An exceptional year for stratospheric ozone
2020 brought prolonged atmospheric vortexes to both poles of the Earth, fuelling the largest sustained ozone holes in years.
The Coronavirus pandemic ensured that 2020 will be remembered as an exceptional year. But if we look at the sky, up into the stratosphere, some people may also remember 2020 for the anomalous behaviour of the Earth’s ozone layer – not only at the South Pole, but in the arctic skies as well.
25 February 2021
16 February 2021
Federico Fierli, Atmospheric Composition Expert at EUMETSAT, says: "This year, we have seen an exceptional destruction of ozone in the Arctic in March 2020 as well as an unusually prolonged polar vortex in Antarctica that lasted until mid-December 2020, both are quite exceptional.”
A prolonged atmospheric vortex over Antarctica
This unusual persistence and extent of the antarctic polar vortex favoured the formation of a “deep” ozone hole, said Diego Loyola, Senior Scientist at DLR (German Aerospace Center). “The area of the ozone hole in 2020 was one of the largest and deepest on record with stratospheric concentration lower than 100 Dobson units (1) in October, the time of year when the maximum ozone destruction occurs. Thanks to the continuity offered by the three Metop missions since 2007, we can contribute to the long-term observations of ozone from satellites and put what happened in 2020 in a wider perspective”. The Atmospheric Composition Satellite Applications Facility (AC-SAF) provides key data from the various EUMETSAT instruments to monitor the atmosphere.
The Global Ozone Monitoring Experiment-2 (GOME-2) observations from aboard Metop show an anomalously strong and lasting stratospheric polar vortex in 2020. This plays a profound role in the formation of the ozone hole. “The polar vortex is a climatological circulation feature that causes air at the poles to become isolated from the rest of Earth’s atmosphere, so that it remains at extremely low ambient temperatures, creating a natural laboratory where chlorofluorocarbons (CFCs) can catalyse ozone destruction,” says Federico Fierli.
GOME-2 is not the only sensor monitoring the health and evolution of Earth’s ozone layer. The AC-SAF also analyses data from the CNES-developed infrared atmospheric sounding interferometer (IASI) instrument. This multi-purpose instrument, primarily dedicated to improving weather forecasts and monitoring atmosphere, is also on board EUMETSAT Metop satellites and has the capability to provide accurate and long-term observations of ozone. The IASI and GOME-2 measurements contribute to the international monitoring programme and provide vital information to the World Meteorological Organization.
Cathy Clerbaux, Senior Scientist at LATMOS/CNRS said in October 2020: “The catalytic destruction of ozone usually takes place in the first two weeks of September, but this process can persist longer in periods of cold temperatures – potentially into October, as we observed in 2020. This is much different and almost the opposite behaviour to what we observed in 2019, when, due to the weakness of the vortex, a small ozone hole, both in depth and extension, was observed.”
Why was the 2020 vortex so persistent when compared to 2019? It was essentially random chance, says Federico, “this persistence pertains to the variability of the global atmosphere, which responds to many complex factors in unpredictable ways each year.”
A northern vortex fuelled ozone depletion there as well
It is worth going back to the beginning of 2020 and looking at the satellite observations from the North Pole as well. IASI data spotted record ozone destruction in the Arctic in February 2020, with the lowest ozone concentration ever recorded by satellite observations.
Cathy Clerbaux adds: “The northern hemisphere vortex is more unstable and warmer when compared to the southern one, due to increased atmospheric disturbances that lead to breaking of the vortex and an influx of mid-latitude air. However, in 2020, an unusually stable and cold vortex developed in the Arctic, acting similarly to the vortex in the southern hemisphere, and leading to the formation of an ozone hole – an uncommon feature compared with previous years.”
Typically, atmospheric waves spread outward and upward into the stratosphere, where they can distort and weaken the polar vortex. During the 2019/2020 winter, such wave activity was unusually weak. In addition, an unusual configuration of the stratospheric polar vortex developed, reflecting atmospheric waves traveling upward from the troposphere back downward. These unique conditions allowed the vortex to remain both strong and cold for several months.
What can we expect in the near future?
The story still needs time to be solved: most CFCs persist in the atmosphere for decades after being released, destroying ozone the whole time – so natural variability means we may still observe conditions like this year in the future. However, the anomalous data from 2020 does not challenge projections of a recovery over the next few decades. Last year, a comprehensive report from the World Meteorological Organization (published every four years) provided the first signs of the ozone recovery long-predicted by models, and confirmed by observations taken within the last decade. This recovery is attributable to the global effort to heavily regulate the ozone depleting substances (such as CFCs) since the Montreal Protocol in 1987 and their further implementations. The reaction of the international community to this threat was rapid, resulting in action taken only a few years after the observational evidence of the ozone hole.
“We should acknowledge the late Paul Crutzen, who sadly passed away early in 2021. He received the Nobel Prize for his work to understand how human activities damage the ozone layer. This body of knowledge about the causes of ozone depletion became the basis for the worldwide observational effort that led to the effective ban – a unique example of how excellent research can directly lead to a global political decision.” said Cathy Clerbaux, Research Director at LATMOS laboratory in Paris.
Federico Fierli and Julia Wagemann will give an online course focusing on last year’s GOME-2 stratospheric ozone datasets on 18 February. Register for the course here.
1 A Dobson unit is a measurement of the mass of ozone in a column of atmosphere, and a measure of 100 Dobson units corresponds to the most severe ozone holes on record (the name “Dobson” comes from the scientist who built the first instrument to routinely measure ozone).
World Meteorological Organization:
Dameris, M., Loyola, D. G., Nützel, M., Coldewey-Egbers, M., Lerot, C., Romahn, F., and van Roozendael, M.: Record low ozone values over the Arctic in boreal spring 2020, Atmos. Chem. Phys., 21, 617–633, https://doi.org/10.5194/acp-21-617-2021, 2021
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