The warm Baltic water has tempted many to go swimming, later in the season than usual, though when this finally comes to an end depends more on winds, than on sunshine.

Meteorologists have pointed to a marine heatwave currently affecting Estonia – a warm spell where the sea's surface temperature is above the mean ambient temperature for at least five consecutive days.

This can happen at any time of the year.

"Whereas at the beginning of September the usual sea temperature has been 15–16 degrees but now we are measuring 17 degrees at the stations for several days in a row, then that equals a heatwave," said Jana Põldnurk, head of the climate department at the Environment Agency

Three weeks in mid-summer with warmer temperatures than those seen earlier in the summer were enough to warm up the sea water, Põldnurk noted, even as this warmer spell was sandwiched between two cooler periods. With water's higher specific heat capacity compared with air's, even cooler air temperatures will not have an immediate effect in the opposite direction.

"If the beautiful summer days the day after tomorrow (Friday – ed.) turn a little cooler, then presumably the sea will not react to the cooling so quickly," she said.
Maritime breezes serve to keep things warm too.

Marine scientist at Tallinn University of Technology (TalTech) Mariliis Kõuts said marine heatwaves like this one have been seen before in recent years. While pleasing to swimmers, the conditions are not ideal for marine life.

"In general, it can be stated that production accelerates, growth and the formation of organic matter, which in turn can lead to unpleasant consequences such as the deepening of oxygen deficiency, which is already a big problem in the Baltic Sea," Kõuts noted.

This can include the re-blooming of the unwelcome blue-green algae (cyanobacteria), particularly on Estonia's more sheltered west coast.

Conversely, marine species normally alien to Estonian waters but which can now be found here, generally from warmer climes, will find the temperatures more amenable.

The phenomenon of a warm early autumn following a cool summer is just one of the effects of climate change, Kõuts added.

"At the start of summer the sea, along with the weather, was cooler than usual. In that sense, this year has been very interesting: On the one hand cooler than usual, on the other hand the end of summer has again turned warmer, while in July it was warmer too. This shows that climate change does not mean that it will get steadily warmer, but that unusual situations will start happening more often," she said.

The warm September means the leaves on the trees will stay green a little longer than usual as well.

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