The ocean currents of the North Sea form a cyclonic circulation. While this is a general pattern, there is considerable variability in the water circulation in response to meteorological variability. The mean residence time of water in the North Sea is about 1 to 2 years, being in general shorter for water in the northern North Sea and longer for water in the southern North Sea. Various water types in different areas characterize the North Sea. The North Sea is a biologically rich and productive region. The North Sea is very productive, due in part to large inputs of nutrients leading to high primary production, the basis for all food chains.

Schematic diagram of the general circulation in the North Sea (after Turrell 1992).