The Southern Hemisphere’s ozone hole reached 27 million km² this month, making it one of the largest ozone holes on record. This year’s large ozone hole is associated with an unusually strong polar vortex, which could also influence Australia’s weather this spring.

What is the ozone hole?

Earth’s atmosphere contains an ozone layer that protects life on the planet’s surface from harmful ultraviolet radiation emitted by the Sun.

Human use of ozone-depleting substances (ODSs) in the late twentieth century depleted stratospheric ozone, leading to the development of the annual Antarctic ozone hole.

The ozone hole is not an actual hole – it’s a region of substantially depleted ozone in the stratosphere high above Antarctica.

The ozone hole develops in late winter and spring when sunlight returns to Antarctica and interacts with ozone-depleting substances on the surface of polar stratospheric clouds – a rare type of cloud that forms in extremely cold air trapped by the polar vortex. These reactions rapidly destroy ozone molecules in the polar stratosphere, creating the ozone hole.

Towards the end of spring, the polar vortex weakens and allows ozone-rich air to mix back into the polar stratosphere, causing the ozone hole to break down.

The ozone hole is measured using Dobson units (DUs), which indicate the total amount of ozone in a vertical column of Earth’s atmosphere. A value of 220 DUs is used as the boundary representing the edge of the ozone hole.

One of the largest ozone holes in decades

NASA satellites have been measuring ozone concentrations over the Antarctic region since the late 1970s.

Earlier this month, the ozone hole reached 27.04 million km². According to data on the NASA Ozone Watch website, this is the 7th largest ozone hole observed since satellite records began in 1979 and the largest since 2015.

At this year-to-date peak on September 18, the ozone hole was around 7.5 million km² larger than average for that date and roughly 3.5 times the size of Australia. The largest ozone holes on record, which occurred in 2000 and 2006, reached close to 30 million km².

Image: Ozone hole on September 18, 2026. This image shows total ozone in Dobson units, with the purple colours showing the least ozone, while green, yellow and red represent more ozone. Source: NASA Ozone Watch.

While the size of this year’s ozone hole has been among the largest ever recorded, other indicators used to monitor the state of the ozone hole – including ozone mass deficit and ozone column minimum value – remain closer to average.

It is also important to point out that this year’s large ozone hole is not a sign that the ozone hole recovery has stalled or slowed down. While there can be a large year-to-year variability in the size of the ozone hole, observations show that the hole in the ozone layer is getting smaller and is on a path towards long-term recovery following the elimination of ODSs under the Montreal Protocol, which was adopted in 1987.

Why is the ozone hole so large this year?

The large size of this year’s ozone hole is associated with a stronger-than-usual polar vortex, which has allowed exceptionally cold air to become trapped in the stratosphere above Antarctica.

Cold air is required to create the polar stratospheric clouds that play an important role in the chemical reactions which lead to ozone depletion above Antarctica. When the air is colder than average, conditions are more suitable for the development of polar stratospheric clouds, and ozone depletion becomes more prominent.

The air temperature in the polar stratosphere became unusually cold in September, which coincided with a sharp increase in the size of the ozone hole.

According to the Copernicus Atmospheric Monitoring Service (CAMS), “in the first half of September, the area of this year’s ozone hole increased sharply, to an estimated 25 million km² on 12 September – around 5 million km² above the average for this period.”

“The rapid development of the 2026 ozone hole coincided with a sudden decrease in the minimum temperature at approximately 20km above the south pole.”

Data from the US Climate Prediction Center shows that temperatures around 20 – 30 km above the Antarctic region have been among the lowest 10 % of observations in the historical record for most of this month.

Image: Mean temperature at the 10 hPa level (around 30 km above the surface) above the Antarctic region, with the red line showing temperatures in 2026 and the grey shaded areas and black lines showing historical observations. Source: NOAA / CPC.

Will Australia see higher UV levels?

Australia is not positioned beneath the ozone hole and is shielded by a protective layer of ozone. While a large ozone hole can make it easier for parcels of ozone-poor polar air to briefly spread northward, the ozone hole is likely to remain positioned over Antarctica this spring.

Any increases in UV level would be most likely to affect Tasmania and far southern parts of the Australian mainland.

While the ozone hole itself isn’t expected to have a major impact on Australia this spring, the strong polar vortex may have more of an influence on weather patterns.

What does the strong polar vortex mean for Australia’s weather?

A strong Antarctic polar vortex can favour positive phases of the Southern Annular Mode (SAM), particularly during spring.

A positive SAM occurs when the belt of westerly winds that surround Antarctica are located further south than usual for that time of year. During the Southern Hemisphere’s spring, a positive SAM typically pushes westerly winds and cold fronts to the south of Australia and increases the likelihood of easterly winds over eastern Australia, which:

Increases the likelihood of drier than average weather over parts of southwestern and southeastern Australia.

Enhances the chances of rain over parts of eastern Australia.

Forecast models indicate that the SAM is expected to be in a neutral to positive phase through the end of September into early October. However, it is important to note that the SAM is only one climate driver that influences Australia’s weather. Other factors, including a strengthening El Niño, will all play a role in Australia's weather in the coming weeks and months.

While this year’s ozone hole is among the largest seen in decades, the more relevant phenomenon for Australia may be the unusually strong polar vortex associated with it, which could influence the country’s weather patterns this spring.