Inflows to the Baltic Sea

It is generally well-known that inflows of so called healthy water into the Baltic have a good effect on the ecosystem and living organisms. This is also the oceanographic process that generates the most interest.

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 for example 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.

Inflow - how does it happen?

The complete context for an extreme inflow is still not completely clear but the following factors all play a role:

• The water level for the southern Kattegat and southwest Baltic
• 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.

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