The deep water in the Baltic Proper is renewed during specific conditions when salt water from the Kattegat flows through the Belt and the Sound and fills in turn the deep areas of the Arkona Basin, the Bornholm Basin, the Eastern Gotland Basin and the western Gotland Basin.
Since the inflows and outflows, salinity and oxygen levels have been monitored in the Baltic for over 100 years by SMHI and other institutes there is a relatively large amount of data available for these events.
Many find it surprising that the measurements show that the inflows are nothing unusual. However the inflows that significantly change the state of the deep waters of the Baltic are rare.
The complete context for an extreme inflow is still not completely clear but the following factors all play a role:
- The water level for Kattegat and the Baltic sea
- The salinity and temperature of the Kattegat water
- Long seiches (oscillations) in the Baltic
- The duration of the flow
- Layering and density of the Baltic deep basins
A powerful saltwater inflow requires special conditions. The sea level in the Baltic needs to be low, i.e. a high pressure period will help. This needs to be followed by a longer period of several weeks with low pressure over the Kattegat and Baltic, giving high water, and strong south-westerly winds.
These winds blow the water onto the west coast and in to the Kattegat at the same time as causing the sea level to be extra low. The inflowing water also needs to be of higher density, i.e. higher salinity (more than 17 ppm) with a low temperature, in order to be able to push out the existing deep water on its way along the bottom of the Baltic.
All these factors, which depend on the short and long term weather, have to occur at the same time in order for an extreme inflow to happen. During the 20th century there were around 100 extreme inflow events.
Large salt water inflows
Salt water inflows are classified according to the volume of water that flows through the Belt and the Sound. Small inflows are less than 100 km3, medium are 100-200 km3, large are 200-300 km3 and very large inflows are over 300 km3.
Very large inflows occurred in 1897 (330 km3), 1906 (300 km3), 1922 (510 km3), 1951 (510 km3) and 1993/94 (300 km3).
Large inflows occurred in 1898 (twice), 1900, 1902 (twice), 1914, 1921, 1925, 1926, 1960, 1965, 1969, 1973, 1976 and 2003.
41 medium inflows have occurred since 1898.
New water from the Kattegat
In order to add new oxygen to the deep basins, new water needs to flow in, and this water must both be heavier (denser, i.e. saltier) than the old water and also have been recently in contact with the atmosphere in order to be oxygen-saturated.
In the Baltic, this type of water can only come from the Kattegat, when there are extreme inflows. The inflowing water is heavier and therefore flows along the sea floor, filling the deep basins and pushing out or mixing with the old water.
When the first basins are filled, the remaining water flows over the edge and continues to the next basin. In this way the deep basins of the Baltic are successively filled with new water from an inflow.
What happens in detail
The high water level in the Skagerrak and Kattegat cause water to flow into the southern Baltic through the Belt and the Sound. The depth of the sill in the Sound is 8 m while the water through the Belt has to get over the Darss Sill which is 18 m.
On its way through the Sound, the inflowing water mixes with the existing water and the salinity level drops. The water running through the Sound has a shorter distance to the deep area (53 m) of the Arkona basin and is therefore less diluted than the water flowing through the Belt.
The inflow volume through the Sound is between a quarter and a third of the total volume. When the water has filled the Arkona basin to 45 m the water starts to flow over the sill to the Bornholm basin which has a depth of 105 m. When this is filled to 60 m the water flows through the Stolpe channel to the Gotland Deep (259 m).
It takes 5-6 months before the Gotland Deep is filled. The very large saltwater inflows reach the Gotland basin after a year, with the Landsort Deep (459 m), the Norrköping Deep (205 m) and the Karlsö Deep (114 m) but do not reach as far as the Bothnian Sea.
If the density of the inflowing water is lower than the existing deep water then the new water is stored in a layer above.
The new bottom water forces out the old bottom water which mixes with the higher water layers, and increases the salinity of the surface water. The new bottom water now remains relatively still until the next salt water inflow. The new oxygen level reduces with time due to the decomposition process of sinking plankton.