Relationship between air-sea gas transfer and short wind waves

Laboratory studies have been conducted in two circular wind wave flumes to investigate the relationship between air-sea transfer velocities of weakly soluble, nonreactive gases and wind-generated surface waves over clean water surfaces and in the presence of surface films. Detailed surface wave measurements have been made using a scanning laser slope gauge. In the circular tanks, longer gravity waves (wavenumber below 12 rad/m) are hardly affected by surfactant, while shorter waves (above 100 rad/m) are significantly reduced. With higher surfactant concentrations, waves above 200-300 rad/m may be completely eliminated. Because of the absence of narrow-banded fetch-limited gravity waves, the wave fields in the circular tanks are significantly different from those in linear wind wave flumes. At a given wind friction velocity, the transfer velocity may decrease by as much as 60% because of surface films. Regardless of the surfactant concentrations, the transfer velocity shows a reasonable correlation with the total mean square slope and With the mean square slope of shorter wind waves (wavenumber above 200 rad/m). However, it shows a poor correlation with the mean square slope of longer wind waves (wavenumber below 50 rad/m). These observations suggest that short wind waves play an important role in air-sea gas exchange.