Bias Evaluation for Sensor-Based Dissolved Oxygen from CTD and Profiling Floats in the World Ocean Database

Abstract Sensor-based oxygen (O 2 ) measurements in the ocean have become the dominant data source for in situ O 2 since 2010. We examine the overall quality of the shipborne conductivity–temperature–depth (CTD)- and Argo-measured O 2 in the World Ocean Database (WOD) and evaluate biases by comparing them to independently measured reference bottle-sampled profiles. No significant O 2 difference is found between CTD and bottle data for the global ocean, suggesting high quality of CTD O 2 observations generally. A negative residual bias is found in both postcorrected delayed-mode (−1.69 ± 5.15 μ mol kg −1 , fitted mean ± standard deviation) and real-time-adjusted (−4.68 ± 6.99 μ mol kg −1 ) Argo O 2 . Residual biases have both spatial (basin scale and vertical) and temporal variations, indicating the complexity of residual biases. We also examine Argo O 2 biases based on the type of calibration methods. The Argo delayed-mode O 2 profiles calibrated using the World Ocean Atlas ( WOA ), on average, are offset by −1.60 ± 5.30 μ mol kg −1 , while a larger bias (−3.29 ± 4.86 μ mol kg −1 ) is found for profiles calibrated using in-air measurements for the global ocean. The Argo delayed-mode data calibrated with O 2 profiles from the WOD have a slight positive bias (0.33 ± 4.16 μ mol kg −1 ). Further analysis demonstrates that the O 2 profiles without matched reference profiles also have negative bias similar to those with matched reference profiles when they are compared to WOA . This analysis suggests the negative residual biases in Argo postcorrected O 2 should be carefully considered when using Argo O 2 data to draw conclusive inferences about anthropogenic impacts on ocean oxygen concentrations and/or derived quantities in a changing ocean. Significance Statement This study evaluates the quality of dissolved oxygen (O 2 ) measurements from CTD and Argo profiling floats in the World Ocean Database, comparing them to reference bottle data. Results show that CTD measurements are highly accurate and serve as a reliable reference for Argo O 2 evaluations. Argo data reveal negative residual biases, both in real-time-adjusted and delayed-mode measurements, with variations across spatial, vertical, and temporal scales. Biases are also influenced by calibration methods, with larger discrepancies found in profiles calibrated using in-air measurements compared to those calibrated with the World Ocean Atlas . These findings suggest the presence of global negative biases in Argo O 2 data, highlighting the need for further corrections to improve the calibration methods of Argo O 2 .