A view on Antarctic ozone 2022

By Anu-Maija Sundström, Julia Wageman, Sabrina Szeto (FMI), Klaus-Peter Heue (DLR), Federico Fierli and Rob Roebeling

Stratospheric ozone protects Earth’s biosphere from harmful ultraviolet radiation. In the past century, the abundant use of human-created ozone-depleting substances caused rapid thinning of the layer of stratospheric ozone in the 1980s. This thinning is largest around poles, in the Antarctic and Arctic regions (see Figure 1).

The vast majority of the atmosphere’s ozone (O₃) is found in the stratosphere (the atmospheric layer between around 10-50km above the Earth’s surface). Ultraviolet radiation from the Sun triggers chemical processes that convert so-called anthropogenic (caused by human activity) halogen source gases (such as chlorine and bromine) to more reactive halogen gases.

During the coldest months of the year, chemical changes that occur on Polar Stratospheric Clouds (PSCs) induce an increased abundance of the most reactive halogen gases (eg chlorine monoxide and bromine monoxide) that are known to effectively destroy ozone (Jacobs, 1999). The abundant availability of these halogens, constrained by the stratospheric polar vortex (strong winds), causes a strong depletion in stratospheric ozone concentrations over large areas around the poles. These areas, also known as the ozone hole, vary in size from year-to-year, and usually reach the largest extent in late winter or early spring. The phenomenon of depletion of stratospheric ozone occurs over both the Antarctic and Arctic. However, the development of an ozone hole is most pronounced over the Antarctic for three reasons:

1. The polar vortex over Antarctica is more stable, this causes an isolation of stratospheric air masses.
2. As a consequence of number 1, the low temperatures allowing the formation of polar stratospheric clouds extent over longer periods.
3. The more persistent chemical destruction of ozone by reactive halogen gases (WMO, 2019).

In 1987, the international community implemented an historic environmental treaty, the Montreal Protocol, that regulates the production and consumption of harmful ozone-depleting substances. The Montreal Protocol is one of world’s most successful international treaties, resulting in phasing out nearly 99% of all ozone-depleting substances and advancing the recovery of the stratospheric ozone layer.

As part of the Montreal Protocol, the World Meteorological Organization (WMO) provides, in successive four-year intervals, scientific assessment reports that highlight the advances and updates in the scientific understanding of ozone depletion, and provide policy-relevant scientific information on current challenges and future policy choices. The latest assessment report, The Scientific Assessment of Ozone Depletion: 2022, highlights that the abundance of total tropospheric chlorine and total tropospheric bromine from long-lived ozone depleting substances have continued to decline, and total column ozone in the Antarctic continues to recover, despite substantial interannual variability in the size, strength, and longevity of the ozone hole (WMO, 2022).

Ozone change and its influence on climate

Apart from a reduced protection against harmful ultraviolet radiation, anthropogenic stratospheric ozone depletion can also impact the climate system as a whole. The Intergovernmental Panel on Climate Change (IPCC) assessments, for example, reported that the depletion of stratospheric ozone was the main driver of cooling of the lower stratosphere in the past century (IPCC, 2021). This, in turn, is known to have affected the climate in the southern hemisphere through, among others, a southward shift of mid-latitude rain, and a widening of the Hadley circulation (NOAA, 2018), and leading to drier weather conditions in Australia and South America (Yongyun et al., 2018). In the near term, the projected impacts of the ozone hole on the climate in the southern hemisphere are expected to revert as the ozone hole recovers (IPCC, 2021). However, in the long term the projected increases in atmospheric concentrations of greenhouse gasses will continue, and will continue to be a key driver of future climate in the southern hemisphere.

Observing ozone from satellites

EUMETSAT's Atmospheric Composition Satellite Application Facility (AC SAF) provides unique climate data records for monitoring the evolution and current state of the ozone layer globally, including that of the ozone hole. These climate records of total column ozone concentration are obtained from merged Global Ozone Monitoring Experiment-2 (GOME-2) observations onboard Metop-A, B, and C satellites since 2007. The global ozone concentrations are monitored on a daily basis, and provide information on the extent of the ozone hole (Hassinen et al., 2016, Hao et al., 2014).

