Relationship between air‐sea gas transfer and short wind waves

TLDR

Circular wind‑wave flumes lack narrow‑band fetch‑limited gravity waves, producing wave fields that differ markedly from those in linear flumes. The study used two circular wind‑wave flumes and a scanning laser slope gauge to measure air–sea transfer velocities of weakly soluble gases over clean and surfactant‑coated water surfaces. Transfer velocities drop up to 60 % with surface films, correlate strongly with short‑wave mean square slope, and short wind waves—especially those above 200 rad m⁻¹—play a key role in air–sea gas exchange, while long waves are largely unaffected.

Abstract

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.