Abstract
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 (Ed(490)) and photosynthetically available radiation (PAR) profiles captured by 103 BGC-Argo floats spanning 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 (Kd). Our results show: (1) MODIS-Aqua Kd(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 Kd(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 Kd(PAR) is underestimated below two penetration depth (2zpd), 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−2 d−1 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 Kd(PAR) at depth.
Highlights
Light from the sun fuels oceanic primary production, heats the upper ocean, and oxidizes chemical compounds such as organic molecules
In this study, based on a recently compiled global BGC-Argo dataset, first, we evaluate several ocean color remote sensing algorithms for the diffuse attenuation coefficient at 490 nm (Kd(490)), including two empirical [28,29] and two semi-analytical algorithms [9,30]; we evaluate the performance of the algorithm for the diffuse attenuation coefficient (Kd(PAR)) of photosynthetically available radiation (PAR) [10], at different optical depths; we evaluate several empirical [29,31] and semi-analytical algorithms [32] for the euphotic depth and the 0.415 mol photons m−2 d−1 isolume depth (z0.415)
Two euphotic layer depth algorithms are evaluated here: (i) zeu-L07, is an operational Moderate Resolution Imaging Spectroradiometer (MODIS) product provided by NASA, based on [32], which is an implementation of the Inherent Optical Properties (IOPs)-Kd(PAR) model [10]; (ii) zeu-M07, is derived from MODIS-retrieved Chlasat, based on an empirical Chla-zeu relationship proposed by Morel et al [29]: log10(z eu−M07) = 1.524 − 0.436X − 0.0145X2 + 0.0186X3 (6)
Summary
Light from the sun fuels oceanic primary production, heats the upper ocean, and oxidizes chemical compounds such as organic molecules. In this study, based on a recently compiled global BGC-Argo dataset (described in [15,26,27]), first, we evaluate several ocean color remote sensing algorithms for the diffuse attenuation coefficient at 490 nm (Kd(490)), including two empirical [28,29] and two semi-analytical algorithms [9,30]; we evaluate the performance of the algorithm for the diffuse attenuation coefficient (Kd(PAR)) of photosynthetically available radiation (PAR) [10], at different optical depths; we evaluate several empirical [29,31] and semi-analytical algorithms [32] for the euphotic depth (zeu) and the 0.415 mol photons m−2 d−1 isolume depth (z0.415). The need for validation stems from the fact that these algorithms have not been validated globally and over the whole year
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
