A Net CO ₂ Sink in the Korea Strait and East/Japan Sea Revealed by Direct Eddy Covariance Measurements

Abstract

The Korea Strait (KS) and East/Japan Sea (EJS), Kuroshio‐influenced regions of the northwestern Pacific, are important yet still poorly constrained components of the regional carbon budget. Air‐sea carbon dioxide (CO ₂ ) fluxes in KS and EJS were quantified using direct eddy covariance measurements collected during seasonal cruises between 2023 and 2025. During the observed spring and autumn periods, both regions acted predominantly as net CO ₂ sinks, with mean CO ₂ fluxes ranging from −0.55 to −1.96 μmol m ⁻²  s ⁻¹ . Random Forest regression revealed distinct seasonal and regional shifts in the variables most strongly associated with CO ₂ flux variability. In KS, spring CO ₂ flux variability was more strongly associated with chlorophyll‐a concentrations under relatively stable atmospheric conditions. In autumn, the dominant association shifted toward friction velocity, indicating a stronger role of wind‐driven turbulence and gas transfer. In EJS, physical predictors dominated in both seasons (Richardson number in spring, friction velocity in autumn), indicating a persistently strong influence of physical forcing. Enhanced air‐sea temperature differences in autumn likely contributed to atmospheric instability and elevated wind‐driven turbulence, supporting continued CO ₂ uptake despite reduced solubility in warmer water. The observed eddy covariance fluxes exceeded regional bulk‐derived estimates by approximately an order of magnitude or more, consistent with independent EC measurements and suggesting that bulk parameterizations may underestimate turbulent CO ₂ exchange in dynamic shelf and gateway environments. These findings highlight the value of direct turbulent flux measurements for capturing short‐term wind forcing in regional carbon‐cycle estimates, particularly in dynamic western boundary current‐influenced systems.