The Influence of a Novel Salinity-Based Solar Attenuation Algorithm on Modeled Temperature in Mobile Bay

The attenuation of solar radiation in the upper ocean and its effect on the vertical temperature structure of the water column is essential for representing physical and biogeochemical models of marine systems. However, the optical properties of water in coastal regions have temporal and spatial scales that are difficult to resolve using common numerical modeling systems. This limits the ability of numerical models to accurately reproduce observed thermal conditions. Here, we tested an empirically derived light attenuation algorithm that is dependent on surface salinity. Using extensive field measurements of vertical photosynthetically available radiation (PAR) profiles in Mobile Bay and the adjacent coastal waters, we found that a single exponential function based on the Beer-Lambert law provided the most precise calculation of the attenuation of downward irradiance. The attenuation coefficients had a non-linear, negative correlation with sea surface salinity, which could be represented by a power function. The negative correlation is consistent with the physical expectation that high attenuation is associated with turbid, freshwater conditions while low attenuation is associated with clearer, marine water conditions. This salinity-based attenuation algorithm will be implemented into the Regional Ocean Modeling System (ROMS) to compare simulated vertical temperature profiles based on our algorithm versus the Jerlov attenuation algorithms provided in ROMS. Our salinity-based attenuation algorithm in conjunction with a previous developed surface heat flux correction term improved the accuracy of the modeled temperature and density profiles in Mobile Bay by better capturing temporal and spatial variations in water mass optical properties.