Evidence of seasonal carbon dioxide uptake by the Southern Ocean from a 10-year record of atmospheric carbon dioxide, observed from coastal Antarctica.

The Southern Ocean plays a critical role in modulating excess atmospheric carbon dioxide, accounting for roughly 40% of global ocean anthropogenic CO₂ uptake since industrialisation. Given its significance in the global carbon cycle, understanding the Southern Ocean carbon sink is important but studies show high uncertainties in the magnitude and evolution of this carbon sink. The Southern Ocean is a remote and challenging region to measure, and the resulting sparsity of observational data is the main cause of uncertainty in air-sea carbon flux in the region. Long term, high-temporal-frequency data sets especially are rare for the Southern Ocean, but these can give valuable insights into the carbon cycle processes occurring in the region.

This work presents ten years of high-temporal-frequency in situ atmospheric carbon dioxide mixing ratios measured from two coastal Antarctic research stations; Halley, operated by the British Antarctic Survey, and the German research station, Neumayer. The coastal location of these stations means they are ideally placed to explore air-sea CO₂ exchange over the Southern Ocean. 

Both the Halley and Neumayer records show short-term fluctuations in CO₂ mixing ratios during austral summer, with over ~0.5 ppm decreases in CO₂ sometimes observed over the course of a day - about one fifth of the average annual growth rate (~2.4 ppm per year^-1 for this 10-year record). Analysis of air mass trajectories reveal that these fluctuations in CO₂ occur when the sampled air has spent considerable time in contact with the Southern Ocean, suggesting CO₂ uptake has occurred, leading to the reduced CO₂ mixing ratios observed.

We present an in-depth analysis of the drivers of the short-term variability observed during austral summer, including the role of mixing height, sea-ice coverage, wind speed and biology. Observational data represent an important tool with which to tease out key factors determining Southern Ocean CO₂ uptake, and thus in assessing how uptake may evolve in the future.