RECCAP2 – Southern Ocean carbon fluxes and storage

Cool temperatures, vigorous overturning circulation, and high biological productivity make the Southern Ocean a key region for the air-sea CO₂ exchanges. It is also the main gateway for anthropogenic CO₂ into the ocean owing to the upwelling of old water masses with low anthropogenic CO₂ concentration, and the transport of the newly equilibrated surface waters into the ocean interior. Here we present results from the Southern Ocean chapter of RECCAP2, which is the Global Carbon Project’s second systematic study on Regional Carbon Cycle Assessment and Processes. We analyse Southern Ocean contemporary carbon fluxes and anthropogenic carbon accumulation in 1985-2018 from a wide range of global ocean biogeochemical models (GOBMs), surface ocean pCO₂-based data products (pCO_2-products), and data-assimilated models, with the aim of identifying patterns of regional and temporal variability, model limitations and future challenges. Our results highlight agreement of GOBMs and pCO₂-products on the mean Southern Ocean contemporary CO₂ uptake (0.75 ± 0.28 PgC yr^-1 and 0.74 ± 0.07 PgC yr^-1 respectively). Compared with RECCAP1 (where the database of model- and observation-based estimates was significantly smaller), the new estimates suggest a weaker sink, possibly due to better representation of winter outgassing. Strong discrepancies exist between GOBMs and pCO₂-products in seasonality and trend estimates between 1985-2018. The pCO₂-products show the presence of a stagnation in uptake through the 1990’s followed by a rapid increase in uptake, while GOBMs show consistent uptake throughout the 1985-2018 period. On a regional level, the subtropical seasonally stratified (STSS) biome has the largest air-sea CO₂ flux with uptake of CO₂ peaking in winter, whereas the ice (ICE) biome is characterised by a generally small magnitude of fluxes into and out of the ocean and a pronounced seasonal cycle with the largest ocean uptake of CO₂ in summer. Connecting these two, the subpolar seasonally stratified (SPSS) biome has intermediate flux magnitude, with GOBMs showing spread in the strength of winter outgassing and difficulties in simulating the strongest CO₂ uptake in summer. The biases in GOBMs originate mainly from the non-thermal component of air-sea CO₂ flux, and in particular from the difficulty in simulating the competing effects of circulation and biology on carbon draw-down in summer. Our analysis reveals a distinct zonal asymmetry (secondary to the latitudinal gradient) between the Atlantic, Pacific and Indian sectors. The zonal asymmetry is observed in the mean uptake and amplitude of the seasonal cycle rather than the phasing of the seasonal cycle. GOBMs show a 20% spread and an overall underestimate of their simulated anthropogenic carbon accumulation, pointing to insufficient water mass formation and interior ventilation. These first results confirm the global relevance of the Southern Ocean carbon sink and highlight the strong regional and interannual variability of the Southern Ocean carbon uptake in connection to physical and biogeochemical processes.