Abstract
AbstractA 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: and . 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 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.
Highlights
Air-sea exchange is an important process in the global budgets of many trace gases, with significant implications for climate, biogeochemical cycles, and pollution transport
HiWinGS meteorological data, sea state, eddy correlation flux measurements, and gas transfer coefficients for CO2 and DMS will be presented
We show summary plots of supporting measurements as necessary, but for additional detail the reader is directed to other HiWinGS publications: Yang et al (2014, 2016) have previously reported measurements and model results for methanol and acetone fluxes, sensible heat flux, and wind stress; Kim et al (2017) present results for air-sea emissions of biogenic organic compounds and their influence on aerosol size distributions; and Brumer et al (2017) report on the wind and sea state dependencies of whitecap fraction
Summary
Air-sea exchange is an important process in the global budgets of many trace gases, with significant implications for climate, biogeochemical cycles, and pollution transport. Modeling air-sea gas fluxes requires an accurate description of the air-sea concentration gradient and the transfer rate coefficient (transfer velocity). The gradient in atmospheric surface layer and surface seawater gas concentration is the driving force for the flux. The transfer coefficient specifies the effects of physical diffusive mechanisms that facilitate gas transfer. Gas concentrations can be estimated from global gridded climatological extrapolations based on historical data sets (e.g., Kettle et al, 1999; Takahashi et al, 2009) or computed with output from ocean biogeochemical models and atmospheric chemical transport models. The transfer coefficient can be represented as a simple empirical function of the primary forcing factor, wind speed, based on laboratory or field measurements, or computed from a parametric description of the physical mechanisms utilizing a broader set of primary forcing factors.
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