Droughts across southern Australia are to continue increasing as the Roaring Forties get stronger and closer to Antarctica, a study has found. It also explains why Antarctica is bucking the global warming trend.



Australian National University researchers looked at the past 1,000 years of Southern Ocean winds for the first time, along with ice core samples and South American tree rings and lakes.

They found increasing greenhouse gases were strengthening the Southern Ocean’s Roaring Forties – known as the Southern Annual Mode (SAM) – that delivered rains to southern Australia. They also found the winds were tightening in over Antarctica.

“As the westerly winds are getting tighter, they’re actually trapping more of the cold air over Antarctica,” said lead researcher Nerilie Abram.

“This is why Antarctica has bucked the trend. Every other continent is warming, and the Arctic is warming fastest of anywhere on Earth.”

This resulted in the decreasing rainfall across southern Australia, particularly in Western Australia.

“This isn't good news for farmers in southern Australia who are reliant on winter rains,” said the report, published on Monday in Nature Climate Change.

The southern region of Western Australia had a 20% decline in rainfall since the 1960s, said Abram.

“The rainfall there has such a strong influence from what’s being thrown up at it by the Southern Ocean. On the south-eastern side, we have a real interaction of the different climate modes. On the east side, we also have a very strong influence from tropical climate modes like El Nino.”

A reconstruction of the SAM showed a 1000-year perspective and gave context to the only other records available that showed prominent increases in wind strength beyond natural variability since the middle of last century, but no further back.

“This is what gives us a really good indicator that greenhouse gases are what’s causing the winds to intensify.

“On average, the strength of the SAM is now at its highest positive value – stronger and closer to Antarctica than they have been at any time over the last 1000 years,” she said.

The winds moved between two and five degrees closer to the South Pole in the past half century alone.

“These winds have been changing over the last few decades and that’s one of the reasons we’ve been having such severe droughts,” said Abram.

The researchers explained that while greenhouse gases impacted mostly in the winter, during summer and spring months the SAM was influenced strongly by depletions in the ozone layer.

As the hole in the ozone layer appeared to be repairing itself – thanks in part to the fall in the use of harmful substances – the greenhouse gas effect was the main cause for concern.

The alterations in the SAM were only slightly mirrored by its northern counterpart, Professor Matthew England from the University of New South Wales' Climate Change Research Centre said.



“As we increase greenhouse gases, different parts of the planet warm differently,” he said.



“The northern hemisphere is continent dominated. In the south we have ocean dominated regions and they don’t warm in consistent ways.”

The researchers said all signs pointed to a continuing increase in these winds, and further concentration over Antarctica, unless greenhouse gases in the atmosphere were reduced.



“The SAM seems to be one part of the weather system which responds quite rapidly,” England told Guardian Australia.



“Unlike things like glacial melt – that’s really hard to slow down – with the SAM it seems you get a payback for reducing your emissions on a quick time scale.”

