Marine heatwaves in the Baltic Sea

IOW researchers investigate causes and effects.

Marine heatwaves – periods in which the upper water layers in the sea temporarily become exceptionally warm – are occurring with increasing frequency worldwide. Recent studies by the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) have now confirmed this trend also for the Baltic Sea. IOW researchers analysed very large meteorological and hydrographic data sets and identified the specific wind and weather conditions that cause Baltic Sea heatwaves. They also analysed for the first time the extent to which heatwaves reach the Baltic Sea floor and discovered that they can lead to severe oxygen depletion even in usually well-aerated coastal waters.

It is clearer than ever that the global climate is heating up – seemingly unstoppable: In this year alone, previous record temperatures were exceeded every month and just recently, on July 21, the European Copernicus Climate Service reported the warmest day ever recorded on Earth. Global warming is not stopping at the oceans either and the Baltic Sea in particular is one of the fastest warming marine regions in the world, having warmed by more than 1 °C in the last three and a half decades.

In addition to general ocean warming, marine ecosystems are also affected by marine heatwaves. These are limited periods in which the temperature of the upper water layers exceeds a threshold value typical for the respective region and season for at least five days. In the last century, such heatwaves have increased worldwide: An international study shows that since 1925 they have occurred more frequently and lasted longer, resulting in over 50 % more marine heatwave days per year.

“On a global scale, the phenomenon of ocean heatwaves has already been studied quite well. Until now, however, there have been no regional-scale studies or studies that specifically address the conditions of shallow marginal seas,” explains Markus Meier, Baltic Sea climate expert at the IOW. “We therefore wanted to know, what exactly promotes the development of heatwaves in the Baltic Sea and what effects they can have,” says Meier, who is investigating Baltic Sea heatwaves together with an IOW group of researchers specialising in dynamics of regional climate systems.

Meier and his team analysed enormous meteorological data sets from the European Meteorological Centre from over three decades, which they incorporated into high-resolution ocean models. They identified distinctive characteristics in the large air pressure patterns and wind systems over the North Atlantic and Europe that lead to heatwaves in the Baltic Sea. “In the summer months, it is the stable high-pressure systems over Scandinavia that cause the heatwaves – not only due to strong solar radiation and high air temperatures, but above all due to the exceptionally weak winds under such conditions,” says Meier. “The latter prevents the increasingly warming surface water from mixing with cold water from the depths, leading to a heat build-up in the upper water layers.”

Baltic Sea heatwaves can also occur in winter: “They develop whenever strong, persistent westerly winds transport warm, humid air masses from the Atlantic to Europe, which means that the Baltic Sea does not cool down as much as usual at this time of year,” explains climate expert Meier. Such warm, persistent winter winds occur when a particularly high difference in air pressure builds up over the Atlantic between the Icelandic low and the subtropical high pressure belt. “It is well known that the Atlantic influences our climate over Europe. But the fact that it also causes extreme events such as winter heatwaves in the Baltic Sea is new,” says Meier. The dataset covering the period between 1980 and 2016 that was analysed in the study also shows that both summer and winter heatwaves in the Baltic Sea are becoming more frequent, last longer and affect increasingly larger areas. “We don’t know yet whether this intensification is due to climate change in general or to natural variability in the climate system – or to both combined. But this is the subject of further research,” summarises Markus Meier.

Another recent study by the IOW shows that heatwaves in the Baltic Sea are not just a statistical phenomenon, but can have a significant impact on vital environmental parameters. The IOW researchers working with Markus Meier analysed model data from five decades (1970 to 2020) from the European Copernicus service for monitoring the marine environment. The data take into account a large number of parameters in high temporal and spatial resolution that are important for temperature and matter exchange processes in the Baltic Sea. The research team also investigated for the first time the extent to which summer heatwaves spread into the deeper waters, with a particular focus on the shallow coastal areas of the Baltic Sea, which are normally considered to be permanently well “aerated” and therefore robust against oxygen deficits.

The results of this study were surprising and alarming. “Our analyses have shown that heatwaves in summer often reach down to the sea floor up to a water depth of around 20 metres, where they can locally reduce the oxygen content of the water by up to 3 ml / litre,” says Meier. This is substantial, as the summer oxygen concentrations can generally be quite low at the sea bed near the coast and now sometimes drop below 2 ml / litre. “This is the critical threshold below which life is no longer possible for more highly developed organisms such as mussels, worms, crabs and fish,” explains Meier. The oxygen-depleted areas at the bottom of the sea are also increasingly located above the stable thermocline, which in summer forms at a water depth of around 20 metres in the Baltic Sea and above which wind-driven mixing should normally ensure sufficient aeration. “As the Baltic Sea is one of the fastest warming regions in the world’s oceans, there is a high risk that marine heatwaves, together with further climate warming, will more and more cause critical oxygen deficits for the bottom fauna. This would have far-reaching consequences for the entire ecosystem,” concludes Baltic Sea climate researcher Markus Meier.

IOW Media contact:
Dr. Kristin Beck, phone: +49 (0)381 – 5197 135 | presse@io-warnemuende.de

The IOW is a member of the Leibniz Association that connects 96 independent research institutions that range in focus from natural, engineering and environmental sciences to economics, spatial and social sciences and the humanities. The institutes are jointly financed at the state and national levels. The Leibniz Institutes employ a total of 20,500 people, of whom 11,500 are scientists. The total budget of the institutes is 2 billion Euros. http://www.leibniz-association.eu

Wissenschaftliche Ansprechpartner:

Prof. Dr. Markus Meier | Head of the IOW section Physical Oceanography
phone: +49 381 5197 150 | markus.meier@io-warnemuende.de

Originalpublikation:

Gröger, M., Dutheil, C., Börgel, F., Meier, HEM (2024): Drivers of marine heatwaves in a stratified marginal sea. Climate Dynamics 7062, DOI:10.1007/s00382-023-07062-5

Safonova, K., Meier, H.E.M. & Gröger, M. (2024): Summer heatwaves on the Baltic Sea seabed contribute to oxygen deficiency in shallow areas. Commun Earth Environ 5, 106. DOI:10.1038/s43247-024-01268-z

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