Features of coastal current instabilities in the North Sea: Results of a numerical model

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Abstract
Coastal currents that are forced by buoyancy discharges from rivers often show a tendency to become unstable under certain conditions. Two such coastal current systems in the North Sea, the German Bight and the Skagerrak, and their instabilities are investigated and compared here with the help of a fully nonlinear, three‐dimensional, baroclinic circulation model. Differences between the two regions were found with respect to three main aspects. First, in the Skagerrak, instabilities contribute to the kinetic energy on a scale that is comparable to the mean kinetic energy. This does not hold for the German Bight. Second, of the two forms of instabilities fed by baroclinic and barotropic energy conversion, in the Skagerrak, barotropic conversion is as important as baroclinic instability, while in the German Bight, baroclinic conversion clearly dominates. Finally, in the German Bight, instability tends to occur only if supported by wind in a direction that helps establishing a steep front against strong tidal mixing. One explanation for the distinction in the features of the two regions is the water depth (German Bight is <40 m; Skagerrak is ∼300 m) and, consequently, the amount of tidal mixing applied to the near‐surface coastal current front. Several deep and shallow water coastal currents outside the North Sea are found to exhibit features similar to those presented for the Skagerrak and German Bight.