Rivo Uiboupin, an associate professor of remote sensing methods for investigating sea surface temperature at Tallinn University of Technology, said that marine heatwaves are rare and extreme situations when the water temperature is significantly higher than the long-term average for at least five days. 

"Most people think of heat waves in the summer. However, a heat wave can also occur in the spring or even in the winter when the water temperature is significantly higher than the long-term average for that time of year," he said.

"If weather conditions have been what they are: relatively windless and with plenty of sunshine, the surface water will warm up. It also surprised me a bit, but still, if you look at the data from the measuring stations in Sõrve, it is 16 degrees Celsious, which is quite a lot for May," Uiboupin said.

In Estonia, the marine heatwave reached the main islands and from there Sweden, but also further south. 

Fig. 1. The graph below shows the marine heatwave on May 21, 2024. The area affected is marked in purple (top right).

In total, about 100,000 square kilometers, about a quarter of the Baltic Sea area, are currently affected by heatwaves. On the west coast of Saaremaa, for example, seawater temperatures have been up to five degrees warmer than normal, reaching over 16 degrees.

According to the professor, there are three to four heatwaves a year in the Baltic Sea, each lasting 15 to 20 days.

According to the model forecast for the next few days, the continued warm weather suggests that the marine heatwave continues. However, stronger northeasterly and easterly winds offshore may slow down the further development of the heat wave. These winds could cause cold water from deeper layers to rise to the surface along the Estonian coast.

The frequency, duration, and magnitude of marine heatwaves in the Baltic Sea have increased in recent decades. To understand their potential impact on the marine ecosystem and the blue economy, scientists at the Tallinn University of Technology (TalTech) have developed a climate service to monitor sea surface temperature and marine heatwaves.

They set up an autonomous data processing model that calculates sea surface temperatures from satellite images and compares them with the long-term average to identify areas affected most by heatwaves.

Fig. 2 displays the spatial map of the marine heat wave and its temporal variability, while Fig. 3 displays the temporal and spatial variability of the sea surface temperature.

The Baltic Sea heatwave is receding, but on land the heat is gaining momentum

This week saw the highest sea temperatures in 30 years, but the marine heatwave that began on May 12 and is now subsiding.

According to Uiboupin, the heatwave in the sea is receding, but it is still gaining momentum on land, with temperatures of up to 31 degrees predicted in the coming days.

"It's not very usual for the end of May to be close to or even over 30 degrees, as expected in the coming days, but it's not unusual either," said forecaster Helve Meitern.

The forecaster predicts that the warm summery weather will last until June 6 or 7. The heat will likely make the sea swimmable in the shallower bays by next weekend, with temperatures in the 20s. In the meantime, at least in the capital, we will have to make do with less.

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