Performance of the MODIS semi-analytical ocean color algorithm for chlorophyll-a

The Moderate Resolution Imaging Spectroradiometer (MODIS) semi-analytical (SA) algorithm calculates the spectral absorption properties of surface waters, splitting them into those associated with phytoplankton, aph(λ), colored dissolved organic matter or gelbstoff, ag(λ), and water, aw(λ). The phytoplankton absorption coefficient, aph(675), is then used to derive the concentration of chlorophyll-a, Chlor_a_3. The SA algorithm is designed to respond to variable ratios of aph(λ) to ag(λ) and to wide ranges in the chlorophyll-specific phytoplankton absorption coefficient, a*ph(λ), for a given chlorophyll-a level. In this paper, the SA algorithm is expanded to include environments consistent with strong upwelling zones and high latitudes. Spatial and temporal differences in MODIS Terra chlorophyll-a retrievals are examined between Chlor_a_3 and an empirical algorithm, Chlor_a_2, developed to mimic the performance of the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) OC-4 chlorophyll-a algorithm. The greatest differences observed are for upwelling regions and for southern high-latitude waters during austral spring where Chlor_a_2 values are on average about half of field and Chlor_a_3 values due to lower chlorophyll-specific phytoplankton absorption coefficients typical of this region. Preliminary match-up results indicate strong linearity and good agreement between in situ chlorophyll-a concentrations and MODIS-derived Chlor_a_3 compared to Chlor_a_2.