Wind Speed and Sea State Dependencies of Air‐Sea Gas Transfer: Results From the High Wind Speed Gas Exchange Study (HiWinGS)

A variety of physical mechanisms are jointly responsible for facilitating air‐sea gas transfer through turbulent processes at the atmosphere‐ocean interface. The nature and relative importance of these mechanisms evolves with increasing wind speed. Theoretical and modeling approaches are advancing, but the limited quantity of observational data at high wind speeds hinders the assessment of these efforts. The HiWinGS project successfully measured gas transfer coefficients (k660) with coincident wave statistics under conditions with hourly mean wind speeds up to 24 m s−1and significant wave heights to 8 m. Measurements of k660for carbon dioxide (CO2) and dimethylsulfide (DMS) show an increasing trend with respect to 10 m neutral wind speed (U10N), following a power law relationship of the form: k660CO2∼U10N1.68and k660dms∼U10N1.33. Among seven high wind speed events, CO2transfer responded to the intensity of wave breaking, which depended on both wind speed and sea state in a complex manner, with k660CO2increasing as the wind sea approaches full development. A similar response is not observed for DMS. These results confirm the importance of breaking waves and bubble injection mechanisms in facilitating CO2transfer. A modified version of the Coupled Ocean‐Atmosphere Response Experiment Gas transfer algorithm (COAREG ver. 3.5), incorporating a sea state‐dependent calculation of bubble‐mediated transfer, successfully reproduces the mean trend in observed k660with wind speed for both gases. Significant suppression of gas transfer by large waves was not observed during HiWinGS, in contrast to results from two prior field programs. Large data set of coincident gas transfer and wave measurements under high wind conditionsCOAREG bulk gas transfer model simulates observed gas transfer coefficientsNo evidence for significant suppression of gas transfer in the presence of large waves