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

Breaking waves at the ocean's surface inject bubbles and turbulence into th e water column. During periods of rough weather the scales of wave breaking will increase with increasing sea states and result in mixing of the surfa ce waters and the turbulent transport of bubbles to depth. Depending on the ir concentrations and size distribution, the entrained bubbles can signific antly change the optical properties of water, introducing potentially signi ficant errors in retrieval of remotely sensed hyperspectral data products. In this paper, the effects of bubbles on optical scattering in the upper oc ean are investigated through optical scattering calculations based on field measurements of bubble populations. The field measurements were obtained o ffshore Point Conception, California, in June 1997, using an acoustical tec hnique which measured the bubble size distribution at 2 Hz from a surface b uoy designed to follow the longer waves. The effects of the bubbles on the bulk optical scattering and backscattering coefficients, b and b(b), respec tively, are determined by using the acoustically measured size distribution s, and size-dependent scattering efficiencies based on Mie scattering calcu lations. Time series of the bubble distributions measured in rough conditio ns (wind speed, U-10 = 15 m/s, significant wave height, H-1/3 = 3.2 m) sugg est that the bubble contribution to light scattering is highly variable nea r the ocean surface, with values spanning roughly 5 decades over time perio ds of O(10) minutes. Bubble size distributions measured at a 0.7-m depth in dicate that the optical effects of the bubbles on b(b), and hence the remot e sensing reflectance, will be significant at bubble void fractions above 1 0(-6) and that the bubble contribution to total b(b) will exceed values of 10(-2) m(-1) inside bubble clouds.