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Stabilization of dense Antarctic water supply to the Atlantic Ocean overturning circulation

Abstract

The lower limb of the Atlantic overturning circulation is resupplied by the sinking of dense Antarctic Bottom Water (AABW) that forms via intense air–sea–ice interactions next to Antarctica, especially in the Weddell Sea1. In the last three decades, AABW has warmed, freshened and declined in volume across the Atlantic Ocean and elsewhere2,3,4,5,6,7, suggesting an ongoing major reorganization of oceanic overturning8,9. However, the future contributions of AABW to the Atlantic overturning circulation are unclear. Here, using observations of AABW in the Scotia Sea, the most direct pathway from the Weddell Sea to the Atlantic Ocean, we show a recent cessation in the decline of the AABW supply to the Atlantic overturning circulation. The strongest decline was observed in the volume of the densest layers in the AABW throughflow from the early 1990s to 2014; since then, it has stabilized and partially recovered. We link these changes to variability in the densest classes of abyssal waters upstream. Our findings indicate that the previously observed decline in the supply of dense water to the Atlantic Ocean abyss may be stabilizing or reversing and thus call for a reassessment of Antarctic influences on overturning circulation, sea level, planetary-scale heat distribution and global climate2,3,8.

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Fig. 1: Pathways of AABW from the Weddell Sea into the world ocean.
Fig. 2: Area of LWSDW from hydrographic sections.
Fig. 3: Normalized areas of water masses on hydrographic sections.
Fig. 4: Northward transport of LWSDW through Orkney Passage.

Data availability

CTD data were collected on UK, US and German research cruises; these data are available at CCHDO (http://cchdo.ucsd.edu) for US and some UK cruises, BODC (http://www.bodc.ac.uk) for UK cruises and PANGAEA (http://www.pangaea.de) for German cruises46,47,48,49,50,51,52,53,54; links to the data are given in Supplementary Table 1. Mooring data from Orkney Passage are available from BODC at https://www.bodc.ac.uk/data/bodc_database/nodb/data_collection/6565/.

The altimeter products were produced by Ssalto/Duacs and distributed by Aviso, with support from CNES (https://www.aviso.altimetry.fr). ERA-interim reanalysis data are available from the ECMWF (https://www.ecmwf.int/en/research/climate-reanalysis/era-interim). SOSE data are available from http://sose.ucsd.edu/. GEBCO_2014 bathymetry data are available from https://www.gebco.net/.

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Acknowledgements

E.P.A., A.C.N.G. and M.P.M. were supported by Natural Environment Research Council (NERC) grant nos. NE/K012843/1 and NE/K013181/1 (Dynamics of the Orkney Passage Outflow). E.P.A., A.S.M., B.A.K., Y.L.F. and M.P.M. were supported by NERC grant no. NE/N018095/1 (Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports, ORCHESTRA). K.P. was supported by NSF grant no. OCE-1536779. A.C.N.G. was supported by the Royal Society and the Wolfson Foundation. Collection of data on A23 and SR1b was supported by NERC National Capability funding including ORCHESTRA. Collection of data in Orkney Passage was supported by NERC National Capability funding including ORCHESTRA and was funded in part by the Climate Observation Division, Climate Program Office (FundRef no. 100007298), National Oceanic and Atmospheric Administration, US Department of Commerce. Computational resources for SOSE were provided by NSF XSEDE resource grant no. OCE130007.

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E.P.A., A.S.M., M.P.M., B.A.K., Y.L.F., J.B.S. and B.A.H. contributed to data collection and interpretation. K.P. instigated this study with questions concerning the interpretation of the diminishing area of LWSDW along SR1b and A23. K.L.S. performed the kinetic energy anomaly analysis and generated Supplementary Fig. 3. E.P.A., A.S.M. and M.P.M. made the remaining figures and wrote the manuscript with contributions from all the remaining authors.

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Correspondence to E. Povl Abrahamsen.

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Peer review information: Nature Climate Change thanks Viviane Menezes and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Table 1 and Supplementary Figs. 1–4

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Abrahamsen, E.P., Meijers, A.J.S., Polzin, K.L. et al. Stabilization of dense Antarctic water supply to the Atlantic Ocean overturning circulation. Nat. Clim. Chang. 9, 742–746 (2019). https://doi.org/10.1038/s41558-019-0561-2

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