Eutrophication and oxygen depletion are well-known threats to the ecological balance of the Baltic Sea, which is increasingly under pressure due to climate change. In this context, large saltwater inflows from the North Sea play a crucial role. They transport oxygen-rich water into the deeper layers of the Baltic Sea, counteracting oxygen deficiency and so-called dead zones.
In a recent study, researchers from Kiel University have now demonstrated for the first time that there are two different types of major saltwater inflows into the Baltic Sea. They arise due to different weather conditions and current patterns. These factors determine the composition of the respective inflows and affect the distribution of salinity and oxygen levels in the Baltic Sea in different ways.
This new understanding explains why not every major saltwater inflow enriches the deep waters of the Baltic Sea with oxygen to the same extent. The study has recently been published in the journal Communications Earth & Environment.
Two types of Major Baltic Inflows identified
Major saltwater inflows into the Baltic Sea, known as Major Baltic Inflows (MBIs), have been well documented in scientific research for years. Studies indicate that they typically occur when prolonged easterly winds are followed by strong westerly winds. However, it has remained a mystery why such massive amounts of water from the North Sea sometimes transport relatively little fresh oxygen into the depths of the Baltic Sea.
In their study, Dr. Ulrike Löptien, Professor Matthias Renz and Dr. Heiner Dietze from the Institute of Computer Science and the Institute of Geosciences at Kiel University have now solved this puzzle using a combination of modern machine learning techniques and numerical ocean circulation models. Specifically, the researchers employed unsupervised learning, a machine learning method without predefined categories.
Based on a long-term reconstruction of major saltwater inflows and historical weather data, the algorithm identifies two distinct types of inflow events, each with its own atmospheric and oceanographic characteristics. Model simulations confirm these clusters and allow a first targeted analysis of their respective impacts on the oxygen supply of the Baltic Sea.
The first cluster, called “Classic,” is characterized by a pronounced high-pressure system over Scandinavia and the northwestern Baltic Sea, followed by moderate westerly winds. The second cluster, “Stormy,” shows a different pattern: it begins with weaker high-pressure influence over the central Baltic Sea, followed by strong to stormy westerly winds.
Two current patterns—and their consequences for the Baltic Sea
In the “Classic” type, a strong high-pressure system initially pushes large amounts of water out of the Baltic Sea. When moderate westerly winds set in, the previously displaced water flows back into the Baltic Sea. The key factor here is that the relatively weak westerly winds do not significantly disrupt the typical stratification in the Kattegat, allowing the saline North Sea water to remain stable beneath a layer of fresher Baltic Sea water.
The “Stormy” type behaves differently: here, the preceding easterly winds displace less water, resulting in a weaker stratification. The subsequent westerly winds are significantly stronger, promoting a more intense mixing of North Sea and Baltic Sea water. As a result, more saline and oxygen-rich water of Atlantic origin ultimately reaches the deep basins of the Baltic Sea.
According to the researchers, the salinity of the inflowing water is crucial for oxygen distribution. It determines how deep the oxygen can actually penetrate into the Baltic Sea. Only highly saline water is dense enough to reach the lower layers, where oxygen is urgently needed for breaking down sinking plankton and other organic substances.
The influence of North Sea water is therefore essential for the ecological balance of the Baltic Sea. Coming from the Atlantic, it is denser, saltier, and richer in oxygen—properties that enable it to reach the deep layers of the Baltic Sea and renew the oxygen-poor water there.
“The methods we used and the insights we gained can also be applied to many other coastal and marginal seas worldwide—wherever different water masses meet. Especially in the context of climate change, which not only raises temperatures but also alters weather patterns and current systems, a deeper understanding of these processes is of crucial importance,” says lead author Dr. Ulrike Löptien, who sees far-reaching potential in the study’s findings.
More information: Ulrike Löptien et al, Major Baltic Inflows come in different flavours, Communications Earth & Environment (2025). DOI: 10.1038/s43247-025-02209-0
Journal information: Communications Earth & Environment
Provided by Kiel University