Despite its importance for the global cycling of carbon, there are still large gaps in our understanding of the processes driving annual and seasonal carbon fluxes in the high-latitude Southern Ocean. This is due in part to an historical paucity of observations in this remote, turbulent, and seasonally ice-covered region. Here, we use autonomous biogeochemical float data spanning 6 full seasonal cycles and with circumpolar coverage of the Southern Ocean, complemented by atmospheric reanalysis, to construct a monthly mixed layer budget of dissolved inorganic carbon (DIC). We investigate the processes that determine the annual mean and seasonal cycle of DIC fluxes in two different frontal zones of the Antarctic Circumpolar Current (ACC)—the Sea Ice Zone (SIZ) and Antarctic Southern Zone (ASZ). We find that, annually, mixing with carbon-rich waters at the base of the mixed layer supplies DIC which is then, in the ASZ, either used for net biological production or outgassed to the atmosphere. In contrast, in the SIZ, where carbon outgassing and the biological pump are weaker, the surplus of DIC is instead advected northward to the ASZ. In other words, carbon outgassing in the southern ACC, which has been attributed to remineralized carbon from deep water upwelled in the ACC, is also due to the wind-driven transport of DIC from the SIZ. These results stem from the first observation-based carbon budget of the circumpolar Southern Ocean and thus provide a useful benchmark to evaluate climate models, which have significant biases in this region.

Much of the salty, high oxygen North Atlantic Deep Water (NADW) leaving the Atlantic flows through the Argentine Basin, where it is diluted by fresher, low oxygen Circumpolar Deep Water (CDW). This mixing of deep water masses is often overlooked in the zonally-averaged description of the overturning circulation. Here, we show that most of the mixing occurs along the western boundary: (i) extreme, isolated oxygen/temperature anomalies recorded by three autonomous biogeochemical floats suggest that subsurface eddies can inject relatively unmodified CDW far into the northwestern Argentine Basin, and (ii) moderate, numerous temperature/salinity anomalies indicate a mixing zone from Rio Grande Rise to the Malvinas Current. This western eddy pathway shortcuts the gyre-scale cyclonic route for CDW inferred from previous studies. Significantly, CDW dilution of NADW affects the properties of deep waters that upwell in the Southern Ocean, and hence the connection between Northern and Southern Hemisphere polar climates.