Coherens North Sea case study

The program has been applied in previous projects to the study of physical and biological processes in coastal areas with an important river input and for the simulation of the advection and dispersion of contaminants in the North Sea. An important objective is to demonstrate its scientific usefulness by simulating a full annual cycle in typical shelf seas and comparing the results with observational data sets.

The area, selected for the study is the part of the North Sea between 4°W and 57°N including the Channel, the southern Bight and the central North Sea. A fine resolution (6-7 km) grid is adopted in the horizontal allowing a realistic simulation to be made and to study the impact of frontal processes on the biological system. The simulation starts at 1 January 1989 when the water column is vertically mixed and ends at 31 December 1989 covering the formation and erosion of the thermocline and the spring and autumn blooms of plankton. The evolution of the salinity field is simulated including input of the most important river discharges and fluxes at the open boundaries.

The model results are compared with the North Sea Project data collected at more than 120 stations in the southern and central North Sea. The data set consists of vertical profiles of temperature and salinity taken at approximatively monthly intervals. Concentrations of chlorophyll, nitrate and suspended sediment concentrations are available for three depths at most of the stations.

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Results from a first run with the model for the year 1989. The first figure shows the depth of the thermocline in August, taken as the depth where the temperature difference with the surface value first equals 0.5°C and taken to be zero if the surface-bottom temperature difference is lower than 0.5°C. The boundary between the (thermally) stratified and mixed areas is marked by a sharp front which extends eastwards from the British coast upto the central North Sea, then curves upwards first northeastwards and finally northwards parallel to the Danish coast. The transition between the mixed and stratified regime occurs at a water depth of ~40 m as can be observed in the temperature contour plot along a vertical transect at 4.5°E.

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The model results will be compared with the data of the North Sea Project. The locations of the data stations are indicated with a "+". A preliminary comparison has been made at station CS (55°30'N, 0°55'E). The time series plot represents the surface and bottom temperatures. Data values are indicated by diamonds (surface) and squares (bottom). Although the initial temperatures are too low, the onset of stratification is well predicted while model and data are afterwards in good agreement up to July. In the late summer and the autumn vertical stratification is underestimated by the model with lower surface and higher bottom temperatures. This may be due to advective processes which need to be further investigated.