Improved state estimations of lower trophic ecosystems in the global ocean based on a Green's function approach

A lower trophic ecosystem model, which was originally developed for describing the variability in plankton distribution in the western North Pacific, is applied to the global ocean. A realistic time series of dynamically self-consistent circulation fields obtained by a 4-dimensional variational ocean data assimilation experiment is used for the environmental fields in the ecosystem model. An optimization of the bio-ecological parameter values in 40 provinces by a Green's function approach enables the ecosystem model to represent the observed variabilities of nutrients and surface chlorophyll concentration with a relatively small number (11) of compartments. In addition, artificial trends of the ecosystem variables due to model errors are generally suppressed by the optimization. The obtained parameter values and ecosystem fields are generally consistent with observations in terms of assimilated and non-assimilated metrics. The integrated lower-trophic ecosystem and physical fields in the global ocean offer important information on the mechanisms (such as El Niños) responsible for the seasonal-interannual variability in the lower trophic ecosystem. Our results demonstrate that, with adequate parameter sets capable of modifying indirectly the model and significance of each pathway to reflect the characteristics of local species, a simple ecosystem model can be applied in quantitative analyses to enhance our understanding of the influence of climate variations on the ocean ecosystem.