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

Abstract

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−1 and significant wave heights to 8 m. Measurements of k660 for 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: k660 CO2∼U10N1.68 and k660 dms∼U10N1.33. Among seven high wind speed events, CO2 transfer responded to the intensity of wave breaking, which depended on both wind speed and sea state in a complex manner, with k660 CO2 increasing 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 CO2 transfer. 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 k660 with 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.

Key Uncertainties in the Recent Air‐Sea Flux of CO2

“…Specifically, Model 8, based on direct flux measurements of CO 2 (McGillis et al, 2001), implies relatively high transfer velocities in strong winds. More recent direct flux measurements of CO 2 report a diversity of results, ranging from further evidence of very high transfer velocities in strong winds (e.g., Blomquist et al, 2017) to values fairly consistent with the DTE-based models (e.g., Bell et al, 2017).…”

Wave‐Related Reynolds Number Parameterizations of CO2 and DMS Transfer Velocities

“…The latest direct eddy correlation fluxes of CO 2 and DMS were made during the High Wind Gas exchange Study (HiWinGS) (Blomquist et al, ; Brumer et al, ; Yang et al, ) at wind speeds up to ~25 m s −1 . The wavefield was monitored with a Riegl laser altimeter and a Datawell DWR‐4G Waverider buoy.…”

Satellites will address critical science priorities for quantifying ocean carbon

“…The eddy covariance results have shown more variability between studies than the gas tracer results; some differences are attributable to experimental artifacts; however, other differences are due to processes that control fluxes but cannot be captured routinely by wind parameterizations (eg Butterworth and Miller ; Blomquist et al . ), or to region‐specific complexities (eg those that occur in polar waters) (WebPanel 2). As such, scientific focus is now moving away from purely wind‐based proxies for characterizing atmosphere–ocean gas exchange.…”