UC San Diego

A thorough understanding of the oceanic light fields is required to support studies of various biological, chemical, and physical processes and phenomena in the ocean. The interaction of light with seawater and its constituents involves absorption (change of radiant energy into another form of energy), elastic scattering (change in light propagation direction but not wavelength), and inelastic radiative processes (change in light wavelength and propagation direction). The absorption and elastic scattering have been the primary research focus for decades. The inelastic processes have been less investigated and often ignored in oceanographic studies or applications. The inelastic processes, including Raman scattering and fluorescence, have been demonstrated to significantly affect the oceanic light fields. However, a systematic examination of these influences within different ocean layers is lacking. I studied the effects of inelastic processes on oceanic light fields in the near-surface (0-10 m), euphotic (0-200 m), and mesopelagic (200-1000 m) layers.

I modeled the upwelling radiance within the top 10 m of the ocean surface layer. The inelastic processes dramatically affect the upwelling radiance and its attenuation coefficient in the red and near-infrared spectral regions, indicating that common approaches for estimating water-leaving radiance from extrapolating measurements of upwelling radiance are inadequate. A new strategy is proposed for more accurate in-situ determinations of water-leaving radiance, which is critical for ocean color applications. Using both a unique field dataset and radiative transfer modeling I examined the effects of inelastic processes in the euphotic layer. I demonstrate distinctive features caused by inelastic processes in the irradiance and radiance fields as well as apparent optical properties for realistic scenarios of optically non-uniform water column. I also demonstrate the role of inelastic processes in photosynthetically available radiation and heating within the upper ocean. Finally, I modeled the mesopelagic light field to comprehensively characterize its magnitude, spectral composition, and angular distribution, which is important for understanding the habitat of deep-sea animals. In contrast to common assumptions, my results show much higher magnitude of green and red light at mesopelagic depths primarily owing to Raman scattering. The results also show a nearly-asymptotic regime of light field below ~400 m.