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Southern Ocean buoyancy forcing of ocean ventilation and glacial atmospheric CO2

Abstract

Atmospheric CO2 concentrations over glacial–interglacial cycles closely correspond to Antarctic temperature patterns1. These are distinct from temperature variations in the mid to northern latitudes2, so this suggests that the Southern Ocean is pivotal in controlling natural CO2 concentrations3. Here we assess the sensitivity of atmospheric CO2 concentrations to glacial–interglacial changes in the ocean’s meridional overturning circulation using a circulation model4,5 for upwelling and eddy transport in the Southern Ocean coupled with a simple biogeochemical description. Under glacial conditions, a broader region of surface buoyancy loss results in upwelling farther to the north, relative to interglacials. The northern location of upwelling results in reduced CO2 outgassing and stronger carbon sequestration in the deep ocean: we calculate that the shift to this glacial-style circulation can draw down 30 to 60 ppm of atmospheric CO2. We therefore suggest that the direct effect of temperatures on Southern Ocean buoyancy forcing, and hence the residual overturning circulation, explains much of the strong correlation between Antarctic temperature variations and atmospheric CO2 concentrations over glacial–interglacial cycles.

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Figure 1: Schematic of the global overturning with emphasis on the Southern Ocean, in the present day and at the LGM.
Figure 2: Forcings and results from the numerical model described in the text and Methods.
Figure 3: Stream function and phosphate concentrations for ‘modern’ and ‘glacial’ forcings, from the model.

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Acknowledgements

We thank J. R. Toggweiler and K. Speer for their constructive reviews. A.J.W. thanks the Royal Society for support under its Research Professorship scheme. G.K.V. acknowledges support from the Royal Society (Wolfson Foundation), a Marie Curie fellowship, and the NSF.

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All authors contributed to the model development and concept. A.J.W. wrote the first draft and all authors contributed to revisions of the manuscript.

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Correspondence to Andrew J. Watson.

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Watson, A., Vallis, G. & Nikurashin, M. Southern Ocean buoyancy forcing of ocean ventilation and glacial atmospheric CO2. Nature Geosci 8, 861–864 (2015). https://doi.org/10.1038/ngeo2538

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