Dangerous Marine Heat Waves In The Baltic Sea – Study Of Causes And Effects

Eddie Gonzales Jr. – MessageToEagle.com – Marine heat waves—periods in which the upper water layers in the sea temporarily become exceptionally warm—are occurring with increasing frequency worldwide.

Image credit: SimoneVomFeld –  Pixabay

Recent studies by the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) have now confirmed this trend also for the Baltic Sea. The first study is published in the journal Climate Dynamics.

IOW researchers analyzed very large meteorological and hydrographic data sets and identified the specific wind and weather conditions that cause Baltic Sea heat waves. They also analyzed for the first time the extent to which heat waves 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 heat waves. 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 heat waves have increased worldwide: An international study shows that since 1925 they have occurred more frequently and lasted longer, resulting in over 50 % more marine heat wave days per year.

“On a global scale, the phenomenon of ocean heat waves 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 heat waves in the Baltic Sea and what effects they can have,” says Meier, who is investigating Baltic Sea heat waves together with an IOW group of researchers specializing in dynamics of regional climate systems.

Meier and his team analyzed 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 heat waves in the Baltic Sea.

“In the summer months, it is the stable high-pressure systems over Scandinavia that cause the heat waves—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 heat waves 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 heat waves in the Baltic Sea is new,” says Meier.

The dataset covering the period between 1980 and 2016 that was analyzed in the study also shows that both summer and winter heat waves 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,” summarizes Markus Meier.

Published in Communications Earth & Environment, another recent study by the IOW shows that heat waves 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 Meier analyzed 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 heat waves 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 heat waves in summer often reach down to the sea floor up to a water depth of around 20 meters, where they can locally reduce the oxygen content of the water by up to 3 ml / liter,” 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 / liter.

“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 meters 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 heat waves, 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 Meier.

The first study is published in the journal Climate Dynamics.

source

Written by Eddie Gonzales  Jr. – MessageToEagle.com Staff Writer