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

[摘要] The Southern Ocean (44–75° S) plays a critical role in the globalcarbon cycle, yet remains one of the most poorly sampled ocean regions.Different approaches have been used to estimate sea–air CO₂ fluxes inthis region: synthesis of surface ocean observations, ocean biogeochemicalmodels, and atmospheric and ocean inversions. As part of the RECCAP (REgionalCarbon Cycle Assessment and Processes) project, we combine these differentapproaches to quantify and assess the magnitude and variability in SouthernOcean sea–air CO₂ fluxes between 1990–2009. Using all models andinversions (26), the integrated median annual sea–air CO₂ flux of−0.42 ± 0.07 Pg C yr⁻¹ for the 44–75° S region, isconsistent with the −0.27 ± 0.13 Pg C yr⁻¹ calculated usingsurface observations. The circumpolar region south of 58° S has asmall 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 fluxlocated in the 44 to 58° S circumpolar band (model and inversionmedian: −0.36 ± 0.09 Pg C yr⁻¹ and observations:−0.35 ± 0.09 Pg C yr⁻¹). Seasonally, in the44–58° S region, the median of 5 ocean biogeochemical modelscaptures the observed sea–air CO₂ flux seasonal cycle, while the medianof 11 atmospheric inversions shows little seasonal change in the net flux.South of 58° S, neither atmospheric inversions nor oceanbiogeochemical models reproduce the phase and amplitude of the observedseasonal sea–air CO₂ flux, particularly in the Austral Winter.Importantly, no individual atmospheric inversion or ocean biogeochemicalmodel is capable of reproducing both the observed annual mean uptake and theobserved seasonal cycle. This raises concerns about projecting future changesin Southern Ocean CO₂ fluxes. The median interannual variability fromatmospheric inversions and ocean biogeochemical models is substantial in theSouthern Ocean; up to 25% of the annual mean flux, with 25% of thisinterannual variability attributed to the region south of 58° S.Resolving long-term trends is difficult due to the large interannualvariability and short time frame (1990–2009) of this study; this isparticularly evident from the large spread in trends from inversions andocean biogeochemical models. Nevertheless, in the period 1990–2009 oceanbiogeochemical models do show increasing oceanic uptake consistent with theexpected increase of −0.05 Pg C yr⁻¹ decade⁻¹. In contrast,atmospheric inversions suggest little change in the strength of the CO₂sink broadly consistent with the results of Le Quéré et al. (2007).

[学科分类] 地球化学与岩石