The Southern Ocean plays an important role in the ocean’s uptake of heat and carbon yet the processes controlling this uptake are not well understood. To date, more than 100 biogeochemical profiling floats that measure water column pH, oxygen, nitrate, fluorescence, and backscattering at 10-day intervals have been deployed throughout the Southern Ocean as part of the Southern Ocean Carbon and Climate Observations and Modeling Project (SOCCOM). Empirical algorithms are developed from shipboard bottle data that estimate pH in the Pacific sector of the Southern Ocean. These algorithms are applied to estimate pH on floats with no pH sensors and to validate and adjust sensor data from floats with pH sensors. The adjusted float data provide, for the first time, full seasonal cycles in surface and water column pH on weekly resolution throughout the Southern Ocean, including under sea ice. These pH data are then used to derive other carbonate system parameters, such as dissolved inorganic carbon, the saturation state of aragonite, and the partial pressure of carbon dioxide (pCO₂). Detailed analysis of the uncertainties in these derived parameters as well as comparisons with existing data and climatologies suggest that despite their increased uncertainty relative to direct measurements, these float-derived carbonate system parameters can be used to improve climatological and model-based estimates for oceanic carbon flux, as well as to increase knowledge of spatial, seasonal, and interannual variability in air-sea carbon flux.
Float-based climatological seasonal cycles for all carbonate system parameters for the years 2014-2017 are calculated and drivers controlling the seasonal cycles are parsed out and examined across the frontal regions. The float-based climatologies are systematically compared with existing climatologies as well as with several fully-coupled Earth System Models (ESMs). Significant differences are found in this comparison suggesting that a previous lack of wintertime data has led to underestimations of the winter outgassing of carbon dioxide south of the Polar Front of the Antarctic Circumpolar Current. These results highlight areas for improvement in the existing climatologies and ESMs while also introducing new questions about the role of spatial, interannual, and decadal variability in these observed differences. These float-based climatological seasonal cycles are then used in combination with ESM projections for increasing atmospheric carbon dioxide under the Representative Concentration Pathways 4.5 and 8.5 to predict the timing of onset of aragonite undersaturation and hypercapnia (seawater pCO₂ > 1000 μatm) across five Southern Ocean frontal regions under a changing climate. Future changes in the seasonality of pH, pCO₂, and the saturation state of aragonite using this method are found to be significantly greater than ESM-based estimates. This information can be used to inform laboratory studies aiming to test the effects of ocean acidification and hypercapnia on marine organisms as well as to improve predictions of how the ocean’s role in climate may change in the future.