1 Department of Hydrodynamics and Water Resocurces, Technical University of Denmark2 Section for Arctic Technology, Department of Civil Engineering, Technical University of Denmark
Surface Fronts in the Kattegat
In stratified waters like those around Denmark there is a close correlation between the biology of the water masses and their structure and currents; this is known as dynamic biologicaloceanography. The currents are particularly strong near the fronts, which can be seen in several places throughout the area. When the rotation of the earth is considered, these frontal flows can account for many hitherto unsolved features in the Kattegat, a sea within the Danish inland waters. Important aspects of the dynamics of rotating stratified systems are reviewed. These include the time scale, which equals the reciprocal of the socalled Coriolis parameter, and the length scale, which is known as the Rossby radius. Also, because of their limited width currents influenced by rotation are quite persistent. The flow which results from the introduction of a surface level discontinuity across a wide channel is discussed; this situation resembles the conditions in the Kattegat where the transition towards the Skagerrak consists of a front---the Kattegat/Skagerrak front. The remarkable difference between changing the upstream and the downstream boundary conditions in a rotating system is discussed.A three-dimensional hydrodynamic model, MIKE 3, is used to show that the features outlined above also apply to highly perturbed systems like the Kattegat/Skagerrak front. Based on the model results, a general flow pattern for the Kattegat is discussed. This picture accounts for several dynamical features of the area which have been unexplained so far. These include that outflowing water from the Great Belt can often be found along the east coast of Jutland, and that a comparatively large part of the flow of brackish water through the Kattegat takes place in the narrow channel west of the island Læsø.In connection with the results of the numerical model a mechanism for the generation of along-frontal instabilities and eddies is suggested. Also, the effect of an irregular bathymetry is studied.Together with observations of wind and water levels some of the oceanographical observations from the old lightvessels are used to study the baroclinic flows of the Kattegat. The observations support the suggested picture of the general circulation. Also, they indicate how weakly outflowing water masses from the Baltic are governed by this circulation. In case of strong outflow, the buoyancy flux from the brackish water overrules this circulation to produce coastal currents flowing cyclonically through the Kattegat.Off the headland Skagen, the lightvessel observations together with earlier studies suggest that strong wind-driven currents are responsible for the location of the Kattegat/Skagerrak frontin this area.Observations from the interior of the Kattegat show that strong baroclinic currents can be found also away from the major frontal areas. This is also the impression when studying the shown satellite image. Mechanisms for generation of these fronts are discussed.Finally, MIKE 3 is used to study a conceptual model of the Kattegat. This model does not include mixing and friction. Because of the limited extent of the model compared to the actual size of the Kattegat, baroclinic instabilities to some extent conceal the general flow patterns. In spite of that, the implications of the Kattegat/Skagerrak front for the dynamics of the area are shown. In connection with this, some results of the DYNOCS project are briefly discussed.It is expected that the results of this work can contribute significantly to a progress of the understanding of the dynamic biological oceanography of the Kattegat. Furthermore, thestratification together with the horizontal extent of many other parts of the Danish inland waters implies that the dynamics of these should also be discussed in terms of rotational effects.
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Department of Hydrodynamics and Water Resources, Technical University of Denmark, 1998