Sea–air CO<sub>2</sub> fluxes in the Southern Ocean for the period 1990–2009

Abstract

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).

The Southern Ocean as a constraint to reduce uncertainty in future ocean carbon sinks

“…We have identified a strong bias in the amplitude of carbon uptake simulated by the CMIP5 models for the period 2001-2010 in the Southern Ocean, ranging from a source to the atmosphere to a sink almost 3 times more powerful than has been observed (1.03 ± 0.09 vs. 0.27 ± 0.13 Pg C yr −1 ; Lenton et al, 2013). Inter-model spread in the SO carbon sink arises from variations in the surface pCO 2 seasonality, which is attributed by the bias in the simulated timing and amplitude of primary production and SST.…”

“…Figure 4 depicts the inter-model relationships in the SO and eSO domains for R SO mean , R eSO mean , R SO cum , and R eSO cum . In SO, four models (BCC-CSM1.1, NorESM1-ME, MPI-ESM-LR, and CESM1-BGC) overestimate the carbon uptake flux from the atmosphere to the ocean when compared with two independent observationally based estimates of about 0.15 ± 0.12 Pg C yr −1 (Landschützer et al, 2014) and 0.27 ± 0.13 Pg C yr −1 (Lenton et al, 2013); the latter is derived from Takahashi et al (2009) data sets. The highest estimates are simulated by the BCC-CSM1 and NorESM1-ME models at 1.03 ± 0.09 and 0.64 ± 0.11 Pg C yr −1 , respectively.…”

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

“…While the magnitude of the seasonal cycle in the Southern Ocean lies within the upper range of CMIP5 again poor phasing is seen. That the seasonal cycle is out of phase suggests that during the summer the solubility response likely dominates over the NPP response, leading to an outgassing in the summer and uptake in the winter, as discussed in Lenton et al (2013). Consequently, we see that the poor global phasing in global sea-air CO 2 fluxes is likely due to the solubility dominated response of the high latitudes during the summer.…”

Marine projections of warming and ocean acidification in the Australasian region

“…This large range in the Tasman Sea likely reflects the variability in the strength of Pacific Western Boundary currents in the ESMs (Hu et al, 2015), which influences the southward extension of the East Australian Current (Ridgway, 2007). In contrast, the differences near southern Australia likely reflect differences in the location of isotherms related to the coarse resolution of current ESMs (Lenton et al, 2013).…”