Abstract

    An adjoint method is applied to a three-dimensional global ocean biogeochemical model to optimize the ecosystem parameters using SeaWiFS surface chlorophyll observation. Identical twin experiments were conducted to test the robustness of the optimization technique in recovering the generated synthetic data, in which it was able to identify the most sensitive parameters to be phytoplankton and zooplankton exudation, herbivore egestion as fecal pellets, zooplankton grazing, and the assimilation efficiency. The assimilation of SeaWiFS chlorophyll data yielded significant reduction of the model-data misfit, especially in high latitude regions.  The regional experiments suggested that the parameters in the high latitudes have higher seasonal variance than in the lower latitudes. The optimized model run yields global net primary production of 36 Pg C yr-1, which lies within the range suggested by Antoine et al. [1996].  Additional constraints of nutrient data from the World Ocean Atlas showed further reduction in the model-data misfit and that assimilation with extensive data sets is necessary.


Schematic diagram of the minimization scheme

    In this study, the seasonal SeaWiFS chlorophyll data is used. We averaged the monthly data from January 1997 to December 2003 into seasonal data sets. The data are interpolated into the model grid for the assimilation purposes. Below are the intrpolated seasonal chlorophyll data for boreal winter (a), spring (b), summer (c), and fall (d).

    The model-data misfit are reduced varying the model control variable at three different region (north, tropic, and south) and different seasons. Below are the optimized model chlorophyll output (a posteriori) for winter (a), wpring (b), summer (c), and fall (d).

Antoine, D., J-M. André, and A. Morel (1996), Oceanic primary production 2. Estimation at global scale from satellite (coastal zone color scanner) chlorophyll, Global Biogeochem. Cycles, 10, 57-69.