Successive mid-latitude cyclones increase the risk of flooding in the Baltic Sea

Coastal flooding in the Baltic Sea is often associated with powerful mid-latitude cyclones which increase the sea level due to their strong on-shore winds and low air pressure. According to a new study, the number of consecutive cyclones also has a significant effect on sea levels. The maximum sea levels observed on the Baltic sea coast are about 2-3 times higher when at least three strong cyclones pass over the area in a short period of time compared to when only one strong cyclone occurs.

"In Helsinki, for example, the maximum sea level caused by a single strong cyclone is about 29 cm above normal, while in the event of at least three successive strong cyclones, the average maximum is 68 cm," says Mika Rantanen, a researcher at the Finnish Meteorological Institute.

The study, published in the journal Geophysical Research Letters, also found that the stronger the successive cyclones, the higher the sea levels. The study estimates that consecutive cyclones are associated with long-lasting westerly winds, which increase the average water level in the Baltic Sea due to water inflow from the North Sea through the Danish Straits. If several cyclones occur in succession, the water volume does not have time to recover.

Mid-latitude cyclones in the Baltic Sea were investigated over a forty-year period

The new study also looked at the weather conditions under which extremely high sea levels occur. It was found that almost half of the extremely high sea level events were associated with clusters of at least three cyclones, and only about 10% with single cyclones.

"Although the frequency of single cyclones was about the same as that of clusters of at least three cyclones, it turned out that almost half of the high sea level events were associated with the clusters," says Mika Rantanen.

The study looked at all strong mid-latitude cyclones in the Baltic Sea area between October and March from 1980 to 2022. The maximum sea levels during each cyclone or cluster were studied at eight different tide gauges in Finland, Sweden, Estonia and Latvia.

The study is part of the ongoing MAWECLI (MArine and WEather events in the changing CLImate as potential external hazards to nuclear safety) project, which investigates extreme marine and weather events in support of nuclear power plant safety in Finland.

Further information

Researcher Mika Rantanen, Finnish Meteorological Institute, puh. 050 475 0756, mika.rantanen@fmi.fi

Researcher Jani Särkkä, Finnish Meteorological Institute, puh. 050 346 9087, jani.sarkka@fmi.fi

The scientific article is freely available in the journal Geophysical Research Letters.

Article reference: Rantanen, M., van den Broek, D., Cornér, J., Sinclair, V. A., Johansson, M. M., Särkkä, J., Laurila T. K., & Jylhä, K. (2024). The impact of serial cyclone clustering on extremely high sea levels in the Baltic Sea. Geophysical Research Letters. https://doi.org/10.1029/2023GL107203