From a parched, hot September and October — on the brink of extreme drought — to flooding rains in summer, the past few months have thrown up a range of weather extremes across the country.
Much of it comes down to the natural variability of weather in general.
But in the background, several climate drivers have been helping guide the weather in certain directions — and not just El Niño.
So how have they each been behaving recently?
Indian Ocean Dipole likely helps with spring dry
The Indian Ocean Dipole (IOD), sometimes referred to as El Niño’s cousin, forms over waters to Australia’s west.
Exerting its influence during winter and spring, it has two phases: negative, which is the wet phase, and positive, the dry phase.
Since August, the IOD has been in a positive phase — peaking at its second-highest values ever recorded by the Bureau of Meteorology, according to climatologist Catherine Ganter.
Based on its typical influence, Ms Ganter said it was likely to have played a role in the intense dry conditions experienced during winter and spring, which set up an early start to the bushfire season, and saw several regions in serious rainfall deficits.
This is especially true given some of the more unusual characteristics of this year’s El Niño may have dampened its drying influence, according to Ms Ganter.
“So this positive IOD was on the strong end, and it would have likely contributed to the below-average rainfall that we saw, particularly in August to October which was that dry three month period,” she said.
The IOD has lingered for longer than normal this year, but is now breaking down, so is no longer having an influence on Australia’s climate, according to Ms Ganter.
Southern Annular Mode encourages summer rainfall
Though September and October were parched, rainfall kicked up a notch in November for large parts of Queensland, NSW, the NT and northern WA.
Since then, the eastern states have seen repeated bouts of heavy rainfall.
Many parts of Victoria and South Australia have recorded an entire summer’s worth of rainfall already, and others face flooding emergencies.
One of the likely reasons for this enhanced summer rainfall — particularly over the south-eastern states — is a driver known as the Southern Annular Mode (SAM), according to Monash University climate scientist Kimberley Reid.
The SAM refers to the north-south position of a belt of westerly winds over the southern ocean, and is linked to rainfall patterns across Australia.
Since early December, the SAM has persistently been in a positive phase, meaning the belt of winds is closer to Antarctica, which tends to favour rainfall in summer for the eastern states.
“What that does in summer, for south-eastern Australia, is it sort of draws tropical, warm, moist air further south,” Dr Reid said.
“It also allows for more onshore winds over the east coast, which again helps push moist air over Australia.
“So that’s probably why we are seeing a higher-than-average number of wet weather systems in summer this year.”
Something a bit unusual about the SAM’s behaviour this summer, according to Dr Reid, is that it’s happening during an El Niño event,
Typically, a persistent positive SAM is associated with the El Niño-Southern Oscillation’s opposite, phase — La Niña, which is the wet phase for Australia.
Monash University climate science professor Julie Arblaster said it may have been pulled further south this year partly due to conditions in the Antarctic region, including the sea ice extent and the ozone hole.
“We do associate the positive SAM with more sea ice, usually. I think the fact that the sea ice has grown back a bit closer to [normal] since we’ve had that positive SAM is consistent with that,” Dr Arblaster said.
“The other thing in the Antarctic region is that we had an ozone hole [that] lasted a lot longer than it usually does.
“[That] can also have an impact on a positive SAM.”
El Niño driving warmth, but not following usual script
An El Niño climate pattern was declared underway by the Bureau of Meteorology in September, after months of speculation.
It is one of the world’s most consequential climate drivers, and is linked to hotter, drier weather in Australia — particularly during winter and spring — with its warming influence continuing into summer.
Though some characteristics of the event have made it unusual, Dr Reid said the driver likely played a part in the warmer, drier conditions in spring, as well as the later onset to the monsoon in northern Australia.
The event is still underway, which means it may be contributing to the warmer-than-average temperatures that have continued into summer too, along with climate change, according to Dr Reid.
El Niño doesn’t have much of an influence on rainfall during summer months, which she said made the recent rainfall over eastern Australia less surprising.
However, there have been some signs El Niño is not playing by its normal rule book this year, which Dr Reid said may have softened its overall influence.
“Typically during an El Niño, you have cooler ocean temperatures to the east, and that’s why we often associate it with less rain,” she said.
“But the fact that there’s warmer ocean temperatures, and warm waters typically help promote rainfall, means that we’re likely seeing a bit more rain than we normally would if was a more traditional El Niño.
“And I think this is probably throwing climate scientists out a bit, that the patterns aren’t looking like what we’re used to seeing.”
Dr Reid said the atmospheric component of El Niño was also struggling to hold strong.
This was occurring at the start of the event too, which was why the Bureau was hesitant to declare the event underway in the first place.
“You really need both the atmosphere and ocean to be speaking the same language to get those strong impacts on Australia,” Dr Reid said.
Monsoon kicks off in tropics
Over recent weeks, heavy rainfall, gusty winds and cooler conditions have kicked in across the tropics, bringing relief to the building humidity.
The seasonal onset of tropical activity, known as the monsoon, was delayed this year compared to its usual late-December onset, which the Bureau said may in part be due to the El Niño event.
The MJO is a recurring area of enhanced cloud and storms that forms in the Indian Ocean, then travels east around the tropics.
When active over northern Australia, it often leads to several days to weeks of enhanced rainfall and flooding.
“The MJO kicking in is probably why we’re seeing a lot of tropical activity,” Dr Reid said.
“And there’s a few tropical lows that may develop in tropical cyclones in both the Indian and Pacific Oceans at the moment.”