Changes in sea level are mainly caused by these two components: Tides - The dominant forcing of the tides is the variation in the gravitational field on the surface of the earth due to regular movements of the earth-moon and earth-sun systems. Weather effects - Changes in sea level mainly caused by changes in atmospheric pressure and wind.
Tidal predictions can be calculated for a location for any time as long as water level observations have been carried out at the location. In Norway tidal predictions are calculated by the Norwegian Hydrographic Service (NHS) and are published in tide tables. Figure 1 presents predicted tides for Oscarsborg (Norwegian port) for the period 19 - 26 February 2000.
Observed Water Level
The Norwegian Hydrographic Service (NHS) is operating a tide gauge network consisting of 23 tide gauges. As Figure 2 presents so often differ the observed water level from the predicted tides.
The difference between observed water level and predicted tides are mainly caused by weather effects, i.e. wind and changes in atmospheric pressure (Figure 3).
At the southern part of Norway (Sørlandet and Oslofjorden) the weather effects can in periods be larger than the tides. For the rest of the Norwegian coast the tides are the dominant part of the sea level changes.