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Anthropogenic carbon dioxide transport in the Southern Ocean driven by Ekman flow

Abstract

The Southern Ocean, with its large surface area and vigorous overturning circulation, is potentially a substantial sink of anthropogenic CO2 (refs 1–4). Despite its importance, the mechanism and pathways of anthropogenic CO2 uptake and transport are poorly understood. Regulation of the Southern Ocean carbon sink by the wind-driven Ekman flow, mesoscale eddies and their interaction is under debate5,6,7,8. Here we use a high-resolution ocean circulation and carbon cycle model to address the mechanisms controlling the Southern Ocean sink of anthropogenic CO2. The focus of our study is on the intra-annual variability in anthropogenic CO2 over a two-year time period. We show that the pattern of carbon uptake is correlated with the oceanic vertical exchange. Zonally integrated carbon uptake peaks at the Antarctic polar front. The carbon is then advected away from the uptake regions by the circulation of the Southern Ocean, which is controlled by the interplay among Ekman flow, ocean eddies and subduction of water masses. Although lateral carbon fluxes are locally dominated by the imprint of mesoscale eddies, the Ekman transport is the primary mechanism for the zonally integrated, cross-frontal transport of anthropogenic CO2. Intra-annual variability of the cross-frontal transport is dominated by the Ekman flow with little compensation from eddies. A budget analysis in the density coordinate highlights the importance of wind-driven transport across the polar front and subduction at the subtropical front. Our results suggest intimate connections between oceanic carbon uptake and climate variability through the temporal variability of Ekman transport.

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Figure 1: Uptake and inventory of anthropogenic carbon.
Figure 2: Northward transport of anthropogenic carbon.
Figure 3: Variability of anthropogenic carbon transport.
Figure 4: Vertical structure of anthropogenic carbon transport.

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Acknowledgements

T.I. and M.W. are supported by the US National Aeronautics and Space Administration (NASA) grant NNX08AL72G and the US National Oceanic and Atmospheric Administration grant NA08OAR4320893. The Southern Ocean State Estimate is supported by NASA and US National Oceanographic Partnership Program contracts to the Massachusetts Institute of Technology. The NASA Advanced Supercomputing division and the San Diego Supercomputer Center provided computing and data storage resources. J. L. Sarmiento and N. Lovenduski provided comments that improved the manuscript.

Author Contributions T.I. designed and performed numerical simulations of the Southern Ocean carbon cycle and analysed the model output; M.W. performed model–data comparison and calculations of carbon transport; M.M. developed the Southern Ocean State Estimate; all authors contributed to the interpretation of the results and writing of the manuscript.

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Correspondence to T. Ito.

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Ito, T., Woloszyn, M. & Mazloff, M. Anthropogenic carbon dioxide transport in the Southern Ocean driven by Ekman flow. Nature 463, 80–83 (2010). https://doi.org/10.1038/nature08687

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