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

Abstract

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, a ph( λ), colored dissolved organic matter or gelbstoff, a g( λ), and water, a w( λ). The phytoplankton absorption coefficient, a ph(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 a ph( λ) to a g( 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.