Different water layers
Saltier water is denser than fresh water, so the water flowing in from the North Sea is relatively dense and tends to sink down into the deeper basins of the very variable bottom topography.
In the northern parts of the Baltic Sea the large volume of river run-off creates a top layer with low salinity that is therefore lighter. This upper layer becomes mixed by the wind while the water underneath remains layered.
The layers are separated by a halocline where the salinity gradient is so large that the mixing created by the wind is not sufficient to break up the layers. Oxygen from the sea surface is only mixed into the upper layer.
The only way that oxygen can be added to the lower layer is by a horizontal inflow of salty water through the Danish straits. This water can be oxygen-rich due to contact with the sea surface in the shallow waters around Denmark.
Aeration from salty inflows
The bottom waters in the deep basins are only aerated when there are large saltwater inflows. These occur randomly with an interval of one or several years, usually between November and January, although they can also occur during autumn and spring.
These large saltwater inflows are preceded by 20-30 days of easterly winds followed by strong westerly winds for the same length of time.
Significance of large-scale atmospheric circulation
Wind conditions are affected by the large-scale atmospheric circulation. The same applies to freshwater input since a strong air pressure gradient over the North Atlantic gives strong westerly winds which transport moisture that in turn leads to precipitation and run-off in the Baltic area.
Effect on the large-scale regional circulation
Climate studies indicate that the large-scale regional circulation may be affected by global warming. This, together with other factors, alters the precipitation and wind conditions that control inflows of fresh water and salty water to the Baltic Sea.
In order to calculate how these changes affect the Baltic Sea, we first need to understand the processes that control the fluxes and state. This is why it is important to study the changes that have been observed over a long period of time, and to try to describe them using models. This provides a general understanding that can be used in future scenarios.
Natural variation can change
The Baltic Sea is characterised by a long-term natural variation. This characterisation will continue in the future, but climate change could eventually affect the type of variation.
Effect on plant and animal life
We know that the conditions in the Baltic Sea, such as sea ice, oxygen, seal level and salinity will be changed by a warmer climate, but there is currently no reliable information about how this will affect the whole ecosystem.
Research is currently being carried out to find out how the food chain in the Baltic Sea could be affected by climate change. Which fish species will be found in the future, and how will algal blooms and the extent of the oxygen-depleted areas be affected by a warmer climate?
Fish are affected by direct factors such as changes in temperature and salinity, but also indirectly by the availability of food and predators. A warmer climate could lead to changes in the food chain causing the disappearance of entire species.
There are already reports of new species and changes in the fish stocks, such as swordfish near Bornholm and tuna in the southern Baltic.
Over 80 researchers from 12 countries summarised the effect of climate change on the whole Baltic Sea region in 2007. This included a compilation of all known effects on the marine environment.
SMHI was one of the partners in the scientific evaluation of the Baltic region that brought together historic, current and expected future climate information. The report presents likely developments due to the effects of climate change on the marine environment, availability of fresh water and land ecosystems.
Scenarios for the future climate in the Baltic area contain significant uncertainties, which mainly depend on the size of future emissions of greenhouse gases.
According to the report, a likely future scenario could be an average warming of 3–5 °C over the next hundred years. The largest rise in temperature would probably occur during the winter in the eastern and northern parts of the Baltic Sea while the summer precipitation patterns would change for the southern Baltic Sea.
One consequence of a warmer climate is a longer growing season, which could be up to 90 days longer than today. Reports indicate that the salinity of the sea will change, which will have a significant effect on the animal and plant life in the Baltic Sea. The change in precipitation patterns could also have a negative effect on the over-fertilisation situation in the Baltic.