Abstract
The proxy for phytoplankton biomass, Chlorophyll a (Chl a), is an important variable to assess the health and state of the oceans which are under increasing anthropogenic pressures. Prior to the operational use of satellite ocean-colour Chl a to monitor the oceans, rigorous assessments of algorithm performance are necessary to select the most suitable products. Due to their inaccessibility, the oligotrophic open-ocean gyres are under-sampled and therefore under-represented in global in situ data sets. The Atlantic Meridional Transect (AMT) campaigns fill the sampling gap in Atlantic oligotrophic waters. In-water underway spectrophotometric data were collected on three AMT field campaigns in 2016, 2017 and 2018 to assess the performance of Sentinel-3A (S3-A) and Sentinel-3B (S3-B) Ocean and Land Colour Instrument (OLCI) products. Three Chl a algorithms for OLCI were compared: Processing baseline (pb) 2, which uses the ocean colour 4 band ratio algorithm (OC4Me); pb 3 (OL_L2M.003.00) which uses OC4Me and a colour index (CI); and POLYMER v4.8 which models atmosphere and water reflectance and retrieves Chl a as a part of its spectral matching inversion. The POLYMER Chl a for S-3A OLCI performed best. The S-3A OLCI pb 2 tended to under-estimate Chl a especially at low concentrations, while the updated OL_L2M.003.00 provided significant improvements at low concentrations. OLCI data were also compared to MODIS-Aqua (R2018 processing) and Suomi-NPP VIIRS standard products. MODIS-Aqua exhibited good performance similar to OLCI POLYMER whereas Suomi-NPP VIIRS exhibited a slight under-estimate at higher Chl a values. The reasons for the differences were that S-3A OLCI pb 2 Rrs were over-estimated at blue bands which caused the under-estimate in Chl a. There were also some artefacts in the Rrs spectral shape of VIIRS which caused Chl a to be under-estimated at values >0.1 mg m-3. In addition, using in situ Rrs to compute Chl a with OC4Me we found a bias of 25% for these waters, related to the implementation of the OC4ME algorithm for S-3A OLCI. By comparison, the updated OLCI processor OL_L2M.003.00 significantly improved the Chl a retrievals at lower concentrations corresponding to the AMT measurements. S-3A and S-3B OLCI Chl a products were also compared during the Sentinel-3 mission tandem phase (the period when S-3A and S-3B were flying 30 sec apart along the same orbit). Both S-3A and S-3B OLCI pb 2 under-estimated Chl a especially at low values and the trend was greater for S-3A compared to S-3B. The performance of OLCI was improved by using either OL_L2M.003.00 or POLYMER Chl a. Analysis of coincident satellite images for S-3A OLCI, MODIS-Aqua and VIIRS as composites and over large areas illustrated that OLCI POLYMER gave the highest Chl a concentrations and percentage (%) coverage over the north and south Atlantic gyres, and OLCI pb 2 produced the lowest Chl a and % coverage.
Highlights
Studying Chlorophyll a (Chl a), a proxy of phytoplankton biomass, is fundamental to understanding the role that phytoplankton play in ma rine biogeochemistry, including the carbon cycle, the flow of energy through the trophic food web and climate feedback (Groom et al, 2019; Werdell et al, 2018)
Of the 751 match-ups with S-3A Ocean and Land Colour Instrument (OLCI) pb 2.23–2.29, over half were in the oligotrophic gyres (N = 429)
We quantify the statistical performance of S-3A OLCI, MODIS-Aqua and Suomi-Visible Infra-red Imaging Radiometer Suite (VIIRS) Chl a products against in situ Chl a esti mated spectrophotometrically using a quasi-autonomous underway sampling system deployed on three Atlantic Meridional Transect cam paigns in 2016, 2017 and 2018
Summary
Studying Chlorophyll a (Chl a), a proxy of phytoplankton biomass, is fundamental to understanding the role that phytoplankton play in ma rine biogeochemistry, including the carbon cycle, the flow of energy through the trophic food web and climate feedback (Groom et al, 2019; Werdell et al, 2018). Since 1997, a succession of ocean colour satellites have been launched, mainly by the National Aeronautics and Space Administration (NASA) and European Space Agency (ESA), and from Indian Space Research Organisation, China National Space Agency and geostationary OC satellites launched by the Korean Ocean Satellite Centre and Japanese Meteorological Agency. These OC missions have provided data at an unprecedented frequency, both in time and space, to enable synoptic tracking of changes in phytoplankton biomass across the world’s oceans, which had not previously been possible (Blondeau-Patissier et al, 2014). The recent Copernicus Sentinel mission was launched in 2015 and will provide a further two decades of ocean colour data (Donlon et al, 2012)
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