Australian weather system affects rain in the north and heatwaves in the south

The last few years have seen horrific floods in the Southern Australian states. But in the northwest part of the continent, rainfall has almost doubled in the past 70 years, causing its own set of problems.

Researchers have now connected this increase in rainfall to the Madden–Julian Oscillation (MJO), in a paper published in Geophysical Research Letters.

The team hope this will provide more information to groups like the Bureau of Meteorology (BoM) to better project extended forecasts.

“The MJO is an eastward travelling region of enhanced cloudiness and rainfall,” University of Melbourne climate scientist Alex Borowiak told Cosmos Science.

“The MJO starts in the Indian Ocean, then travels across the top of the north of Australia and brings heavier rainfall. Cyclones are more likely to form if the MJO is present over northern Australia.

“Then it reaches the cool waters of the eastern Pacific where it disappears. It takes about 30 to 60 days to do this.”

The MJO is enormous – it travels approximately 12,000 to 20,000 kilometres, more than a quarter of the circumference of the Earth. It affects not just northwestern Australia but also the Northern Territory, Queensland, PNG, Indonesia and the Pacific Islands. It also aids in the formation of tropical cyclones.

This was the case in early 2023, when the Fitzroy River experienced devastating flooding after Tropical Cyclone Ellie dropped 800 mm of rainfall.

Forecasters already knew that rain had been increasing in the northern part of Australia since the 50s, but it wasn’t confirmed if the MJO – or some other weather phenomena – was causing the increase. However, there were some clues.

“The MJO is also a useful tool for weather forecasting in northern Australia,” says Borowiak.

“[Forecasters have] found that these seasonal forecasts are actually more accurate when the MJO is present, because it follows such a predictable path.”

The team used satellite and rainfall data from the BoM since 1974 to track how the MJO was moving and when the rain was falling.

They found that the rain only increased when the MJO was over northwest Australia, and when the MJO wasn’t present, rainfall levels in the top end have actually been decreasing.

Borowiak also notes that when the MJO is increasing rainfall in the northwest, it is also worsening heatwaves in southern Australia by collecting the moisture from the lower half and redistributing it.

“It’s also got really, really wide-reaching impacts in southern Australia as well,” he says.

“The MJO is pulling moisture away from the southern part of Australia. So, heat waves are actually more intense when the MJO is above northern Australia.”

Unfortunately, like many of these climate change phenomena, the MJO is likely changing due to climate change.

The MJO is connected to a warm section of water just north of Australia called the Indo Pacific Warm Pool, and as that’s growing it has “warped” the cycle of the MJO. This means the MJO now speeds over the Indian Ocean and slows down over northern Australia.

“There’s a lot of important questions about what that means for the ecosystems out there, there’s plenty of unique plant and animal species that only live up there,” says Borowiak.

“The rainfall has increased by over one and a half times the amount in some regions – what does such a large rainfall change mean?”