Optical Measurements of Whitecaps and Bubbles During Large Scale Wave Breaking in the Southern Ocean
Date of Completion
bubbles, whitecaps, ocean color, wave breaking
Heidi M. Dierssen
Michael S. Twardowski
Edward C. Monahan
James B. Edson
Field of Study
Doctor of Philosophy
Wave breaking contributes to climate relevant processes, such as the production of sea salt aerosols and the exchange of gas (e.g. CO2, CH4, DMS, water vapor) and heat between the ocean and atmosphere. Quantifying these processes, however, has been hampered by the lack of field data under high wind conditions and the inherent challenges in measuring whitecaps. Here, optical tools were developed to estimate metrics of whitecaps and bubbles associated with wave breaking along the polar front zone of the Atlantic sector of the Southern Ocean. In this study, wind speeds exceeded 15 m s-1, significant wave heights routinely surpassed 4 m, fractional whitecap coverage exceeded 5%, and bubble plumes penetrated to over 10 m depth in the water column. With a single channel above-water radiometer mounted on a ship, metrics were developed to quantify wave breaking intensity, duration and decay rate, and fractional whitecap coverage. Radiometric estimates of whitecap coverage followed a cubic dependence with wind speed and captured more of the decaying bubble plume area than methods using high-resolution digital imagery. Optical measurements of the near forward volume scattering function and the critical scattering angle for bubbles (~80°) were used to detect deeply penetrating bubbles ranging from 0.5 to 125 μm radius. When extrapolated to 4 m depth, our estimates suggest that the small bubbles here could be supplying ~36% of the total void fraction and likely contributed to the supersaturation of low solubility gases. Finally, preliminary results from a least-squares inversion technique applied to measurements of the bulk optical volume scattering function suggest that a persistent, small (mode radius~0.2 μm) bubble population with a narrow size distribution contributed between 5% (low wind) and 20% (high wind) to the total backscattering during the experiment. Under high wind conditions, foam and bubbles serve to enhance the magnitude and alter the spectral distribution of light leaving the surface ocean, which can impact penetration of light to depth and associated heating rates. These optical tools can be used to better quantify air-sea processes related to wave breaking throughout the ocean and the potential impact on global climate.
Randolph, Kaylan L., "Optical Measurements of Whitecaps and Bubbles During Large Scale Wave Breaking in the Southern Ocean" (2015). Doctoral Dissertations. 901.