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What controls the warming of the Antarctic Bottom Water supply to the Atlantic Ocean?


Project Description

During the last three decades, the Antarctic Bottom Water (AABW) that fills the bulk of the ocean abyss has exhibited a striking warming and contraction in volume over much of the world ocean, particularly in the Atlantic basin. While the causes of these changes are unknown, available evidence suggests that, in the Atlantic Ocean, the warming and contraction of AABW may be caused by changes in the strength and configuration? of winds over the Weddell Sea, where much AABW is produced. AABW is the densest of the world’s water masses, and is particularly important to global climate because of its role in drawing down heat and carbon into the depths of the ocean.

This project will test and quantify this hypothesis by (i) determining how the flow and properties of the AABW entering the Atlantic Ocean from the Weddell Sea respond to wind forcing, and (ii) assessing the dynamical processes connecting wind forcing perturbations to changes in AABW. The project will generate a thorough understanding of the ways in which changes in Antarctic winds may shape the heat content of the deepest layers of the global ocean abyss.

The relationship between variations in Antarctic winds and changes in AABW will be investigated through the analysis of measurements from a set of moorings that have been deployed by the British Antarctic Survey in Orkney Passage (the main pathway of AABW out of the Weddell Sea) over the last 10 years. This mooring dataset is the longest continuous record of AABW characteristics anywhere in the Southern Ocean. This time series will be analysed in conjunction with atmospheric reanalyses and ocean state estimates (models that are constrained to fit observations while remaining dynamically consistent). Once the wind–AABW relationship has been thoroughly documented, the dynamical processes through which wind forcing modulates AABW flow and properties will be assessed by analysing two unique data sets:

• A detailed survey of the AABW flow through Orkney Passage acquired using novel ship-deployed and autonomous instrumentation during the DynOPO project (https://oceanmixing.github.io/projects/dynopo/);

• A set of ultra-high-resolution simulations of the AABW flow through the region run by Dr. Sonya Legg, a collaborator at Princeton University.

The newly gained dynamical insights will be used to define the ways in which changes in Antarctic winds can modulate deep-ocean heat content across the world ocean.

The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton (UoS), and co-hosted at the British Antarctic Survey and the UoS. Specific training will include:

- Techniques to handle and analyse large observational and model data sets
- Opportunities to attend modules in ocean physics
- Opportunities to present research results in international conferences
- Possible opportunities to participate in an Antarctic research cruise. This is not a prerequisite for completing the PhD, and is subject to ship scheduling, medical clearance, and physical fitness requirements

Funding Notes

UK and EU students who meet the UK residency requirements will be eligible for a full NERC studentship. Students from EU countries who do not meet the residency requirements may still be eligible for a fees-only award. More information can be found in the UKRI Training Grant Terms and Conditions

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References

Meredith, M. P. et al. (2011): Synchronous intensification and warming of AABW outflow from the Weddell Gyre. GRL 38, L03603.
Naveira Garabato et al. (2019): Rapid mixing and exchange of deep-ocean waters in an abyssal boundary current. PNAS 116 (27), 13233-13238.
Purkey, S. G. & G. C. Johnson (2013): AABW warming and freshening: Contributions to sea level rise, ocean freshwater budgets, and global heat gain. J. Clim. 26, 6105-6121.

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