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.

Profiles of oxygen measurements from Argo profiling floats now vastly outnumber shipboard profiles. Air calibration of a float’s oxygen optode upon surfacing enables accurate measurements in the upper ocean but does not necessarily provide similar accuracy at depth. In this study we use a quality controlled shipboard dataset to show that, on average, the entire Argo oxygen dataset is offset relative to shipboard measurements (float minus ship) at pressures of 1450 to 2000 db by -1.9 ± 4.7 µmol kg-1 (95% confidence interval around the mean of {-2.3, -1.5}) and air calibrated floats are offset by -3.1 ± 5.3 µmol kg-1 (95% CI: {-3.7, -2.4}). The difference between float and shipboard oxygen is most likely due to offsets in the float oxygen data and not due to oxygen changes at these depths or biases in the shipboard dataset. In addition to posing problems for the calculation of long-term ocean oxygen changes, these float oxygen offsets impact the adjustment of float nitrate and pH measurements and therefore bias important derived quantities such as the partial pressure of CO2 (pCO2) and dissolved inorganic carbon. Correcting floats with air-calibrated oxygen for the float-ship oxygen offsets changes float pH by 3.2 ± 3.8 mpH and float-derived surface pCO2 by -3.3 ± 4.1 µatm. This adjustment to float pCO2 represents half, or more, of the bias in float-derived pCO2 reported in studies comparing float pCO2 to shipboard pCO2 measurements.

We assess the Southern Ocean CO2 uptake (1985-2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project phase 2 (RECCAP2). The Southern Ocean acted as a sink for CO2 with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75±0.28 PgCyr-1) and pCO2-observation-based products (0.73±0.07 PgCyr-1). This sink is only half that reported by RECCAP1. The present-day net uptake is to first order a response to rising atmospheric CO2, driving large amounts of anthropogenic CO2 (Cant) into the ocean, thereby overcompensating the loss of natural CO2 to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO2-products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26±0.06 and 0.11±0.03 PgCyr-1 decade-1 respectively). This is despite nearly identical pCO2 trends in GOBMs and pCO2-products when both products are compared only at the locations where pCO2 was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non-thermal processes of biology and mixing/circulation. Ocean interior accumulation of Cant points to an underestimate of Cant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push towards better process understanding of drivers of the carbon cycle.

The Southern Ocean Seasonal Sea Ice Zone (SSIZ) is characterized by the development of spring phytoplankton blooms following retreating sea ice. Until recently, assessing SSIZ bloom carbon export has been limited by a lack of under ice observations. Here, we relate the timing of phytoplankton growth to the drawdown of surface nutrients and sea ice cover and estimate spring bloom net community production (bNCP) using biogeochemical profiling float observations. The onset of biological production follows initial sea ice breakup with 64% of bNCP under partial sea ice cover. Estimates of bNCP range from <1 to >4 mol C m-2 bloom-1, with earlier sea ice breakup associated with higher bNCP, and the highest bNCP where micronutrient supply is likely enhanced by topographic-driven mixing. These results indicate that satellite-derived export estimates will underestimate bNCP in the SSIZ and have implications for future carbon export in a changing Southern Ocean sea ice regime.