Just days after some of the worst bushfires ever seen in Australia, the Bureau of Meteorology tweeted on January 8 that a climate driver called the Indian Ocean Dipole had returned to neutral.
Around the same time, another climate driver, the Southern Annular Mode, returned to neutral as well.
Then the rains came in eastern Australia. And hail. And floods. And dust storms. And lightning.
"It's really happened with a bang, or a real switch from unusually dry and stable conditions to this really significant event," said meteorologist Diana Eadie.
"The system that brought a supercell to the eastern suburbs of Melbourne is the same system that brought the large hail to Canberra, and it's the same system that's also been aiding storm development in Queensland."
Senior BOM climatologist Felicity Gamble said the change in the state of the climate drivers had not caused the wild weather — it just made conditions more favourable for rain.
"You still need to have those synoptic systems come through," she said.
"But given that the drivers are now neutral, we're in a much more favourable position for that to occur than we were, say, a couple of months ago."
Record climate driver events
The IOD has been identified as one of the main culprits behind the drought and this summer's terrible bushfires.
When waters are cooler than usual off Western Australia and warmer off east Africa (as they were this past spring), scientists call this a positive IOD.
In times of positive IOD, Australia typically experiences drought — last year's positive IOD event was the second strongest ever recorded.
At the same time, Australian weather has also been under the influence of a negative Southern Annular Mode (SAM), bringing strong, dry westerly winds — bushfire winds — further north.
Last year's negative SAM event was the strongest on record.
Climate drivers change simultaneously
Ms Gamble said that around the start of the year, three big climate drivers changed.
"SAM and the IOD did both decline at the same time, and, tied in with that, the southern movement of the monsoonal trough — it did all just line up at the start of 2020."
Ms Eadie said easterly winds had begun flowing again across the eastern seaboard.
"We're seeing a lot more moisture now streaming off the Coral Sea and the Tasman Sea," she said.
"We've been in easterly winds along the east coast for the better part of the last couple of weeks, and with that moisture is increased as well.
"So unlike previous systems coming through with a lot of wind but very dry conditions, this particular low was able to tap into a lot more moisture along the entirety of the eastern seaboard."
Positive IOD linked to bushfires, drought
Ms Gamble said there was a strong correlation between positive IODs and rainfall deficiencies, and then negative IODs and above-average rainfall.
"You say 'El Nino' and people know, 'Oh, that means drought in Australia', or vice versa, people tend to know La Nina means wetter-than-average conditions as well," she said.
"But you say 'Indian Ocean Dipole', and most people wouldn't really know what a positive IOD means or what a negative IOD means.
"And yet it can have as big, sometimes bigger impact on our climate than an El Nino or La Nina event — perhaps because it doesn't have quite such a catchy name."
The drought is not over
The BOM is predicting an equal chance of a wetter or drier-than-average period from February to April for most of Australia.
In other words, with climate drivers returned to neutral, the conditions favour an average amount of rain in most of Australia over the next few months.
Despite recent rain, senior BOM climatologist Andrew Watkins cautioned the drought was not over, tweeting:ABC