Mean and eddy motion in the Skagerrak/northern North Sea: insight from a numerical model

Insight regarding the mean and eddy motion in the Skagerrak/northern North Sea area is gained through an analysis of model-simulated currents, hydrography, kinetic energy and relative vorticity for the 2 years 2000 and 2001. In this a -coordinate ocean model is used. Since the tidal currents are generally strong in the area, care is exercised to distinguish the mesoscale (eddy) motion from higher-frequency motion such as tides, before computing the mean and eddy kinetic energy. The model-simulated response is first compared with available knowledge of the circulation in the area, and when available, also with sea-surface temperature obtained from satellite imagery. It is concluded that the model appears to faithfully reproduce most of what is known, in particularly the upper mixed layer circulation. An analysis of the mean and eddy kinetic energy reveals that many of the mesoscale structures found in the area are recurrent. This is particularly true for the structures off the southern tip of Norway. Also in general, areas of strong mean and eddy kinetic energy are co-located. The exception is the area off the southern tip of Norway, where the eddy kinetic energy is much larger than its mean counterpart. An analysis of the relative vorticity reveals that the variability found is due to the occurrence of recurrent anticyclonic eddies. It is hypothesized that these eddies are generated due to an offshore veering of the Norwegian coastal current (NCC) as it reaches the eastern end of the Norwegian Trench plateau. Here it becomes a free jet, which is then vulnerable to either barotropic instability caused by the horizontal shear in the jet-like structure of the NCC at this point, or a baroclinic (frontal) instability. The latter may come into play when the NCC veers offshore and its relatively fresh water meets the inflowing saline water of Atlantic origin, a frontogenesis that may become strong enough for cyclogenesis to take place. Due to the depth-independent nature of the model-generated eddies, the barotropic instability is the most likely candidate. It remains to resolve the reason for the offshore veering of the NCC. The most likely candidate mechanisms are vortex squeezing or simply that the coastline curvature is large enough for the NCC to separate from the coast in a hydraulic sense.Responsible Editor: Phil Dyke     

The age as a diagnostic of the dynamics of marine ecosystem models

The constituent-oriented age theory (CAT) worked out by Delhez et al. (1999) is a flexible tool that can be applied to diagnose complex models. It is shown here how this can be used to quantify the pace at which an ecosystem model works. At the cost of the introduction of one additional evolution equation for each compartment of the ecosystem model, the mean age of the biological material forming these compartments can be computed. The information obtained in this way complements the information provided by the concentration data; while the latter measures the standing stocks, the former provides an integrated assessment of the interaction rates and matter fluxes. The benefits of the method are demonstrated with a simple Lotka–Volterra system and a one-dimensional vertical model of the nitrogen cycle in the Ligurian Sea. The theory can be used to study the biological compartments individually or the ecosystem as a whole. In particular, the age is a valuable tool to quantify the overall cycling rate of nitrogen in the food web.Responsible Editor: Phil Dyke