Comparing float pCO2 profiles in the Southern Ocean to ship data reveals discrepancies

The Southern Ocean plays a crucial role in the global carbon cycle. Recently, the utilization of biogeochemical (BGC) Argo float data has provided valuable insights into the uptake and release of carbon dioxide (CO2) by this region. However, significant uncertainty remains regarding the accuracy of pCO2 (partial pressure of CO2) values derived from float data. In this study, we compared pCO2 estimates obtained from float pH data with those from ship-collected data across the Southern Ocean, employing pCO2-depth, pCO2-O2 and CO2-O2 vs saturation plots to assess the degree of agreement between these two datasets. Our findings reveal significant systematic differences. A preliminary analysis, ignoring other factors, found that the float data is consistently higher, on average, than the ship data at equivalent depths and oxygen levels. We tested the hypothesis that inaccurate float pH data or float pCO2 correction process is the main cause of the pCO2 difference, by quantifying other factors that could produce systematic differences, including: (i) spatial sampling bias, (ii) seasonal bias, (iii) errors in estimated alkalinity, (iv) errors in carbonate system constants, and (v) higher levels of anthropogenic CO2 in float data. However, none of the other factors were found to be able to fully account for the discrepancies, suggesting issues with float pH data quality and/or the float pCO2 correction process. Additional analysis included refinements to ship-based and float-based pCO2 before intercomparison. Overall, we estimate that, in the Southern Ocean, surface pCO2 from floats is biased high by, on average, at least 10 μatm.