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

• Main Authors: Zhang, Chuqing, Wu, Yingxu, Brown, Peter J., Stappard, David, Silva, Amavi N., Tyrrell, Toby
• Format: Text
• Language: English
• Published: 2024
• Subjects: Southern Ocean
• Online Access: https://doi.org/10.5194/egusphere-2023-3143; https://egusphere.copernicus.org/preprints/2024/egusphere-2023-3143/

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 (CO 2 ) by this region. However, significant uncertainty remains regarding the accuracy of p CO 2 (partial pressure of CO 2 ) values derived from float data. In this study, we compared p CO 2 estimates obtained from float pH data with those from ship-collected data across the Southern Ocean, employing p CO 2 -depth, p CO 2 -O 2 and CO 2 -O 2 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 p CO 2 correction process is the main cause of the p CO 2 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 CO 2 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 p CO 2 correction process. Additional analysis included refinements to ship-based and float-based p CO 2 before intercomparison. Overall, we estimate that, in the Southern Ocean, surface p CO 2 from floats is biased high by, on average, at least 10 μatm.