A meso-scale model of the central and southern North Sea: Consequences of an improved resolution

Abstract

Results of a meso-scale model with a resolution of approximately 3 km of the central and southern North Sea are presented. The effect of the meso-scale resolution is depicted by comparing these results with data obtained from a 20 km resolution model. The validation of the model by means of observed SST data and of temperature data from a hydrographic transect demonstrates that the meso-scale model is able to reproduce the observations reasonably well. A comparison of the 3 km resolution results with 20 km results additionally shows that the large-scale model is not able to reproduce all the observed features of the SST with the same accuracy, in particular in the near-coastal areas along the Dutch and German coast. The parameters, which are investigated in more detail in this study, are temperature and salinity as well as the mean and eddy kinetic energy of the residual flow. The comparison between meso-scale and large-scale results demonstrates that not only the temperature and salinity distribution but also the kinetic energy is strongly affected by the chosen grid resolution. With respect to the generated mean kinetic energy of the residual flow the refined model resolution produces an increase up to 100% in the eastern parts of the North Sea, whereas in the Humber/Wash and Southern Bight region a decrease by up to 200% was obtained. In the northern part of the North Sea the finer resolution does not lead to large changes in the mean kinetic energy because here the large-scale features of the circulation dominate which also can be resolved by the large-scale model. This also explains that the mean kinetic energy differences between small- and large-scale model results are larger in summer than in winter because in winter due to the stronger winds the large-scale circulation is more dominant than in summer. The additional eddy kinetic energy of the residual flow (defined here as the kinetic energy contained in the spatial scales between 3 and 20 km) in general is around two orders of magnitude smaller than the mean kinetic energy. However, there are several areas where the eddy kinetic energy reaches the same order of magnitude as the mean kinetic energy, for instance in the Southern Bight. From the fact that largest eddy kinetic energy rates occur in areas of strongest gradients of the mean flow it could be deduced that barotropic instabilities play the dominant role. Contrarily, baroclinic instabilities seem to be only of minor importance since areas of strongest density gradients only exhibit very localised maxima.