The Baltic Sea has been at an uncommonly high level over a long period. The main reason is the predominance of low pressure in Northern Europe .
“When the sea level is only a few tens of centimetres above the average level few people notice, but if it rises by over 40 centimetres it starts becoming obvious to most people. So far this winter we've had three such long periods at Landsort, which paints a relatively accurate picture of the general levels of the Baltic over the long term. This latest period has been exceptionally long,” says oceanographer Pia Andersson.
Even the brief raised sea levels have been extreme this winter. Record levels were reported in Forsmark and Barsebäck, up to 144 cm above average. But Landsort and Skeppsholmen in Stockholm have also seen significant results. However, there is still a long way to go to reach the 420 cm raised level measured in St. Petersburg in 1824.
These changes are led by many complex processes. Along Swedens coasts it is mainly air pressure and the winds on the North and Baltic Seas that are predominant, but there are also periodic fluctuations in the sea basins. The main result is that once strong winds have pushed the water up against a coast, the water will recede.
The movement in the water continues right up until the opposite coast is reached and the water level rises there. This movement can oscillate back and forth a number of times.
“The observed water level is a combination of several basic processes that take place concurrently. These processes are then compounded by shorter waves created directly by the wind,” says Pia Andersson.
The forces of the oceans are very noticeable. Apart from direct damage to buildings and facilities near the sea, there is also a risk of erosion. The southern coast of Skåne in southern Sweden is susceptible to high sea levels combined with high waves which put pressure on the shoreline. This was made very clear when areas of land in Skåne plunged into the sea in January, and similarly at the Møns Klint cliffs in Denmark .
But the sea can also affect conditions further inland, as demonstrated by the high floods in western Sweden before Christmas. The high sea level put a stop to the water flows from overfull rivers, water which can normally run out into the sea.
This resulted in severe floods in Kungsbacka and other places. When the water finally recedes, there is a risk of the land around the watercourses collapsing, resulting in a landslide. The lower part of the Göta Älv river valley is particularly sensitive.
“Many local authorities are currently adapting their planning to the risks and raising the safety levels when issuing building permits,” says Sture Lindahl, oceanographer.
“It's all about building on high enough ground. It's particularly important to prevent water getting into the surface water system, water catchments and springs, and also energy supply plants such as transformer stations.”
New service for construction in coastal areas
Building soundly is increasingly important as our climate changes. In the spring SMHI is launching a new support service for building in areas close to the sea. It provides consultancy firms, companies, local authorities and other social planners with data for dimensioning their developments correctly.
It provides details of historical, current and conceivable future seawater levels for different locations. The average water level and different variations can be calculated. The service also includes frequency indications, i.e. risk indications for extreme situations in the future.
An equivalent hydrological dimensioning service for building close to lakes and watercourses is already available.
“Temperature and precipitation are slowly changing. All long-term planning that's dependent on weather and water should be approached with this in mind,” says Anna Karlsson, consultant oceanographer at SMHI.