Ocean scalar irradiance near-surface maxima

Estimates of the in-water spectral scalar irradiance, E-0, are needed to quantify the photosynthetically available radiation. When a highly scattering, optically thick medium is illuminated at its surface, it is possible under certain conditions for E-0 to increase with penetration depth near the surface, even if there are no internal sources at the wavelength of interest. Analysis and numerical examples presented here help to explain and quantify the magnitude and location of potential subsurface E-0 peaks in source-free ocean waters and the dependence of the phenomenon on the seawater optical properties and surface conditions. Peaking in E-0 most likely occurs when the incident illumination is strongly directed at the zenith angle, and the location of the maximum is deepest when the asymmetry of the scattering phase function is large. The presence of surface waves and internal reflection greatly reduces the chance of E-0 peaks being present, making a maximum in E-0 below the surface possible only if the single scattering albedo, omega(0), is >0.95 in homogeneous waters or at potentially smaller values if omega(0) increases with depth.