Pekka Alenius and Riku Lumiaro / FIMR
The pdf-files below descirbe the saline pulses and their strength arriving from the North Sea to the Baltic Sea in the periods 1897-1939 and 1946-2003. During the Second World War it was impossible to collect saline pulse data. The graphics are based on data collected by German marine scientists, and the original sources are included with them.
The Baltic Sea is semi-enclosed, and it is connected to the Atlantic Ocean via the narrow and shallow Danish straits. The Baltic Sea has a positive water balance, i. e. more fresh than salt water reaches it, and the superfluous water flows out into the North Sea.
There is a permanent halocline, i. e. a layer separating waters of different salinity and density, between the lighter, less salty Baltic Sea water and the denser, saltier water of the Atlantic Ocean. These layers don’t mix much, and for this reason the only way oxygen reaches the bottom layers is through the saline and oxygen rich pulses of surface water from the North Sea.
Although the Baltic Sea has a positive water balance, water normally also flows through the Danish straits into the Baltic Sea. This water, however, is a mixture of brackish water from the Baltic Sea and seawater from the North Sea, because the long, narrow and shallow Danish straits slow down the flow of water towards the Baltic Sea.
Occasionally, however, oxygen rich, dense and salty North Sea surface water reaches the Baltic Sea. This occurrence is preceded by lower sea levels in the Southern Baltic Sea and a heavy storm of long duration from the west or south-west in Denmark.
The tracks of storms, or rather more accurately depression centres are important for the formation of saline pulses. The denser water from the North Sea has to sort of flow against the current far enough into the Baltic Sea to prevent its flow back to the North Sea. In addition, the storm also has to last long enough for large watermasses to flow into the Baltic Sea. Such conditions are relatively rare, so that few saline pulses reach the Baltic Sea.
Saline pulse research has been done for decades. German scientists have developed an index of saline pulses, so that different pulses can be compared. During saline pulses, the salinity, amount and duration of flow of water into the Baltic Sea in days is tracked.
During large saline pulses, seawater from the North Sea flows into the Baltic Sea continuously for two weeks. The effects of the saline pulse also partly depend on the amount of saltwater flowing in, because the Southern Baltic Sea consists of deeper basins separated by more shallow thresholds. The seawater called a saline pulse has thus a long and difficult journey along the basins of the Baltic Sea before its effects can be seen on the Finnish coast. The saline pulse easily takes six months, often longer, to reach Finland.