Toward a mechanistic understanding of the decadal trends in the Southern Ocean carbon sink
Figure S1: Ten-year trends in the spatially integrated (<35°S), deseasonalized natural CO2 fluxes, calculated using the sliding window method described in the text. (455.0Kb)
Figure S3: Trends in the spatially integrated Southern Ocean (<35°S) fluxes of natural, anthropogenic, and contemporary CO2. (334.6Kb)
Figure S4: (a) Linear trends in the air-sea flux of natural CO2 and (b) trends linearly congruent with the wind speed index from 1958–2004. (3.245Mb)
Figure S5: Ten-year trends in spatially averaged (<35°S) CO2 fluxes and their estimated contributions, as in equation (4). (361.6Kb)
Figure S6: Temporal evolution of the ACC strength, defined as the mean x-direction velocity between 40°S and 60°S, 0 and 1000 m, and around the entire globe. (256.4Kb)
Figure S7: Linear trends in the (a) mixed layer depth and (b) export of particulate organic carbon from 1979 to 2004. (1.839Mb)
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We investigate the multidecadal and decadal trends in the flux of CO2 between the atmosphere and the Southern Ocean using output from hindcast simulations of an ocean circulation model with embedded biogeochemistry. The simulations are run with NCEP-1 forcing under both preindustrial and historical atmospheric CO2 concentrations so that we can separately analyze trends in the natural and anthropogenic CO2 fluxes. We find that the Southern Ocean (<35°S) CO2 sink has weakened by 0.1 Pg C a−1 from 1979–2004, relative to the expected sink from rising atmospheric CO2 and fixed physical climate. Although the magnitude of this trend is in agreement with prior studies (Le Quéré et al., 2007), its size may not be entirely robust because of uncertainties associated with the trend in the NCEP-1 atmospheric forcing. We attribute the weakening sink to an outgassing trend of natural CO2, driven by enhanced upwelling and equatorward transport of carbon-rich water, which are caused by a trend toward stronger and southward shifted winds over the Southern Ocean (associated with the positive trend in the Southern Annular Mode (SAM)). In contrast, the trend in the anthropogenic CO2 uptake is largely unaffected by the trend in the wind and ocean circulation. We regard this attribution of the trend as robust, and show that surface and interior ocean observations may help to solidify our findings. As coupled climate models consistently show a positive trend in the SAM in the coming century [e.g., Meehl et al., 2007], these mechanistic results are useful for projecting the future behavior of the Southern Ocean carbon sink.
Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 22 (2008): GB3016, doi:10.1029/2007GB003139.
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