Articles | Volume 10, issue 6
Biogeosciences, 10, 4037–4054, 2013

Special issue: REgional Carbon Cycle Assessment and Processes (RECCAP)

Biogeosciences, 10, 4037–4054, 2013

Research article 19 Jun 2013

Research article | 19 Jun 2013

Sea–air CO2 fluxes in the Southern Ocean for the period 1990–2009

A. Lenton1, B. Tilbrook1,2, R. M. Law3, D. Bakker4, S. C. Doney5, N. Gruber6, M. Ishii7, M. Hoppema8, N. S. Lovenduski9, R. J. Matear10, B. I. McNeil11, N. Metzl12, S. E. Mikaloff Fletcher13, P. M. S. Monteiro14, C. Rödenbeck15, C. Sweeney16, and T. Takahashi17 A. Lenton et al.
  • 1Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia
  • 2Antarctic Climate Ecosystems Co-operative Research Centre, Hobart, Tasmania, Australia
  • 3Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia
  • 4School of Environmental Sciences, University of East Anglia Research Park, Norwich NR4 7TJ, UK
  • 5Marine Chemistry and Geochemistry Dept., Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
  • 6Institute of Biogeochemistry and Pollutant Dynamics and Center for Climate Systems Modeling, ETH Zurich, Zurich, Switzerland
  • 7Meteorological Research Institute, Tsukuba 305-0031, Japan
  • 8Alfred Wegener Institute, Bremerhaven, Germany
  • 9Department of Atmospheric and Oceanic Sciences, Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA
  • 10Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia
  • 11Climate Change Research Centre, University of New South Wales, Sydney, Australia
  • 12Laboratoire d'Océanographie et du Climat, LOCEAN/IPSL, CNRS, Université Pierre et Marie Curie, Paris, France
  • 13National Institute of Water and Atmospheric Research, Wellington 6021, New Zealand
  • 14Department of Oceanography, University of Cape Town, Rondebosch 7700, South Africa and Ocean Systems & Climate Group, CSIR, Stellenbosch, South Africa
  • 15Max-Planck-Institute for Biogeochemistry, 07745 Jena, Germany
  • 16Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO 80305, USA
  • 17Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA

Abstract. The Southern Ocean (44–75° S) plays a critical role in the global carbon cycle, yet remains one of the most poorly sampled ocean regions. Different approaches have been used to estimate sea–air CO2 fluxes in this region: synthesis of surface ocean observations, ocean biogeochemical models, and atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Southern Ocean sea–air CO2 fluxes between 1990–2009. Using all models and inversions (26), the integrated median annual sea–air CO2 flux of −0.42 ± 0.07 Pg C yr−1 for the 44–75° S region, is consistent with the −0.27 ± 0.13 Pg C yr−1 calculated using surface observations. The circumpolar region south of 58° S has a small net annual flux (model and inversion median: −0.04 ± 0.07 Pg C yr−1 and observations: +0.04 ± 0.02 Pg C yr−1), with most of the net annual flux located in the 44 to 58° S circumpolar band (model and inversion median: −0.36 ± 0.09 Pg C yr−1 and observations: −0.35 ± 0.09 Pg C yr−1). Seasonally, in the 44–58° S region, the median of 5 ocean biogeochemical models captures the observed sea–air CO2 flux seasonal cycle, while the median of 11 atmospheric inversions shows little seasonal change in the net flux. South of 58° S, neither atmospheric inversions nor ocean biogeochemical models reproduce the phase and amplitude of the observed seasonal sea–air CO2 flux, particularly in the Austral Winter. Importantly, no individual atmospheric inversion or ocean biogeochemical model is capable of reproducing both the observed annual mean uptake and the observed seasonal cycle. This raises concerns about projecting future changes in Southern Ocean CO2 fluxes. The median interannual variability from atmospheric inversions and ocean biogeochemical models is substantial in the Southern Ocean; up to 25% of the annual mean flux, with 25% of this interannual variability attributed to the region south of 58° S. Resolving long-term trends is difficult due to the large interannual variability and short time frame (1990–2009) of this study; this is particularly evident from the large spread in trends from inversions and ocean biogeochemical models. Nevertheless, in the period 1990–2009 ocean biogeochemical models do show increasing oceanic uptake consistent with the expected increase of −0.05 Pg C yr−1 decade−1. In contrast, atmospheric inversions suggest little change in the strength of the CO2 sink broadly consistent with the results of Le Quéré et al. (2007).

Final-revised paper