The Effect of Water Temperature on Air Entrainment, Bubble Plumes, and Surface Foam in a Laboratory Breaking-Wave Analogue

Sea surface temperature (SST) is a key property of the ocean surface which helps dictate the magnitude of turbulent and radiative energy fluxes between the ocean and atmosphere, and as such plays a key role in regulating the climate and its variability. The injection of air below the ocean surface by breaking waves also plays an important role in climate regulation through bubble-mediated gas exchange of important greenhouse gases such as CO2, through sea spray aerosol production flux from bursting bubbles, and through increases in ocean albedo due to surface whitecap foam. However, the role variable water temperature (Tw) plays in driving changes in air entrainment, bubble plume dynamics, and foam evolution generated by wind-driven oceanic breaking waves has not been well characterized. Because the global distribution of SST spans a range from about 0°C close to the poles to about 30°C nearer the equator, any temperature dependent properties of air entrainment, bubble plume dynamics, and foam evolution resulting from breaking waves are likely to exhibit systematic global variation, which is currently not well understood. However, the effect of varying sea surface temperature on air entrainment, sub-surface bubble plume dynamics, and surface foam evolution intrinsic to oceanic whitecaps has not been well studied.

Here we present results of a laboratory study which investigated the effect of variable water temperature on air entrainment, bubble plume evolution, and surface whitecap foam are evolution by using a breaking wave analogue in the laboratory over a range of water temperatures (Tw = 5 °C to Tw = 30 °C) and different source waters. For filtered seawater, air entrainment was estimated to increase by 6 % between Tw = 6 °C and Tw = 30 °C, driven by increases in the measured surface roughness of the plunging water sheet. After 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. After the first 0.5 s, the trend reversed and bubbles degassed more quickly in warmer water. Due to this time and temperature dependent fractionation of the submerged bubble distribution, large differences in the distribution of sub-surface air volume between the two water temperature extremes were observed to emerge during the bubble plume degassing phase. The largest observed temperature-dependent differences in sub-surface bubble densities occurred at radii greater than about 700 μm. Temperature-dependent trends observed in the sub-surface bubble plume were mirrored in the temporal evolution of the surface whitecap foam area demonstrating the intrinsic link between surface whitecap foam and the sub-surface bubble plume. The surface foam area integrated over the duration of the observational period increased with increasing water temperature by about 10% between the two temperature extremes examined. Differences in foam and plume characteristics due to different water sources were greater than the temperature dependencies for the filtered seawater examined.