| Summary: | The vertical distribution of irradiance in the ocean is a key input to quantify processes spanning from radiative warming, photosynthesis to photo-oxidation. Here we use a novel dataset of thousands local-noon downwelling irradiance at 490 nm (E<sub>d</sub>(490)) and photosynthetically available radiation (PAR) profiles captured by 103 BGC-Argo floats spanning nearly three years (from October 2012 to January 2016) in the world’s ocean, to evaluate several published algorithms and satellite products related to diffuse attenuation coefficient (K<sub>d</sub>). Our results show: 1) MODIS-Aqua K<sub>d</sub>(490) products derived from a blue-to-green algorithm and two semi-analytical algorithms show good consistency with the float-observed values, but the Chla-based one has overestimation in oligotrophic waters; 2) The K<sub>d</sub>(PAR) model based on the Inherent Optical Properties (IOPs) performs well not only at sea-surface but also at depth, except for the oligotrophic waters where K<sub>d</sub>(PAR) is underestimated below two penetration depth (2z<sub>pd</sub>), due to the model’s assumption of a homogeneous distribution of IOPs in the water column which is not true in most oligotrophic waters with deep chlorophyll-a maxima; 3) In addition, published algorithms for the 1% euphotic-layer depth and the depth of 0.415 mol photons m<sup>−2</sup> d<sup>−1 </sup>isolume are evaluated. Algorithms based on Chla generally work well while IOPs-based ones exhibit an overestimation issue in stratified and oligotrophic waters, due to the underestimation of K<sub>d</sub>(PAR) at depth.
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