Sea level variations on the Finnish coast
Water level measurements
The Finnish Meteorological Institute (FMI) operates fourteen tide gauges, mareographs, on the Finnish coast. Measurement started at Hanko in 1887 and at Helsinki in 1904. The other stations were established in the 1920s and 1930s, and the latest at Porvoo in 2014.
Water level variations
The difference between the highest and the lowest measured water level is over two meters at all the stations except at Föglö in the Archipelago Sea, where it is a bit lower. The water level fluctuations are strongest in the ends of the Bay of Bothnia and the Gulf of Finland: at Kemi, Oulu and Hamina the total variation is over three meters. The extreme sea levels are measured during autumn or winter.
Water level variations in the Baltic Sea are influenced by both the slow changes in the total water amount in the Baltic Sea and the short-term local variations. The local variations are mainly controlled by wind and air pressure as well as the ice conditions in winter. The tide has an effect of only a few centimeters at the Finnish coast.
- The inward and outward currents through the Danish Straits change the amount of water in the Baltic Sea, and affect thus the water level at the coasts. The water flow is caused by sea level differences between the Baltic Sea and the North Sea, as well as the wind conditions over the Straits.
- Wind affects sea levels by piling up water into certain parts of the Baltic Sea. This piling up is especially evident at the ends of the bays. Thus, the most pronounced extreme sea level values are obtained at these regions. Wind may have a very local effect.
- A high air pressure pushes the sea level down, while a low pressure raises it up. A pressure change of 1 mbar corresponds to a sea level change of about 1 cm. The common air pressure variations may thus cause a sea level variation of several tens of centimeters.
- A compact ice cover affects the short-term sea level variations by preventing the effect of wind on the water surface. When wind cannot pile up water against the coastline, extremely high sea level values are not reached as easily as in open water conditions.
The standing waves, seiches, in the sea are mainly caused by the wind and the air pressure. The seiche is a typical phenomenon for the combined basin of the Baltic Proper and the Gulf of Finland, and its effect on the sea level is emphasized in the ends of the bays. On a small scale, a similar phenomenon can be seen in a bathtub.
If a low pressure is dominating on the area, and a strong wind is blowing from the right direction simultaneously with a seiche piling water up towards the end of a bay, the water level may rise very high in the bay. If the total water amount in the Baltic Sea has accumulated several tens of centimeters over the average in advance, a possibility for an exceptionally high water level exists. The water level rose extremely high in the Gulf of Finland and on the east coast of the Baltic Proper in this kind of circumstances on 9th Jan 2005.
The sea level variations have seasonal differences because of the annual periodicities in the behaviour of the wind and the air pressure. The average sea level is at its highest in December and at its lowest in April-May. The short-term variability of the sea level is also strongest in winter, November-January, and smallest during summer, in May-July. Individual years, however, may differ greatly from each other, and this kind of average annual periodicity is not evident every year.