The tidal currents may reach a speed of a few dm s−1 and dominate any other flow, the more so as they move the whole water column. They give rise to strong mixing of water masses, preventing thermohaline stratification in the shallow southern North Sea. In the Wadden Sea tides cause the periodic exposure of large areas of the sea bed. The shallow topography of the North Sea supports nonlinear effects caused by energy dissipation at the bottom and changing depths due to tidal waves. These processes are stronger than the nonlinearity due

selleck screening library to advection. As a result tidal curves and figures are severely non-harmonic. Averaging the tidal flow results in significant residual currents, which means a permanent displacement of water masses that is independent of any wind or density forcing (see Figure 9). This permanent flow system supports the cyclonic circulation

of the North Sea. It may be mentioned in passing that model calculations with a random forcing at the North Sea’s open boundaries yielded a similar system of residual currents (Günther Radach, personnal communication). Obviously, the specific topography of the North Sea together with nonlinear effects leads to a rectification of chaotic movements. Another example of nonlinearity is Stem Cell Compound Library the superposition of the wind- and density-driven circulation on the tidal flow. Figure 10 shows the propagation of a wind surge in the North Sea with and without considering tidal interaction. The generation of secondary waves around the basin is strongly reduced by tidal dissipation. Backhaus et al. (1986) have shown that whenever Ureohydrolase a constant flow component is combined with a time-dependent periodic tide, there is a considerable reduction in the resulting residual flow. They explained this process by the presence of a much higher energy dissipation due to bottom friction when the actual tide is included, compared to the linear superposition

of constant residual flow fields. In the following, the relevance of the fundamental oceanic forcing mechanisms (geostrophy, Ekman flow, Joint Effect of Baroclinicity and Relief JEBAR) for the North Sea will be examined. The numerical simulations are based on the Hamburg Shelf-Ocean Model HAMSOM, a three-dimensional, baroclinic circulation model with a free surface (Backhaus 1985). For details, see Sarkisyan & Sündermann (2009). Pohlmann (2003) extracted the baroclinic part υ_g=(υg,ug) of the geostrophic flow from the results of the complete circulation model HAMSOM. First, the temperature and salinity fields T  , S  (x  , y  , z  , t  ) were computed on the three-dimensional model grid, next the density ρ  (x  , y  , z  , t  ) was determined by the equation of state, and finally υ_g was calculated: υg(x,y,z,t)=gρf∂∂x(∫z0gρ(x,y,z′,t)dz′), ug(x,y,z,t)=−gρ0f∂∂x(∫z0gρ(x,y,z′,t)dz′),where g is the acceleration due to gravity, f is the Coriolis parameter and ρ0 is a reference density.