The Baltic Sea is one of the fastest-warming seas in the world, so reports about rising water temperatures and marine heat waves have become increasingly common. Surprisingly, however, this same warming sea is also seeing the opposite phenomenon: increasingly intense cold spells have begun occurring in its coastal waters in recent years.

A new study found that while long-term climate warming has reduced the overall number of cold spells in the Baltic Sea, those affecting coastal waters have become more intense. Researchers at Tallinn University of Technology (TalTech) reached this conclusion after analyzing 42 years of satellite data covering the period from 1982 to 2023.

The analysis showed that cold spells in the open sea are generally milder, lowering water temperatures by an average of about 3 degrees Celsius below normal. Near the coast, however, the same events can reduce water temperatures by as much as 4 degrees Celsius. Since 2006, the intensity of cold spells in coastal areas has also shown a clear upward trend. This is notable because, over the same period, the Baltic Sea warmed by an average of about 0.05 degrees Celsius per year.

Overall, the warming trend has significantly shortened the duration of cold spells in the western Baltic Sea, while the number of cold events has declined most sharply in the shallower eastern basins, such as the Gulf of Riga and the Gulf of Finland. Sharp drops in water temperature can have major impacts on marine ecosystems, altering the distribution of species or even causing mass fish die-offs in some areas.

How, then, can cold spells become more intense in a warming sea? The explanation lies in the wind and the fact that the Baltic Sea's coastal waters are shallow. When strong winds push surface water away from the coast, they trigger upwelling that brings colder, deeper water to the surface. Because coastal waters are shallow, the cooling effect often extends throughout the entire water column down to the seabed.

As a result, shallow coastal waters cool more quickly than the open sea, much like a shallow bowl cools faster than a deep one. The effect is further amplified by large-scale atmospheric circulation over the Atlantic Ocean, which in recent years has favored intrusions of cold Arctic air. Together, these factors — the wind, the physics of shallow coastal waters and cold air — help explain why sharp temperature drops are becoming more intense despite the Baltic Sea's overall warming trend.

The researchers published their findings in the journal Environmental Research: Climate.

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