A. L., Neumann, N. K., & Solodoch, A. (2023). “Eddy” Saturation of the Antarctic Circumpolar Current by Standing Waves. Journal of Physical Oceanography, 53(4), 1161–1181. (doi:10.1175/jpo-d-22-0154.1)
5. Cimoli, L., Gebbie, G., Purkey, S. G., & Smethie, W. M. (2023). Annually resolved propagation of CFCs and SF6 in the global ocean over eight decades. Journal of Geophysical Research: Oceans, 128, e2022JC019337. (doi:10.1029/2022JC019337)