Simulating biomass assimilation in a Mediterranean ecosystem model using SOFA: setup and identical twin experiments

Assessing the potential improvement of basin scale ecosystem forecasting for the Mediterranean Sea requires biochemical data assimilation techniques. To this aim, a feasibility study of surface biomass assimilation is performed following an identical twin experiment approach. NPZD ecosystem data generator, embedded in one eighth degree general circulation model, is integrated with the reduced-order optimal interpolation System for Ocean Forecasting and Analysis. The synthetic “sea-truth” data are winter daily averages obtained from the control run (CR). The twin experiments consist in performing two runs: the free run (FR) with summer-depleted phytoplankton initial conditions and the assimilated run (AR), in which, starting from the same FR phytoplankton concentrations, weekly surface biomasses averaged from the CR data are assimilated. The FR and AR initial conditions modify the winter bloom state of the phytoplankton all over the basin and reduce the total nitrogen, i.e. the energy of the biochemical ecosystem. The results of this feasibility study shows good performance of the system in the case of phytoplankton, zooplankton, detritus and surface inorganic nitrogen. The weak results in the case of basin inorganic nitrogen and total nitrogen, the latter nonperformant at surface, are discussed.