OCB Newsletter: Climate variability and Southern Ocean carbon uptake

Since the beginning of the industrial revolution, anthropogenic emissions of carbon dioxide (CO2) have increased atmospheric CO2 concentrations, driving increases in global atmospheric temperature. Only about half of the anthropogenic CO2 emissions have remained in the atmosphere, while the remainder have been absorbed by natural carbon sinks: the ocean and the terrestrial biosphere. Modeling studies suggest that nearly half of the global oceanic anthropogenic CO2 uptake has occurred in the Southern Ocean (Mikaloff Fletcher et al., 2006). As such, the Southern Ocean is an important regulator of atmospheric CO2 and the global climate system.

The physical circulation of the Southern Ocean governs the exchange of CO2 across the air-sea interface. South of the Antarctic Circumpolar Current (ACC), the circulation is characterized by divergence and upwelling of deep water to the surface. The upwelled deep water is enriched in dissolved inorganic carbon (DIC), and given the inefficient DIC uptake via the biological pump, the high latitude Southern Ocean tends to lose natural CO2 to the atmosphere (Figs. 1a, 3; Mikaloff Fletcher et al., 2007). North of the ACC, subduction and mode water formation lead to substantial oceanic uptake of natural carbon from the atmosphere. This pattern of oceanic release and uptake of natural carbon is overlain by a pattern of uptake of anthropogenic CO2, which is largest north of the ACC (Khatiwala et al., 2009). The resulting pattern of the contemporary CO2 fluxes is thus the superposition of these two component fluxes, with reduced outgassing south of the ACC relative to pre-industrial times, and enhanced uptake north of the ACC (Gruber et al., 2009).

The Southern Ocean sink for atmospheric CO2 has exhibited significant interannual to multidecadal variability over the past few decades. Coarse-resolution physical and biogeochemical ocean models yield large interannual variability in high-latitude sea-air fluxes of natural CO2 (Fig. 2; Lenton and Matear, 2007; Lovenduski et al., 2007; Verdy et al., 2007). Studies based on ocean models (Lovenduski et al., 2008) and the inversion of atmospheric CO2 data (Le Quéré et al., 2007) have revealed a significant multi-decadal trend in sea-air fluxes of natural CO2 over the past 30 to 50 years (Fig. 2) that has substantially weakened the Southern Ocean’s capacity to absorb CO2 from the atmosphere. It has been suggested that a large fraction of this variability is driven by the Southern Annular Mode (SAM).

Nicole Lovenduski (University of Colorado, Boulder), OCB Newsletter, Fall 2011. pp 9-11.  Newsletter and full article.

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