On the consistency of MODIS chlorophyll $a$ products in the northern South China Sea

[摘要] Chlorophyll a (Chl) concentrations derived from satellite measurements havebeen used in oceanographic research, for example to interpret eco-responsesto environmental changes on global and regional scales. However, it isunclear how existing Chl products compare with each other in terms ofaccuracy and consistency in revealing temporal and spatial patterns,especially in the optically complex marginal seas. In this study, we examinedthree MODIS (Moderate Resolution Imaging Spectroradiometer) Chl data productsthat have been made available to the community by the US National Aeronauticsand Space Administration (NASA) using community-accepted algorithms anddefault parameterization. These included the products derived from the OC3M(ocean chlorophyll three-band algorithm for MODIS), GSM(Garver–Siegel–Maritorena model) and GIOP (generalized inherent opticalproperties) algorithms. We compared their temporal variations and spatialdistributions in the northern South China Sea. We found that the threeproducts appeared to capture general features such as unique winter peaks atthe Southeast Asian Time-series Study station (SEATS, 18° N,116° E) and the Pearl River plume associated blooms in summer. Theirabsolute magnitudes, however, may be questionable in the coastal zones.Additional error statistics using field measured Chl as the truthdemonstrated that the three MODIS Chl products may contain high degree ofuncertainties in the study region. Root mean square error (RMSE) of theproducts from OC3M and GSM (on a log scale) was about 0.4 and averagepercentage error (ε) was ~ 115% (Chl between0.05–10.41 mg m⁻³, n = 114). GIOP with default parameterization ledto higher errors (ε = 329%). An attempt to tune thealgorithms based on a local coastal-water bio-optical data set led to reducederrors for Chl retrievals, indicating the importance of local tuning ofglobally-optimized algorithms. Overall, this study points to the need ofcontinuous improvements for algorithm development and parameterization forthe coastal zones of the study region, where quantitative interpretation ofthe current Chl products requires extra caution.

[学科分类] 地球化学与岩石