Estimating the Transmittance of Visible Solar Radiation in the Upper Ocean Using Secchi Disk Observations

Penetration of visible solar radiation (VSR) drives heating and phytoplankton photosynthesis in the upper water column; thus, it is always important to accurately describe the vertical distribution of VSR in the oceans. Before the invention and application of modern optical-electronic instruments to measure the vertical profiles of VSR, the transmittance of VSR from surface to deeper ocean (T-VSR) was commonly estimated based on water types and subsequently incorporated in dynamic ocean circulation models. However, the measurement of Secchi disk depth (Z(SD)) has been carried out since the 1860s and there are about a million of Z(SD) data available for the global oceans. Because Z(SD) represents a measure of water's transparency, here we present a scheme based on radiative transfer to accurately estimate T-VSR with Z(SD) as the sole input. It is found that the median ratios between modeled and measured T-VSR are similar to 0.8-1.0 for T-VSR in a range of 1-100% for measurements made in coastal waters and oceanic gyres. However, this median ratio spans similar to 0.04-1.0 for the same measurements when the classical water-type-based model was applied. These results suggest a great advantage, and potentially significant impact, in incorporating the volumetric Z(SD) data to model the dynamic ocean-atmosphere systems in the past 100+ years.