Climate: What will largest Antarctic ozone hole in years mean for New Zealand?

The ozone hole over Antarctica has swollen to its largest size and deepest level in years. Image / ECMWF

The ozone hole over Antarctica has swollen to its largest size and deepest level in years - but it's not yet clear what that might mean for stormy weather and sunburn risk here in New Zealand.

The European Union's Earth observation programme this week reported a strong, stable and cold polar vortex has driven the expansion of the hole to a size among the largest seen in 15 years.

The ozone hole begins to expand every August - at the start of the Antarctic spring - and reaches a peak around October.

This year, it grew "rapidly" from mid-August and peaked in early October at about 23 million sq km, the World Meteorological Organisation announced on Tuesday.

Energy from the sun as it rises over the pole releases chemically-active chlorine and bromine atoms into the polar vortex, which quickly destroy ozone molecules - causing the hole to form.

Despite the growing hole, experts still believe the ozone layer is slowly recovering after adoption of the Montreal Protocol, a treaty signed in 1987 that aims to phase out ozone-depleting substances.

Niwa scientist Dr Olaf Morgenstern said the latest observations were consistent with a gradual recovery of ozone.

"We know for certain that the total amount of ozone-depleting substances in the atmosphere is decreasing, a result of the implementation of the Montreal Protocol," he said.

"Illegal emissions of CFC-11 in China, reported in recent years, have only slowed down but not reversed this development."

Meanwhile, increasing levels of carbon dioxide was leading to a cooling of the polar vortex, which could sometimes lead to unusually cold temperatures, and deep ozone holes like that seen this year.

"We can note, however, that only last spring we had an unusually weak polar vortex and a shallow ozone hole."

2020's #OzoneHole is both larger and deeper than most in recent years. This is being driven by a strong, stable and cold polar vortex, with all signs pointing to continued ozone depletion in the coming days.#Ozone monitoring➡️https://t.co/c2tPB8OBuQ pic.twitter.com/MPiae3I5tL

— Copernicus ECMWF (@CopernicusECMWF) October 1, 2020

While annual changes in the depth of the ozone hole were considered naturally-driven, Morgenstern said it was possible human activity might have had an influence.

"I'd say the jury is out on this one. It's hard to tell, given the relatively short observational record that we have. Some models suggest this much."

Would the wider hole have any implications for New Zealand over coming months?

Morgenstern explained that, when the vortex broke up - likely some time between now and November - ozone-depleted fragments of the polar vortex could drift over NZ, causing spikes in UV that could cause a heightened risk for sunburn.

🌐2020's #OzoneHole has reached its maximum size, according to our #CopernicusAtmosphere Monitoring Service. It is one of the larger and deeper ones of recent years, with minimum #ozone columns falling below 100 DU on 2 Oct

Ozone monitoring➡️https://t.co/c2tPB8OBuQ pic.twitter.com/JfvSHgq6OH

— Copernicus ECMWF (@CopernicusECMWF) October 6, 2020

"Also, polar ozone depletion has contributed to a general ozone deficit over the Southern Hemisphere compared to pre-ozone hole times, but it's not enormous, thanks to the Montreal Protocol," he said.

"The strong vortex is probably a sign of a strong, positive state of the Southern Annular Mode, which correlates with a strengthening and southward displacement of circumpolar winds in the lower atmosphere.

"Perhaps we are in for a stormier than usual spring."

- Additional reporting: AP