Optical measurements of small deeply penetrating bubble populations generated by breaking waves in the Southern Ocean

Bubble size distributions ranging from 0.5 to 125 mu m radius were measured optically during high winds of 13 m s(-1) and large-scale wave breaking as part of the Southern Ocean Gas Exchange Experiment. Very small bubbles with radii less than 60 mu m were measured at 6-9 m depth using optical measurements of the near-forward volume scattering function and critical scattering angle for bubbles (similar to 80 degrees). The bubble size distributions generally followed a power law distribution with mean slope values ranging from 3.6 to 4.6. The steeper slopes measured here were consistent with what would be expected near the base of the bubble plume. Bubbles, likely stabilized with organic coatings, were present for time periods on the order of 10-100 s at depths of 6-9 m. Here, relatively young seas, with an inverse wave age of approximately 0.88 and shorter characteristic wave scales, produced lower bubble concentrations, shallower bubble penetration depths, and steep bubble size distribution slopes. Conversely, older seas, with an inverse wave age of 0.70 and longer characteristic wave scales, produced relatively higher bubble concentrations penetrating to 15 m depth, larger bubble sizes, and shallower bubble size distribution slopes. When extrapolated to 4 m depth using a previously published bubble size distribution, our estimates suggest that the deeply penetrating small bubbles measured in the Southern Ocean supplied similar to 36% of the total void fraction and likely contributed to the transfer and supersaturation of low-solubility gases.