Variations in the bathymetry and strength of tidal currents are responsible for the substantial subregional differences observed in seasonal temperature stratification within the ecoregion. Greater North Sea oceanographic conditions are largely determined by the inflow of saline and nutrient-rich Atlantic Water (Figure 12) and the tightly coupled ocean–atmosphere heat exchange.
There is a pronounced annual cycle in the temperature of the near-surface water layer, which reaches a maximum of about 16°C in summer (averaged over the North Sea area) and a minimum of about 6°C in late winter. Climatological large-scale distributions of the near-surface temperature reveal a northwest-southeast gradient over the winter months, with higher temperatures in the northwestern North Sea and lower temperatures at the German and Danish coasts in the southeast. Summer distributions show a reversed pattern with the highest temperatures in the English Channel and the German Bight and the lowest temperatures near Scotland.
The circulation (Figure 12) is influenced by the bottom topography and the import of saline water from the Atlantic Ocean, low salinity water from the Baltic Sea and freshwater from rivers. The latter is especially prominent in the English Channel, the southern North Sea, and the German Bight. The Atlantic water mixes with river run-off and Baltic outflow along the Norwegian coast creating the Norwegian coastal current. Atlantic water inflow through the northern entrances and, to a lesser degree, through the English Channel can be strongly influenced by the North Atlantic Oscillation (NAO).
Figure 12: Circulation system of the North Sea.