Microscale Wave Breaking and Its Effect on Air-Water Gas Transfer Using Infrared Imagery

Abstract : Air-sea gas exchange plays a crucial role in geochemical cycling, and a robust parameterization of the gas transfer velocity, k, is needed to improve models of global fluxes. However, the dependence of k on wind speed or wind stress has been shown to be a function of the concentration of surfactants, which are ubiquitous in nature. Laboratory measurements indicate that a wave-related mechanism regulates gas transfer because the correlation of k with wave slope is unaffected by the presence of surfactants. Microscale wave breaking has been proposed as the underlying physical process that determines k at low to moderate wind speeds. However, quantifying the effects of microscale wave breaking has been difficult because the phenomenon lacks the visible manifestation of whitecapping. Measurements here show that microscale breaking waves produce thermal surface signatures that can be quantified by infrared imaging techniques. Simultaneous and co-located infrared and wave slope imagery of laboratory wind waves show that distinct areas of the water surface where the thermal boundary layer is disrupted occur coincidentally with waves that have a steep forward face and a dimpled bore-like crest The measurements show that these wave-related areas of enhanced surface renewal are the turbulent wakes of microscale breaking waves. The local k within the wakes, measured using the controlled flux technique, is on average 3.5 times greater than k outside the wakes.