Ocean radiant heating. Part II: Parameterizing solar radiation transmission through the upper ocean

Abstract

Accurate determination of sea surface temperature (SST) is critical to the success of coupled ocean-atmosphere models and the understanding of global climate. To accurately predict SST, both the quantity of solar radiation incident at the sea surface and its divergence, or transmission, within the water column must be known. Net irradiance profiles modeled with a radiative transfer model are used to develop an empirical solar transmission parameterization that depends on upper ocean chlorophyll concentration, cloud amount, and solar zenith angle. These factors explain nearly all of the variations in solar transmission. The parameterization is developed by expressing each of the modeled irradiance profiles as a sum of four exponential terms. The fit parameters are then written as linear combinations of chlorophyll concentration and cloud amount under cloudy skies, and chlorophyll concentration and solar zenith angle during clear-sky periods. Model validation gives a climatological rms error profile that is less than 4 W m-2 throughout the water column (when normalized to a surface irradiance of 200 W m-2). Compared with existing solar transmission parameterizations this is a significant improvement in model skill. the two-equation solar transmission parameterization is incorporated into the TOGA COARE bulk flux model to quantify its effects on SST and subsequent rates of air-sea heat exchange during a low wind, high insolation period. The improved solar transmission parameterization gives a mean 12 W m-2 reduction in the quantity of solar radiation attenuated within the top few meters of the ocean compared with the transmission parameterization originally used. This results in instantaneous differences in SST and the net air-sea heat flux that often reach 0.2°C and 5 W m-2, respectively.