The results from analyzing the satellite imagery have been confirmed by data from coastal monitoring stations. At present, water temperatures are significantly higher than average for Septembers over the last 30 years. Usually, the average sea water temperature in mid-September is close to 14 or 15 degrees, depending on the exact location.

Since the end of August, there has been a heatwave in the eastern Baltic Sea. It reached its highest level to date on September 10, when water temperatures were four degrees above normal levels, hovering around 19-20 degrees Celsius. The heatwave affected approximately 230,000 square kilometers of the Baltic Sea, more than half its total area.

As of Tuesday September 17, the area affected by the marine heatwave is 80,000 square kilometers in size. According to the forecast for the coming days, the warm weather will persist. This also suggests that the marine heatwave will continue and the water will be swimmable for some.

According to Rivo Uiboupin, Associate Professor of modeling and remote sensing at Tallinn University of Technology's (TalTech) department of marine systems, marine heat waves are extreme situations during which, for a period of at least five days, the water temperature is significantly higher than the 30-year average on those specific days.

"Usually, we talk about marine heatwaves in summer when high absolute temperatures occur. However, a marine heatwave can also occur in fall or even in winter, when the water temperature is significantly above the long-term average for a particular day," said Uiboupin.

The frequency, duration and magnitude of marine heatwaves in the Baltic Sea has increased in recent decade, Uiboupin explained.

"What is interesting about the current marine heatwave is that it is an event that, on several days, has exceeded the September marine heatwave intensity record for the last 30 years," Uiboupin added.

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