The basic ideas behind the shelf sea circulation model of the Institut für Meereskunde Hamburg (IFMH) are outlined. The stability and the computational performance of the numerical scheme are essentially based upon implicit algorithms or second order approximations introduced for those terms in the primitive equations which are likely to produce instability. As a result the scheme turns out to be rather stable and much faster than conventional, solely explicit numerical schemes. The application of the IFMH-model on the determination of the low frequency variability of the North European shelf sea provides insight into a frequency domain where information from observational data is rather scarce.