ALBERT

Description: A marine ecosystem model, that had previously been calibrated in a one-dimensional (1D) mode against observations at three time-series and process-study sites simultaneously, is coupled to a three-dimensional (3D) circulation model of the North and Equatorial Atlantic. Compared to an experiment with a previously employed subjectively tuned ecosystem model, the new 3D-model does not only reduce the model-data misfit at those locations at which observations entered the 1D optimization procedure, but also at an oligotrophic site in the subtropics that had not been considered in the 1D calibration. Basin-scale gridded climatological data sets of nitrate, surface chlorophyll, and satellite-derived primary production also reveal a generally lower model-data misfit for the optimized model. The most significant improvement is found in terms of simulated primary production: on average, primary production is about 2.5 times higher in the optimized model which primarily results from the inclusion of a phytoplankton recycling pathway back to dissolved inorganic nitrogen. This recycling pathway also allows for a successful reproduction of nonvanishing surface nitrate concentrations over large parts of the subpolar North Atlantic. Apart from primary production, the parameter optimization reduces root-mean-square misfits by merely 10–25% and remaining misfits are still much larger than observational error estimates. These residual misfits can be attributed both to errors in the physical model component and to errors in the structure of the ecosystem model, which an objective estimation of ecosystem model parameters by data assimilation alone cannot resolve.