Beschreibung: Physical influences on biological primary production in the North Atlantic are investigated by coupling a four-component pelagic ecosystem model with a high-resolution numerical circulation model. A series of sensitivity experiments demonstrates the important role of an accurate formulation of upper ocean turbulence and advection numerics. The unrealistically large diffusivity implicit in upstream advection approximately doubles primary production when compared with a less diffusive, higher-order, positive-definite advection scheme.This is of particular concern in the equatorial upwelling region where upstream advection leads to a considerable increase of upper ocean nitrate concentrations. Counteracting this effect of unrealistically large implicit diffusion by changes in the biological model could easily lead to misconceptions in the interpretation of ecosystem dynamics. Subgrid-scale diapycnal diffusion strongly controls biological production in the subtropical gyre where winter mixing does not reach the nutricline. The parameterization of vertical viscosity is important mainly in the equatorial region where friction becomes an important agent in the momentum balance.

Beschreibung: A means of assimilating simulated satellite ocean color data with a coupled physical-biological model of the North Atlantic Ocean is implemented, allowing the relative sensitivities of different biological parameters to those data to be investigated. The model consists of an eddy-permitting general circulation model derived from the WOCE Community Modeling Effort and a nitrogen-based, four-compartment NPZD marine ecosystem model. Many of the parameters in marine ecosystem models are poorly known and via assimilation, we hope to better constrain their values. The control parameters chosen for the variational assimilation are the model parameters involved in parameterizations of recycling as these are the most poorly known. Simulated observations are taken while following several floats seeded in varying dynamical biogeochemical provinces of the North Atlantic model domain over a six-month period. Twin experimental results show that, for the given functional forms of growth, mortality and grazing, the following parameters can be successfully recovered from simulated satellite ocean color data: nitrate and detrital recycling parameters in the trade wind domain, zooplankton parameters at higher latitudes (westerly wind and polar domains), and the phytoplankton mortality rate in all regions. By simultaneously assimilating ocean color data in different biological provinces, it becomes possible to successfully constrain all ecosystem parameters at once.

Beschreibung: A model of biological production in the euphotic zone of the North Atlantic has been developed by coupling a Nitrate, Phytoplankton, Zooplankton, Detritus (NPZD) nitrogen-based ecosystem model with an eddy-permitting circulation model. The upper ocean physical and biological results are presented for an experiment with monthly climatological forcing. A comparison with satellite ocean color data shows that the model is capable of a realistic description of the main seasonal and regional patterns of surface chlorophyll. Agreement is also good for primary production except in the subtropical gyre where the model produces values more than an order of magnitude smaller than derived from satellite observations. In situ data available at Joint Global Ocean Flux Study (JGOFS) time series and local study sites (Bermuda Atlantic Time-series Study (BATS), 32°N, 65°W; North Atlantic Bloom Experiment (NABE), 47°N, 2O°W; EUMELI oligotrophic, 21°N, 31°W) are used for a more detailed analysis of the model's capability to simultaneously reproduce seasonal ecosystem dynamics in different biological provinces of the North Atlantic Ocean. The seasonal cycle of phytoplankton biomass and nitrate is simulated quite realistically at all sites. Main discrepancies between model and observations are a large zooplankton peak, required by the model to end the phytoplankton spring bloom at the 47°N, 20°W site, and the underestimation of primary production at EUMELI and under oligotrophic summer conditions at BATS. The former model deficiency can be related to the neglect of phytoplankton aggregation; the latter is caused by too inefficient recycling of nutrients within the euphotic zone. Model improvements are suggested for further steps toward a realistic basin-wide multiprovinces simulation with a single ecosystem model.