Optical Impacts on Solar Transmission in Coastal Waters

The quantification of transmission of solar energy is important for assessment of ocean radiant heating and primary productivity. Upper ocean physical processes and optical parameters have been shown to affect solar transmission through the alteration of the in-water solar attenuation coefficient and the sea surface albedo. In the open ocean, models indicate that cloud cover, solar zenith angle, and chlorophyll concentration have the largest effects on solar transmission. Studies of the influences of coastal ocean processes on solar transmission have been limited until recently. We utilize an extensive time series data set of physical and optical measurements to characterize the physical processes and optical parameters that contribute to the variability of solar transmission through the upper water column in coastal waters. The data were collected on a mooring in 24 m water depth during the Hyperspectral Coastal Ocean Dynamics Experiment (HyCODE) in summer 2001. Coherence analyses of daily noontime values over a 41-day period indicate that cloud cover, chlorophyll concentration, and particulates plus gelbstoff have the greatest effects on solar transmission in the upper ocean in the visible wavelengths. All of these parameters are significantly coherent (negative phase) with solar transmission on a scale of about 1 week. Values of solar transmission at the mixed layer depth (MLD; between 1 to 15 m) range from 0.7 to 63% (average is 19%). The average decrease in solar radiation over the 41-day period is 188 W m (wavelength range of 400-800 nm; average solar radiation is 239 W m and average MLD is 6 m). The maximum decrease in solar radiation is 285 W m (for surface solar radiation of 287 W m, transmission of 0.7%, cloud cover of 6%, and MLD of 13 m).