Ambient and transient bubble spectral densities in quiescent seas and under spilling breakers

Recently published observations, using laser holography near the ocean surface, have shown that the densities of 10 to 15 micron radius bubbles can be as high as 106 (per cubic meter per micron radius increment) within 3 m of the surface of quiescent seas, thereby supporting the acoustically derived values of two decades ago. The accumulated evidence suggests that these quiescent microbubble densities off the coast of California are proportional to a−4 for radius range 10 < a < 60 microns and a−2.5 for larger bubbles. A calibrated, floating, multi-frequency, acoustical resonator has now been used to obtain the bubble spectrum for 9 radii from 30 to 240 microns, 25 cm under breaking waves in the open sea. At this depth, bubbles larger than 60 microns have a radius dependence approximately a−2.5 and smaller bubble densities vary approximately as a a−4. Our densities are in good agreement with recently published laboratory studies of bubbles, using laser scattering under wind-blown surfaces with about the same surface frictional velocity U*. Older, photographically derived densities at ocean depth 75 cm are close to the new data only over the limited range 50 to 100 microns. Past inconsistencies between acoustical results and optical results were apparently due to an inability of some optical techniques to identify small bubbles in the difficult ocean environment.