As an example, Figure 11 shows the baroclinic current υ_^g and the difference υ_^−υ_^g of the total flow υ_ and the baroclinic flow part (both monthly means) for August 1991. It is seen that the baroclinic part generally forms a basin-wide anticyclonic circulation, which is opposite to the known cyclonic gyre. The speed increases towards the continental coast (Figure 11a). The difference plot (Figure 11b) shows that the baroclinic component amounts to 10% of the total flow weakening the cyclonic circulation. The Ekman regime is characterized by the balance of the Coriolis force and the vertical exchange of momentum:

−fυe(x,y,z,t)=∂∂z(Aυ(z)∂ue(x,y,z,t)∂z), fυe(x,y,z,t)=∂∂z(Aυ(z)∂ue(x,y,z,t)∂z). GSI-IX In the work of Pohlmann (2003) the terms on the right-hand side are again calculated by means of the complete circulation model HAMSOM. From this forcing the Ekman flow (ue, υe) is deduced. MK2206 Aυ(z) is the vertical eddy coefficient

and depends on depth. Stronger currents (not Ekman balanced) are now appearing along the Norwegian coast. Figure 12a shows by way of example for August 1991 the monthly mean of the Ekman currents at 5 m depth. It has maximum values along the British coast with an onshore direction. Owing to stronger winds it is higher in winter. The difference plot (total current minus Ekman current) in Figure 12b exhibits a residual flow of equal magnitude, but directed offshore (which means a compensation of the Ekman current). The JEBAR term is a component of the oceanic vorticity balance; it describes how baroclinic pressure gradients force the flow in the case of a non-uniform bottom topography. Pohlmann (2003) analysed the vorticity balance of the North Sea for a certain time period, calculating separately the β-term, the vortex stretching and the JEBAR term.

From this study, Figure 13 shows the spatial structure of JEBAR for August 1991: J(χ,1H)=−fH(υ_g∇_H),withχ≡gρ0∫−H0zρdz. Maximum values are seen in the regions where density and topography gradients intersect. Examples are the outer estuaries of the Rivers Rhine and Elbe, the Norwegian Trench and the Fair Isle Passage. During summer the JEBAR gradients, which are directed towards the centre Idelalisib nmr of the North Sea, are enlarged as a result of the joint action of temperature and salinity gradients. Of the remaining terms of the vorticity balance, the temporal derivative and the β-term are smaller than JEBAR by one to two orders of magnitude, whereas the vortex stretching is equally important. Here we present some results of research work done at the Institute of Oceanography, University of Hamburg, within the last two decades. They concern storm surges and the budgets of heat and fresh water in the North Sea.