SIMULATIONS OF THE NORTH-SEA CIRCULATION, ITS VARIABILITY, AND ITS IMPLEMENTATION AS HYDRODYNAMICAL FORCING IN ERSEM

The rationale is given of how the gross physical features of the circu lation and the stratification of the North Sea have been aggregated fo r inclusion in the ecosystem box model ERSEM. As the ecosystem dynamic s are to a large extent determined by small-scale physical events, the ecosystem model is forced with the circulation of a specific year rat her than using the long-term mean circulation field. Especially the ve rtical exchange processes have been explicitly included, because the p rimary production strongly depends on them. Simulations with a general circulation model (GCM), forced by three-hourly meteorological fields , have been utilized to derive daily horizontal transport values drivi ng ERSEM on boxes of sizes of a few 100 km. The daily vertical transpo rts across a fixed 30-m interface provide the necessary short-term eve nt character of the vertical exchange. For the years 1988 and 1989 the properties of the hydrodynamic flow fields are presented in terms of trajectories of the flow, thermocline depths, of water budgets, flushi ng times and diffusion rates. The results of the standard simulation w ith ERSEM show that the daily variability of the circulation, being sm oothed by the box integration procedure, is transferred to the chemica l and biological state variables to a very limited degree only.