Optical, physical, and biological data collected in the western Pacific warm water pool (WWP) as part of the Tropical Ocean-Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) are used to assess variations in the transmission of solar radiation through the water column, to investigate factors that regulate in-water solar transmission, and to examine the sensitivity of upper ocean thermal processes to solar transmission parameterizations. Solar transmission within the upper ocean mixed layer, below 10 m, can be accurately parameterized (within 10%) by using a single exponential profile. Variations in the in-water transmission profile are explained primarily by estimates of the upper ocean chlorophyll concentration (r2=0.83). Mixed layer chlorophyll concentration influences attenuation of the in-water light field. Clouds play a secondary role by altering the shape of the incident solar spectrum. A comparison of solar transmission parameterizations indicates that use of a Jerlov type parameterization for the WWP can lead to a 15 W m-2 error in the solar flux at 30 m (based on a climatological surface irradiance of 220 W m-2). Application of a one-dimensional ocean mixed layer model gives a mean sea surface temperature (SST) error of 0.15 ¿C when the model is forced with a typical Jerlov type solar transmission profile. Instantaneous SST differences nearly reach 1.0 ¿C. An increase in solar flux divergence for a mixed layer results in enhanced stratification and mixed layer shoaling. As mixed layer depth decreases, the quantity of solar radiation penetrating beyond the mixed layer increases, resulting in a destabilization of thermocline waters and a mixed layer depth increase. This feedback mechanism helps regulate both mixed layer depth and the solar irradiance lost to the mixed layer through penetration. ¿ 1998 American Geophysical Union