What Controls Air-Sea Gas Exchange at Extreme Wind Speeds? Evidence from Laboratory Experiments

Abstract

In 1983 Ed Monahan presented the paper “The role of oceanic whitecaps in air-sea gas exchange” at the first International Symposium on Gas Transfer at Water Surfaces. He predicted a steep increase in the gas transfer velocities once the whitecaps can act as low impedance vents. He also postulated that the main role of bubbles is not their additional surface for exchange but that bubbles act as “stirring devices”. In 1983 no suitable experimental data was available to test this simple but keen and visionary idea. Now more than 30 years later laboratory measurements in high wind speed facilities show that a new regime is established at wind speeds beyond 35 m/s, in which the gas transfer increases very steeply with wind speed. This steep increase is independent of the additional flux across bubble surfaces, because it was also observed for gases with moderate solubility including DMS. In fresh water bubble-induced gas transfer is not dominant even at hurricane wind speeds. For low solubility gases such as SF6 and He, an enhancement of at most 25% was found. In sea water transfer across bubble surfaces contributes at most 2/3 of the flux.