The ICES-boxes approach in relation to results of a North Sea circulation model

Based on the division of the North Sea into the ICES boxes, often used as a tool to present data aggregated from measurements, the results of a 3-D baroclinic model for 11 years of simulation are presented. Since the information on the ICES boxes from the 3-D model is available as synoptic property, a comprehensive description of the water budget, the transport through the boxes as well as the flushing times is attained. Moreover, the results of the 3-D model allows for a quantification of the horizontal diffusion acting between the ICES boxes. Integrated properties of the boxes are obtained from comparing the depth of the thermocline derived by vertical diffusion and by the gradient in the temperature profile. Considerable changes can be observed for the period of stratification within the boxes from the gradient in the temperature profile compared to potential energy calculations. The properties of the ICES boxes as the basis for a box model are presented by means of the ecosystem model ERSEM. A tracer study, using freshwater as a conservative tracer, compares the results of the ERSEM box model with the results aggregated from a gridded dispersion model, using the same physical forcing. From this comparison, an appropriate transport representation for the box model is derived. Furthermore the results of ERSEM demonstrate that a modification of the boxes, i.e., by introducing a vertical separation for those boxes which are stratified in summer, can improve the representation of the biological processes influenced by the thermocline dynamics.