A year ago, Canberra was surrounded by fire and smoke. The city had just sweated through its hottest year on record and the region was in drought.
The past month was a stark contrast. Temperatures have been high this week but summer, to date, has been wet and wintry. Canberra is green rather than the dry yellow of 2019.
So why is this happening in a warming planet? (And yes, it is still warming: 2020 was the fourth-hottest year on record.)
The answer lies thousands of kilometres away on the far side of the Pacific Ocean.
The swinging weather in this case is not climate change — it is the regular, but barely predictable, ebb and flow of La Niña and El Niño phenomena.
Eastern Australia is currently undergoing its most intense La Niña event in a decade.
In short, this means the weather is cooler and wetter than usual.
But this particular event is unlikely to last much longer — it is probably already at its peak — nor is it as severe as it may seem.
Who is La Niña and where is she from?
La Niña and her brother El Niño arrive when the tropical waters off South America's west coast are unusually cool (La Niña) or warm (El Niño).
Their arrival was traditionally marked by the Southern Oscillation Index. This measures the difference in air pressure between Darwin and Tahiti — because barometric records in those places are accurate back to the 19th century.
La Niña occurs when Tahiti's air pressure is significantly higher than Darwin's air pressure, over a sustained period.
Bureau of Meteorology senior climatologist Blair Trewin said the change in water temperature "has broader impacts on climate, including eastern Australia".
"During a La Niña year, you get stronger easterly winds through the tropics, which is connected to those pressure patterns," he said.
"You also get changes in the position of weather systems, which tends to push more moisture into eastern Australia."
Swings that defy prediction
About two La Niña and two El Niño events happen each decade, on average.
However, the events are not cyclical — a La Niña does not necessarily follow an El Niño, nor vice-versa.
Forecasts of a La Niña or El Niño's arrival, intensity and duration would be extremely valuable to many industries, especially agriculture.
But despite cutting-edge modelling and more than 140 years of data, meteorologists cannot yet make accurate, long-term predictions.
Dr Trewin said modern forecasts focus on sea temperature rather than air pressure (though the two are linked), and are improving.
"We are pretty reasonably skilled at predicting three to six months ahead," he said.
There are limits, however. One is the "autumn predictability barrier", which means it is harder to make a forecast in February than it is, say, in June, because these events tend to break down (or not) during the southern hemisphere's autumn.
La Niña set to fade after summer
Despite these volatilities, Dr Trewin said he is "relatively confident" of Canberra's medium-term outlook.
"We've been predicting a high probability of La Niña development until about the middle of the year," he said.
So rain and cooler temperatures will likely persist over the next few months at least.
But Dr Trewin says the current event seems to have peaked and is likely to peter out soon afterwards.
In other words, it is nothing like the La Niña that struck a decade ago, which precipitated the devastating Queensland floods of 2010 and 2011 that killed at least 33 people.
"The 2010-11 event was exceptionally strong — one of the two or three strongest events of the last 60 years," Dr Trewin said.
"The event we've seen this time around is not on that level of intensity, but it's still significant."
Think this summer is cold? It's not really
Canberrans may feel the recent weather has cheated them out of a second summer in a row, after bushfires and smoke stole some of the pleasures of last year's.
However, the official records tell a different story — yes, last month was a little cooler than usual, but only just.
Dr Trewin said the ACT's average maximum temperature in December 2020 was only 1 degree Celsius less than the historical average.
"A lot of people would perceive December as being unusually cool," he said.
"I think it stands out more because we've had a run of very hot Decembers.
"You have to go back to 2011 to find a December that wasn't at least 1C above average."
It may be more a case of the ever-warming climate changing our perceptions of what is "normal" — so perhaps this week's 'unbearably hot' will be the 'pleasantly mild' days of the next generation.ABC