|Title||The effect of water temperature on air entrainment, bubble plumes, and surface foam in a laboratory breaking-wave analog|
|Publication Type||Journal Article|
|Year of Publication||2014|
|Authors||Callaghan AH, Stokes MD, Deane GB|
|Journal||Journal of Geophysical Research-Oceans|
|Type of Article||Article|
|Keywords||air entrainment; breaking waves; bubbles; conditions; dependence; energy-dissipation; field; foam; generation; liquid jets; mechanisms; oceanic whitecap coverage; plunging jet; sea spray aerosol; variability; Water temperature; whitecaps|
Air-entraining breaking waves form oceanic whitecaps and play a key role in climate regulation through air-sea bubble-mediated gas transfer, and sea spray aerosol production. The effect of varying sea surface temperature on air entrainment, subsurface bubble plume dynamics, and surface foam evolution intrinsic to oceanic whitecaps has not been well studied. By using a breaking wave analog in the laboratory over a range of water temperatures (T-w=5 degrees C to T-w=30 degrees C) and different source waters, we have examined changes in air entrainment, subsurface bubble plumes, and surface foam evolution over the course of a breaking event. For filtered seawater, air entrainment was estimated to increase by 6% between T-w=6 degrees C and T-w=30 degrees C, driven by increases of about 43% in the measured surface roughness of the plunging water sheet. After active air entrainment, the rate of loss of air through bubble degassing was more rapid at colder water temperatures within the first 0.5 s of plume evolution. Thereafter, the trend reversed and bubbles degassed more quickly in warmer water. The largest observed temperature-dependent differences in subsurface bubble distributions occurred at radii greater than about 700 m. Temperature-dependent trends observed in the subsurface bubble plume were mirrored in the temporal evolution of the surface whitecap foam area demonstrating the intrinsic link between surface whitecap foam and the subsurface bubble plume. Differences in foam and plume characteristics due to different water sources were greater than the temperature dependencies for the filtered seawater examined.
|Short Title||J Geophys Res-Oceans|
This study has further highlighted the rapid evolution of the bubble plumes [see Deane and Stokes, 2002]. Within 2 s of the end of active air entrainment, nearly all the supramillimeter radius bubbles have degassed. These large bubbles play an important role in the air-sea gas exchange of CO2 and other highly soluble gases [Keeling, 1993]. To faithfully quantify the presence of these large bubbles due to oceanic breaking waves, measurements must be made rapidly within the first seconds of wave breaking [e.g., Stokes et al., 2002]. Given the difficult challenges in making such field measurements, the majority of historical oceanic bubble data emphasizes the background bubble population or plumes later in their evolution.