Bubble entrainment by breaking waves and their influence on optical scattering in the upper ocean

Abstract

Breaking waves at the ocean's surface inject bubbles and turbulence into the water column. During periods of rough weather the scales of wave breaking will increase with increasing sea states and result in mixing of the surface waters and the turbulent transport of bubbles to depth. Depending on their concentrations and size distribution, the entrained bubbles can significantly change the optical properties of water, introducing potentially significant errors in retrieval of remotely sensed hyperspectral data products. In this paper, the effects of bubbles on optical scattering in the upper ocean are investigated through optical scattering calculations based on field measurements of bubble populations. The field measurements were obtained offshore Point Conception, California, in June 1997, using an acoustical technique which measured the bubble size distribution at 2 Hz from a surface buoy designed to follow the longer waves. The effects of the bubbles on the bulk optical scattering and backscattering coefficients, b and b b , respectively, are determined by using the acoustically measured size distributions, and size‐dependent scattering efficiencies based on Mie scattering calculations. Time series of the bubble distributions measured in rough conditions (wind speed, U 10 = 15 m/s, significant wave height, H 1/3 = 3.2 m) suggest that the bubble contribution to light scattering is highly variable near the ocean surface, with values spanning roughly 5 decades over time periods of O(10) minutes. Bubble size distributions measured at a 0.7‐m depth indicate that the optical effects of the bubbles on b b , and hence the remote sensing reflectance, will be significant at bubble void fractions above 10 −6 and that the bubble contribution to total b b will exceed values of 10 −2 m −1 inside bubble clouds.