Abstract
In global climate model pre-industrial control simulations the Southern Hemisphere westerly winds show a systematic bias in position and strength relative to estimates of their actual position and strength. These wind-stress biases impact the simulated transport of the Antarctic Circumpolar Current1 and the nature of Southern Ocean water-mass formation1 and may affect the rate of meridional overturning of the global ocean2. The effect they have on oceanic carbon uptake and storage is unknown, however. Here we demonstrate, using a coupled carbon–climate model3, that the wind-stress biases reduce equilibrium ocean carbon storage, redistribute carbon within the ocean and increase oceanic carbon uptake in climate change simulations. The wind-stress biases act directly by influencing Ekman pumping dynamics in the Southern Ocean and also seem to have an indirect effect on the overturning circulation and carbon distribution through the Agulhas leakage and Indo–Atlantic salt flux. Our results indicate that carbon–climate model simulations with the typical pre-industrial wind-stress bias will over-estimate ocean carbon sequestration, and thereby under-estimate atmospheric carbon dioxide concentrations in the twenty-first century, relative to unbiased simulations. The new generation of coupled carbon–climate models may be subject to these wind biases, which could alter their carbon–climate response, although it is worth noting that the uncertainty arising from wind biases that we demonstrate here is one of several uncertainties that affect modelled ocean carbon uptake4.
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Acknowledgements
N.C.S. received financial support from the National Research Foundation of South Africa, and through the NSERC CREATE training programme in interdisciplinary climate science at the University of Victoria, Canada. We thank the UVic Climate Modelling Group, in particular M. Eby, A. Weaver and E. Wiebe, for help with the UVic ESCM. This research has been enabled by the use of computing resources provided by WestGrid and Compute/Calcul Canada. We acknowledge the modelling groups, the Program for Climate Model Diagnosis and Intercomparison and the WCRP’s Working Group on Coupled Modelling for their roles in making available the WCRP CMIP3 multi-model data set. Support of this data set is provided by the Office of Science, US Department of Energy. J. Christian, M. Eby, N. Gillet and J. Scinocca provided useful comments on an early draft of the manuscript.
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J.C.F. conceived of the experiment, advised on the analysis, helped to interpret the results and edited the paper. N.C.S. designed the experiment, conducted the model runs, analysed the output, interpreted the results and wrote most of the paper.
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Swart, N., Fyfe, J. Ocean carbon uptake and storage influenced by wind bias in global climate models. Nature Clim Change 2, 47–52 (2012). https://doi.org/10.1038/nclimate1289
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DOI: https://doi.org/10.1038/nclimate1289
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