TalTech research: Warming Baltic Sea brings stronger coastal cold waves

Scientists at Tallinn University of Technology have discovered a paradoxical phenomenon: although the Baltic Sea is among the world's fastest-warming seas, cold waves in its coastal waters have become increasingly intense. Analysis of 42 years of satellite data shows that the combined effect of winds, shallow coastal waters, and cold Arctic air can lower water temperature by several degrees in a short time.

Scientists at Tallinn University of Technology have published a study that reveals a surprising contradiction in the Baltic Sea's climate change dynamics: despite the sea's rapid warming, cold waves in coastal areas have become increasingly intense.

42 years of data paint a clear picture

The scientists analysed Baltic Sea satellite data from 1982 to 2023, spanning 42 years in total. The results show that long-term climate warming has reduced the overall number of cold waves, but precisely in coastal areas these events have become significantly stronger than before. Since 2006, the intensity of coastal cold waves has shown a clear upward trend, even as Baltic Sea water warmed by an average of approximately 0.05 degrees per year.

In the open sea, cold waves typically lower water temperature an average of three degrees below normal. In coastal waters, the drop can be as much as four degrees, which represents a substantial shock to the sea's ecosystems.

Wind, shallow water and Arctic cold

The physical explanation for the phenomenon lies in the interaction of winds and the peculiarities of coastal waters. When strong winds blow surface water towards the open sea, deeper, cooler water rises to the surface due to upwelling. Because coastal areas are shallow, the cooling effect of this upwelling often extends through the entire water column to the bottom, so shallow coastal water cools faster than deep open sea — similar to how a bowl and a deep pot behave differently.

The situation is amplified by large-scale atmospheric circulation over the Atlantic Ocean, which in recent years has favoured the intrusion of Arctic cold air into the Baltic Sea region. Thus, behind the more intense coastal cold waves lies the combined effect of three factors working together: wind, coastal water physics, and cold Arctic air.

Consequences for marine life

Overall, warming reduces the duration of cold waves noticeably in the western part of the sea, while the number of cold periods has declined most in shallow eastern waters, such as in the Gulf of Riga and the Gulf of Finland.

Strong water temperature drops, however, have serious impacts on marine life: they can alter species distribution and in some areas even cause mass fish die-offs. The Baltic Sea is one of the fastest-warming seas in the world, which is why understanding such counterintuitive and unexpected phenomena is particularly important both for fisheries and marine conservation.

The study results have been published in the international scientific journal Environmental Research: Climate.