Satellite observations of water transparency from VIIRS in global aquatic ecosystems

This study presents a new satellite ocean color data record of Secchi depth (Z_SD) observations from the Visible Infrared Imaging Radiometer Suite (VIIRS). As part of the NOAA enterprise satellite data processing, the decade-long Z_SD data are derived from the visible and near-infrared reflectance measurements over oceanic, coastal, and inland waters. Based on in situ data, the model is excellent in generating low-uncertainty Z_SD data with an absolute percentage difference (APD) of 15%–29%. The satellite and in situ matchups confirm reliable satellite retrievals with APD = 19%–26% over the Z_SD range of 0.1–60 m. Although the product uncertainties are dependent on optical water types, assessments show that the satellite Z_SD estimations are very reliable, especially where Z_SD ≥ 1 m. This new satellite product has enabled the ability to access Level-2 daily Z_SD imagery and information as well as Level-3 data aggregated on daily to monthly scales. Our examination indicates that the ocean transparent windows are situated at 443 and 486 nm in the vast open oceans and 551 nm for most coastal waters. They shift to the red band at 671 nm for extremely turbid environments, such as large river estuaries. From a global perspective, the Z_SD data extend from less than half a meter in nearshore environments to >70 m in the South Pacific Gyre, while demonstrating a strong dependency on the optical water types. Short-term fluctuations over time are registered in the satellite Z_SD daily and monthly data from almost every aquatic environment. Trend analyses reveal significant increases in water transparency over many regions, especially the open ocean. We stress the necessity of normalizing satellite Z_SD estimations to eliminate the uncertainties induced by different solar-zenith angles. The present satellite products can be further improved by accounting for the limitations imposed by the multispectral reflectance data with hyperspectral ocean color spectra.