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We consider results from two 27-year-long simulation pairs derived using two different ocean models. We focus on the Skagerrak/North Sea area. Each pair consists of the two terrain-following coordinate models ROMS and MIPOM. The first pair utilizes an eddy-permitting grid, that is, a grid in which the Rossby radius is barely resolved. The second pair utilizes an eddy-resolving grid in which the Rossby radius is truly resolved. The goal is to compare the quality of the two models and the two pairs. To this end we derive statistical properties such as probability density functions and compare them with similar statistics derived from observations. Thereby we obtain insight into whether a truly eddy-resolving model is required to realistically capture the mesoscale statistics. We find that eddy resolution is critical to get the mesoscale statistics correct, in particular, the strength of the current jets. Our results also indicate that the improvement gained by employing the eddy-resolving grid is mostly due to a better resolved topography. In particular, we find that this is the case in areas exhibiting prominent topographic features, such as the deep Norwegian Trench cutting into the heart of the northern North Sea/Skagerrak area. The results also highlight the advantage of first performing quality assurance investigations when implementing a new model for a new area. [ABSTRACT FROM AUTHOR]

We assess the performance of an eddy-recognizing numerical ocean model in simulating the pattern and variability of the hydrography in the Skagerrak/northern North Sea area. The model we use is a version of the widely used Princeton ocean model employing a terrain-following vertical coordinate. Results from a series of five multi-year simulations of the mesoscale response are described. The simulations differ in their representation of the lateral freshwater supply to the model ocean of which the first is a reference simulation. The next four are variations in which the river discharges and/or the Baltic outflow are given more realistic representations. For validation, we have used in situ hydrographic data. A novelty is that we use the concepts of freshwater height and potential energy anomaly as objective validation tools. We find that, in general, the model faithfully reproduces many of the observed hydrographic features including their mean patterns and their variance. Not surprisingly, we find that the Baltic outflow is by far the most significant freshwater source in terms of its influence on the hydrography in the area, a result corroborating earlier findings. The best validation is obtained when all freshwater supply is made as realistic as possible, in particular the Baltic outflow. We also find that the large scale cyclonic circulation and the location of fronts are robust characteristics of the Skagerrak/northern North Sea circulation given the impact changes in the freshwater input has on the hydrography. Finally, we find that a further exploration of the impact of the lateral open boundary forcing, e.g., the input of Atlantic water, is needed. [ABSTRACT FROM AUTHOR]

We perform eddy-permitting to eddy-resolving simulations of the Skagerrak/northern North Sea with a terrain-following numerical ocean model. We demonstrate that realistic representations of freshwater input are not required when the focus is on modelling mesoscale structures such as meanders and eddies. To arrive at this conclusion, we analyze the results using a recently developed energy diagnostic scheme to study the sensitivity to realistic representations of the lateral freshwater flux provided to the area from the Baltic Sea and by the major rivers. The scheme is suitable for analysis of growth of instabilities, and it has four basic instability processes prominent. We recognize both horizontal and vertical shear instabilities. There are two processes where average potential energy is converted to eddy kinetic energy, and they are related to the mean gradient in surface elevation and the mean lateral density gradient, respectively. The latter process is known as frontal instability. We demonstrate that the change in the eddy kinetic energy field is small, despite the large variations in the hydrographic properties from experiment to experiment. Moreover, generation of eddy activity appears at the same locations and with approximately the same strength regardless of actual representations of freshwater input. Furthermore, we find that vertical shear instability dominates the energy conversion processes in the Norwegian Coastal Current. Finally, we find that the areas off the northwest coast of Denmark recognized with enhanced eddy kinetic energy level is not caused by instability processes but eddy–eddy interaction rooted in variations in the sea level. [ABSTRACT FROM AUTHOR]