On the Parameterization of Irradiance for Open Ocean Photoprocesses

Measurements of downwelling spectral irradiance were made during the fall of 1982 in the North Pacific Ocean (near 33øN, 142øW). These data are used to produce vertical profiles of downwelling irradiance for visible energy (400-700 nm) and photosynthetic available radiation (PAR). The profiles demonstrate that the attenuation of visible energy and PAR are primarily functions of chlorophyll pigment concentration and depth. Commonly used parameterization methods for radiant fluxes and their attenuation, which are based upon either optical water type or mean chlorophyll pigment concentration, do not yield results which compare favorably with the observed profiles. A new parameterization method for the estimation of open ocean in situ radiant fluxes is presented. This method assumes that chlorophyll pigments and pure seawater control the attenuation of spectral irradiance and that the downwelling radiant fluxes just beneath the sea surface are directly proportional to the incident solar flux. Results obtained with this method are in good agreement with the observations. The use of pigment concentrations to characterize attenuation eliminates the need to rely upon empirical optical water mass types, which do not account for temporal or spatial variations of attenuating materials. The present parameterization enables a direct optical coupling between physical and biological processes and is applicable to open ocean observational and modeling studies.