A view on the ozone year 2022

IPCC mentions in its latest assessment report that the strongest ozone loss in the stratosphere continues to occur in austral spring over Antarctica (ozone hole), but that there are emergent signs of recovery after 2000 (IPCC, 2021). Figure 2 shows a merged GOME-2 time series, starting from 2007, for the spring months of the northern hemispheric (March) and southern hemispheric (October). Typically, the lowest values ozone concentrations are observed during these months. Over the last three years, the mean spring ozone concentration remained low in the southern hemisphere, whereas, after a dip in 2020, corresponding concentrations in the northern hemisphere remained higher.

Globally, in the year 2022, the mean total column ozone concentration remained close to the level where it has been for the past two decades. The observed changes in pattern were attributed to dominant weather regimes in 2022. Figure 3 shows the global difference between 2022 and long-term mean (2007–2020) of total column ozone obtained from GOME-2 instruments. The observed changes in pattern were attributed to dominant weather regimes in 2022.

Over the Antarctic, the minimum ozone concentration in 2022 was lower than the long-term average, but not as low, for example, as in 2020. In 2020, the ozone hole was the 12th largest since the start of the NASA satellite observational record in 1979, with an average area of 23.5 Mkm² and a minimum total column ozone concentration of 94 Dobson units (DU) in late winter or early spring (September and October). This was due to a very strong polar vortex over Antarctica that lasted longer than normal. Read further about the exceptional year of stratospheric ozone here.

Figure 4 shows the development of the ozone hole in the southern hemisphere spring, starting from August 2022, based on merged Metop GOME-2 and Copernicus Sentinel 5P Tropospheric Monitoring Instrument (TROPOMI) total column ozone concentration observations. Both instruments use reflected solar radiation at ultraviolet wavelengths to determine the total column ozone. Shaded areas of the time series plot represent long-term average (dark grey), as well as minimum and maximum bounds (light grey) for the minimum ozone concentration and the ozone hole extent. At the beginning of the animation, satellite observations are missing over Antarctica (white area) due to the lack of sunlight. In general, the lack of satellite observations at this time can cause an underestimation of the real size of the ozone hole, as well as overestimation of the minimum values of total ozone concentrations. After mid-September the minimum ozone column concentration started to decrease and the ozone hole area to increase. In early October, the minimum total ozone column concentrations of about 220DU and maximum extent of the ozone hole (about 25 million km^2) were reached. Even though the extent of the ozone hole area over Antarctica was at the larger end of the long-term variability, the ozone year 2022 was not as exceptional as 2020.

Benefits

This study shows the synergistic use of Atmospheric Composition and Climate SAF data to deepen our understanding on the temporal and spatial variations in the total ozone concentration and how these variations are linked to climate.

Training material

A practical workflow outlining how the GOME-2 data onboard of the Metop-A, -B and -C satellites can be used to animate the stratospheric ozone in 2020 over the Antarctic can be found on EUMETSAT's gitlab: https://gitlab.eumetsat.int/eumetlab/atmosphere/atmosphere/-/blob/master/30_case_studies/333_stratospheric_ozone_Antarctic_2020_Metop-ABC_GOME-2_O3_L2.ipynb

Data used

For this Climate Use Case the following data records were used:

Satellite based climate data record of atmospheric ozone profiles, derived from grating spectrometer observations (GOME-2), over the period 2007-2022 by the AC SAF that is cited as:

→ AC SAF (2022): GOME-2 Level 2 Vertical Ozone Profiles High Resolution Release 1 - Metop-A and -B, EUMETSAT SAF on Atmospheric Composition Monitoring, DOI: http://doi.org/10.15770/EUM_SAF_AC_0037 (accessed 2023-01-24)

References

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