Sea-air CO 2 fluxes in the Southern Ocean for the period 1990-2009

• Published in: Biogeosciences
• Main Authors: Lenton, A, Tilbrook, BD, Law, RM, Bakker, D, Doney, SC, Gruber, N, Ishii, M, Hoppema, M, Lovenduski, NS, Matear, RJ, McNeil, BI, Metzl, N, Mikaloff Fletcher, SE, Monteiro, PMS, Rodenbeck, C, Sweeney, C, Takahashi, T
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2013
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Southern Ocean; Austral
• Online Access: https://doi.org/10.5194/bg-10-4037-2013; http://ecite.utas.edu.au/87467

The Southern Ocean (4475 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 seaair 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 seaair CO2 fluxes between 19902009. Using all models and inversions (26), the integrated median annual seaair CO2 flux of −0.42 0.07 Pg C yr−1 for the 4475 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 4458 S region, the median of 5 ocean biogeochemical models captures the observed seaair 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 seaair 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 (19902009) of this study; this is particularly evident from the large spread in trends from inversions and ocean biogeochemical models. Nevertheless, in the period 19902009 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 Qur et al. (2007).