Since 2010, the ozone layer over the Arctic has shrunk dramatically three times: in 2011, 2016 and 2020, when even 95 percent of the ozone in the core area was degraded. According to a study by Marina Friedel from ETH Zurich and her colleagues in "Nature Geoscience", this ozone hole seems to influence the weather over Central Europe. The team had observed that in the same years in which the shrinkage was particularly pronounced, equally strong weather anomalies were found in northern and Central Europe as well as in Siberia. On the other hand, it was wetter around the North Pole than in the long-term average.
It was initially unclear to the working group whether one triggers the other or whether both have a common cause: for example the dynamic variability of the lower stratosphere. Both would then be a symptom of this atmospheric change. Friedel and Co therefore simulated the ozone mining in two different climate models, which has mostly failed to materialize because the additional consideration of the stratospheric ozone would cost too much computer capacity.
However, their calculations then clearly showed that the two weather anomalies considered in 2011 and 2020 were largely due to the respective ozone holes over the Arctic. The simulations therefore corresponded strongly with the observation data from the two years as well as with eight other such events from the last 40 years, which were used for comparison purposes. However, if the working group switched off the ozone destruction in the models, it could not reproduce the observations.
At the beginning of the phenomenon, ozone mining is in the stratosphere: it begins when the temperatures in the Arctic are very deep. "The ozone destruction only runs if it is cold enough and the polar vortex in the stratosphere is around 30 to 50 kilometers above the ground," says Friedel.
Normally, ozone absorbs the UV radiation emitted by the sun, thereby warming the stratosphere. This contributes to the disintegration of the polar vortex in the spring. If the ozone is missing, the stratosphere cools down further and the vortex intensifies, which strikes through to the Earth's surface. "A strong polar vortex then produces the observed surface effects," says Gabriel Chiodo from ETH, who participated in the study. The vortex remains stable for longer than in years without an ozone hole, thereby stabilizing the cold high over the Arctic.
Ozone therefore makes a significant contribution to changing the temperatures and circulation around the North Pole. However, only a part of the weather anomalies goes back to the dynamic changes in the air currents, i.e. to climatic influences that are independent of the ozone hole.
Conversely, climate change favors the formation of ozone holes over the Arctic more often: due to changed air currents, the polar stratosphere stays cold longer in the spring and it is prevented that warm air masses penetrate to the north at these altitudes. And this, in turn, promotes ozone depletion. At the same time, the same gases that cause global warming at the Earth's surface promote a cooling of the higher layers of air in the stratosphere. In general, there are sometimes warm and sometimes cold stratospheric winters in the polar vortex. However, an analysis of meteorological data from the last 56 years has shown a trend towards lower temperatures in the cold stratospheric winters in 2021.
Despite the ban on FCKW in 1987, the concentrations of ozone -reducing substances above the North Pole had increased until 2000. Only since then did they slowly sink to 90 percent of the maximum values. However, together with the falling temperatures in the stratosphere, this is still sufficient to destroy the ozone. According to computer models, this can continue until 2100.
