Near-surface stratification due to ice melt biases Arctic air-sea CO2 flux estimates

• Published in: Geophysical Research Letters
• Main Authors: Dong, Yuanxu, Yang, Mingxi, Bakker, Dorothee C. E., Liss, Peter S., Kitidis, Vassilis, Brown, Ian, Chierici, Melissa, Fransson, Agneta, Bell, Thomas G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2021
• Subjects: Arctic; Arctic Ocean; Sea ice
• Online Access: https://ueaeprints.uea.ac.uk/id/eprint/82091/; https://ueaeprints.uea.ac.uk/id/eprint/82091/1/Accepted_Manuscript.pdf; https://doi.org/10.1029/2021GL095266

Air-sea carbon dioxide (CO2) flux is generally estimated by the bulk method using upper ocean CO2 fugacity measurements. In the summertime Arctic, sea-ice melt results in stratification within the upper ocean (top ∼10 m), which can bias bulk CO2 flux estimates when the seawater CO2 fugacity is taken from a ship’s seawater inlet at ∼6 m depth (fCO2w_bulk). Direct flux measurements by eddy covariance are unaffected by near-surface stratification. We use eddy covariance CO2 flux measurements to infer sea surface CO2 fugacity (fCO2w_surface) in the Arctic Ocean. In sea-ice melt regions, fCO2w_surface values are consistently lower than fCO2w_bulk by an average of 39 µatm. Lower fCO2w_surface can be partially accounted for by fresher (≥ 27%) and colder (17%) melt waters. A back-of-the-envelope calculation shows that neglecting the summertime sea-ice melt could lead to a 6–17% underestimate of the annual Arctic Ocean CO2 uptake.