Four main climate drivers that are influencing Australia's wet weather

Large parts of Queensland, NSW, Victoria, and eastern Tasmania are predicted to receive above median rainfall from December through February, and northern Queensland has at least twice the average chance of unusually high rainfall.

For most of Western Australia and the western Top End however, rainfall will be below the median.

Monash University research fellow at the school of earth atmosphere and environment Dr Kim Reid said ongoing wet weather forecasts this year had helped familiarise Australians with some climate drivers.

"The public's kind of getting their heads around El Nino and La Nina and might have heard of Indian Ocean Dipole, but [Southern Annular Mode] and [Madden-Julian Oscillation] are for the more intense weather nerds," Dr Reid said with a laugh.

"But these are the four main climate drivers that impact Australia and funnily enough, they occur at four different compass points, so [El Nino Southern Oscillation] is to our east, IOD's to our west, SAM is to our south and MJO is to our north."

As a result of ongoing rainfall, BoM has also forecast high stream flows likely for November to January along the east and south-east coast of the mainland, and the north coast of Tasmania.

Last month, flows were high at 77 per cent of locations across Australia, mainly along the east and south-east coast of the mainland, and the north coast of Tasmania, median at 9 per cent of locations across Australia, and low at 14 per cent, mainly on the south-west coast of Western Australia, scattered areas in the north and central parts of the mainland, and central Tasmania.

You may have heard lots about this climate driver of late, as Australia finds itself in the third consecutive La Nina weather event in three years.

It's one phase of what's called the El Nino Southern Oscillation (NSO). The other phase is called El Nino.

"It's to do with the temperature difference across the Pacific Ocean, so when there's cold ocean temperatures near South America and warm ocean temperatures near Australia, this is called La Nina and what it does is it enhances the strength of the winds that come off the Pacific Ocean towards Australia," Dr Reid explained.

These winds from the east result in more moisture over eastern Australia, which, when paired with warmer ocean temperatures around the continent, means even more moisture is available in the atmosphere.

Dr Reid said the Indian Ocean Dipole (IOD) was the younger sibling of El Nino Southern Oscillation (NSO).

"It's the same kind of idea where it's about the temperature differences across the ocean and so when there's warm waters to the northwest of Australia and cooler waters near Africa, we call that a negative Indian Ocean Dipole and that's what we're seeing at the moment," she said.

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A negative Indian Ocean Dipole has been associated with more moisture in the atmosphere, such as in the Northwest Cloud Band; a continuous, moist stretch of clouds that crosses the whole continent.

"And that's where we see enhanced rainfall, in particular, over central Australia," Dr Reid said.

"Working together these two systems just point to more rainfall."

Conversely, a positive IOD, which Australia saw in the lead up to the 2019 to 2020 Black Summer bushfires, results in hotter, drier conditions with less rainfall.

The Southern Annular Mode (SAM) is the north-south movement in winds that spin around Antarctica, meaning it controls the position of weather such as cold fronts and low pressure systems that can bring rain to southern Australia.

The SAM index is positive at the moment and will likely remain positive until at least December, according to BoM.

"In a positive SAM these storm tracks shift towards Antarctica and what that does is it allows more warm tropical air to shift south as well as enhanced likelihood of rainfall over eastern Australia,' Dr Reid said.

However, for Victoria, South Australia and south-west WA a positive SAM means a reduction in rainfall.

"The Madden-Julian Oscillation (MJO) is a pulse of thunderstorm activity that begins in the Indian Ocean and then traverses along the equator and disappears in the Pacific ... where it's active can influence the weather systems that we see in Australia," Dr Reid said.

"For example, when it's active in northwestern Australia, we might get more north-west cloud bands, if it's more active in the north-east, we might get more atmospheric rivers hitting the east coast of Australia."

Atmospheric rivers are narrow regions of intensive water vapor transport in the atmosphere.

Dr Reid said they were quite common, but can get "stuck".

"We saw this in February and March earlier this year, this atmospheric river that kind of got stuck by a big blocking high pressure system in the Tasman Sea, and there was constant rainfall forced over south-east Queensland, northern NSW, and that was associated with that," Dr Reid explained.

"We again also saw one of these systems in March 2021 with the flooding then and so these systems, they're usually quite beneficial, but they can also cause quite intense rainfall."

The MJO can also impact on the onset of the monsoon season in northern Australia.

It's complicated.

University of New South Wales climate change research centre Professor Jason Evans said the increased rainfall we're seeing and how it is connected to a warming climate is still being researched.

"I can tell you with 100 per cent certainty that all the rain we're seeing would happen in a world that has warmed by nearly a degree since pre-industrial, right? Because that's what's actually happened," he said.

"But the question then is, 'if there was a world that had not warmed, how much of this rain would we have seen? That is not a simple straightforward question."

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Oceans around Australia have reached record warm temperatures this year which interacted with each of these weather systems, making them more moist.

A warmer atmosphere is also able to hold more moisture, he said.

"And that's part of the reason that we got such a lot of rain through this year," Professor Evans said.

"One of the main things these different [weather systems] do is that they change the circulation of the atmosphere, so the atmosphere has more moisture, in general, and then if we have a climate mode, in the right phase ... that will give us more rain.

"And when rain happens, because you're starting with more moisture, you'll get more rain."