Data-assimilation studies of marine, nitrogen based ecosystem models in the North Atlantic Ocean

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Schartau, M. (2001): Data-assimilation studies of marine, nitrogen based ecosystem models in the North Atlantic Ocean , PhD thesis, Christian-Albrechts Universitaet zu Kiel.
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The overall goal of this work is to investigate the performance of ecosystemmodels and to relate their results to existing observations in the NorthAtlantic. Therefore different data assimilation methods are applied. Avariational adjoint technique and a micro-generic algorithm ($\mu$GA) areutilized to estimate model parameters, such that the misfit between modelresults and observations is minimised. Data assimilation experiments areperformed with nitrogen based ecosystem models, comprising three and fourstate variables (NPZ- and NPZD models): dissolved inorganic nitrogen (N),phytoplankton (P), herbivorous zooplankton (Z) and detritus (D). TheNPZ-model simulates mean concentrations of the different variables withinthe upper mixed layer, while the NPZD-model has a vertically resolved grid.Physical boundary conditions are obtained from three-dimensional simulationsof the ocean's circulation in the North Atlantic, with daily mean atmosphericforcing from ECMWF-reanalysis data.First, data assimilation experiments are conducted with observations fromthe Bermuda Atlantic Time-series Study (BATS) in order to optimise theNPZ-model. While applying the adjoint method different optimal parametersets are obtained when starting from different initial parameter sets. It isshown that for parameter optimisation of an ecosystem model, theapplication of the $\mu$GA is superior to the performance of the adjointmethod.Second, simultaneous assimilation experiment are performed with theNPZD-model using observational data from three locations in the NorthAtlantic: BATS, the site of the North Atlantic Bloom Experiment (NABE) andthe Ocean Weather Ship-India (OWS-INDIA). The simultaneous optimisationyields a best parameter set, which can be utilized for basin wide simulationsin coupled physical-biological (general circulation) models of the NorthAtlantic.The parameter set retrieved from the simultaneous optimisations producessubstantial differences in the biogeochemical fluxes when compared withmodel results using previously published parameters. In contrast to earliermodels the rapid cycling of organic matter for sustaining primary productionis emphasized. Furthermore, systematic discrepancies between$^{14}$C-fixation rates and modelled primary production are identified.It is suggested that carbon based primary productivity may not beadequately represented by ecosystem models when a constant nitrogento carbon conversion factor is assumed.The chosen physical boundary conditions are adequate to simulate thebiogeochemical fluxes at the BATS and NABE sites. At high latitudes(OWS-INDIA), however, the physical-biological interactions in themodel cannot represent the observed chlorophyll distribution in spring.It is suggested that during this period short-termed alterations ofstratification, rapid biological response and deep mixing of phytoplanktonare necessary in order to reproduce chlorophyllconcentrations at depths of 150-200m.

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