An empirical estimate of the Southern Ocean air‐sea CO₂ flux

Abstract

Despite improvements in our understanding of the Southern Ocean air‐sea flux of CO 2 , discrepancies still exist between a variety of differing ocean/atmosphere methodologies. Here we employ an independent method to estimate the Southern Ocean air‐sea flux of CO 2 that exploits all available surface ocean measurements for dissolved inorganic carbon (DIC) and total alkalinity (ALK) beyond 1986. The DIC concentrations were normalized to the year 1995 using coinciding CFC measurements in order to account for the anthropogenic CO 2 signal. We show that independent of season, surface‐normalized DIC and ALK can be empirically predicted to within ∼8 μ mol/kg using standard hydrographic properties. The predictive equations were used in conjunction with World Ocean Atlas (2001) climatologies to give a first estimate of the annual cycle of DIC and ALK in the surface Southern Ocean. These seasonal distributions will be very useful in both validating biogeochemistry in general circulation models and for use in situ biological studies within the Southern Ocean. Using optimal CO 2 dissociation constants, we then estimate an annual cycle of pCO 2 and associated net air‐sea CO 2 flux. Including the effects of sea ice, we estimate a Southern Ocean (>50°S) CO 2 sink of 0.4 ± 0.25 Pg C/yr. Our analysis also indicates a substantial CO 2 sink of 1.1 ± 0.6 Pg C/yr within the sub‐Antarctic zone (40°S–50°S), associated with strong cooling and high winds. Our results imply the Southern Ocean CO 2 flux south of 50°S to be very similar to those found by Takahashi et al. (2002), but on the higher end of a range of atmospheric/oceanic CO 2 inversion methodologies. This paper estimates for the first time basic seasonal carbon cycle parameters within the circumpolar Southern Ocean, which have up to now been extremely difficult to measure and sparse. The application of such an empirical technique using more widely available hydrographic parameters in the Southern Ocean provides an important independent estimate to not only CO 2 uptake, but also for other future biogeochemical studies. Refining and testing these empirical methods with new carbon measurements will be important to further reduce uncertainties and extend our understanding of Southern Ocean CO 2 dynamics.