Southern Annular Mode Influence on Wintertime Ventilation of the Southern Ocean Detected in Atmospheric O2 and CO2 Measurements

The Southern Annular Mode (SAM) is the dominant mode of climate variability in the Southern Ocean, but only a few observational studies have linked variability in SAM to changes in ocean circulation. Atmospheric potential oxygen (APO) combines atmospheric O2/N2 and CO2 data to mask the influence of terrestrial exchanges, yielding a tracer that is sensitive mainly to ocean circulation and biogeochemistry. We show that observed wintertime anomalies of APO are significantly correlated to SAM in 25‐ to 30‐year time series at three Southern Hemisphere sites, while CO2 anomalies are also weakly correlated. We find additional correlations between SAM and O2 air‐sea fluxes in austral winter inferred from both an atmospheric inversion of observed APO and a forced ocean biogeochemistry model simulation. The model results indicate that the correlation with SAM is mechanistically linked to stronger wind speeds and upwelling, which brings oxygen‐depleted deep waters to the surface. Plain Language Summary The Southern Annular Mode (SAM) is characterized by variability in the strength of the westerly winds that encircle Antarctica. A more positive SAM index is associated with stronger westerly winds over the ocean at about 60°S latitude. Previous studies based mostly on model simulations have suggested that a positive SAM index is also associated with enhanced upwelling of carbon‐rich waters in the Southern Ocean, which influences the uptake and/or release of carbon dioxide to the atmosphere. The same deep waters that have high carbon also have low concentrations of dissolved oxygen, which can cause subtle variations in atmospheric oxygen levels measured at surface stations. We present an analysis of data sets from three Southern Hemisphere stations where air samples have been collected for 25–30 years. We find that during the Southern Hemisphere winter, anomalies in the atmospheric oxygen record are correlated to the SAM index; we find weaker correlations between anomalies in atmospheric carbon dioxide and the SAM index. These results are consistent with variability in air‐sea oxygen fluxes from a model simulation. The model results indicate that the observed relationship between SAM and air‐sea oxygen fluxes is due to both stronger wind speeds and increased upwelling of oxygen‐depleted deep waters. Key Points Significant correlations are found between winter atmospheric potential oxygen and the Southern Annular Mode at three long‐term observing sites A similar relation