Persistent Antarctic ozone loss this spring
A sign of things to come, or just a flash in the pan?
Every couple of weeks each spring I receive an emailed update on the status of the Antarctic ozone hole. The summaries are prepared by Paul Krummel, a colleague at Australia’s CSIRO. They draw heavily on satellite data products from NASA. His 10th report for this season arrived in my mailbox last week. I’ve copied a couple of images from it below.
By most metrics the ozone hole this year is only slightly stronger than average - ranking around 12th strongest of the 40-plus years since satellite data have been available. But, as for the last couple of years, the main characterising feature is its longevity. Ozone amounts for late October (thick black line) were close to their all-time low, and quite similar to those in 2020 and 2021 (green and purple lines).
The reason for its persistence is the stable meteorological conditions that prevailed over the month of October, which had the effect of confining the ozone-depleted air to high latitudes. As the images below show, in late October the ozone hole - defined as the area within the red 220 DU contour - is still quite symmetrical. And there’s still a large area with ozone amounts less than 150 DU (shaded purple).
Despite those low ozone values in Antarctica, you can see that New Zealand remains under a ridge of relatively high ozone, and the amount of overhead ozone close to the maximum on any of those images (despite the relatively low ozone values. As I mentioned last week, you can see that ozone amounts are much lower at low latitudes too.
From a UV perspective, these lingeringly long events are important because ozone is still low when sun elevations become higher.
In 2020 and 2021, these low ozone values persisted right through December, leading to unprecedentedly high UV levels in Antarctica. It’s conceivable that those new records will be exceeded this year.
The reasons for the persistence in recent years aren’t fully understood (at least by me), but could be a consequence of climate change. For example, it’s possibly due to the lower stratospheric temperatures that come with that. Associated changes in wind patterns could also be important. But other effects, like aerosols from forest fires in recent years (exacerbated by climate change) or unusually large amounts of water vapour this year injected by the Tongan eruption may also play a part.
I was disappointed to see that the Ozone Science Assessment Panel (SAP) now focuses on ozone depletion just in September (rather than over a longer period) as the preferred metric to characterise chemical ozone loss from ozone-depleting-substances. While that period may give a better correlation with the concentrations of chlorine in the atmosphere, it’s not very relevant for a UV perspective and it may be missing something important. The ozone hole has persisted longer than ‘usual’ in 4 of the last 5 years. We can’t continue to ignore that for ever.