Near‐Surface Stratification Due to Ice Melt Biases Arctic Air‐Sea CO<sub>2</sub> Flux Estimates
Yuanxu Dong, Mingxi Yang, Dorothée C. E. Bakker, Peter S. Liss, Vassilis Kitidis, Ian Brown, Melissa Chierici, Agneta Fransson, Thomas G. Bell
Abstract
Abstract Air‐sea carbon dioxide (CO 2 ) flux is generally estimated by the bulk method using upper ocean CO 2 fugacity measurements. In the summertime Arctic, sea‐ice melt results in stratification within the upper ocean (top ∼10 m), which can bias bulk CO 2 flux estimates when the seawater CO 2 fugacity is taken from a ship's seawater inlet at ∼6 m depth ( f CO 2w_bulk ). Direct flux measurements by eddy covariance are unaffected by near‐surface stratification. We use eddy covariance CO 2 flux measurements to infer sea surface CO 2 fugacity ( f CO 2w_surface ) in the Arctic Ocean. In sea‐ice melt regions, f CO 2w_surface values are consistently lower than f CO 2w_bulk by an average of 39 μatm. Lower f CO 2w_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 CO 2 uptake.