Positive phase of SAM influences winter APO anomalies over the Southern Ocean

The Southern Ocean is an important regulator of global climate because of its role in driving the global thermohaline circulation and the related fluxes of carbon and nutrients between deep and surface waters. Given this important role, understanding the response of physical and biogeochemical processes within the Southern Ocean to climate variability is vital to improving predictions of future climate change. The Southern Annular Mode (SAM) is the dominant mode of variability in the Southern Ocean but only a few observational studies have linked variability in the SAM to changes in ocean circulation. Atmospheric Potential Oxygen (APO) is an atmospheric tracer that responds mainly to variability in ocean circulation and biogeochemistry. In this study, we examine long-term (20-30 year) records of APO based on samples collected at approximate two-week intervals from observing sites including Palmer Station and South Pole, Antarctica and Cape Grim, Tasmania. We show that the depth of the seasonal minimum in APO which occurs during austral winter is significantly correlated to the SAM index for all three sites. We find weaker correlations between seasonal maximums in atmospheric CO2 and the SAM index. We show a similar correlation in APO air-sea fluxes inferred from an atmospheric inversion of observed APO and in air-sea oxygen fluxes from a forced ocean biogeochemistry model simulation. Model results indicate that the correlation between air-sea oxygen fluxes and the SAM index is mechanistically linked to stronger wind stress and stronger upwelling, which brings oxygen-depleted deep waters to the surface.