About a year ago, Canberra was covered by hot fire and smoke, having been smouldered by its hottest year ever recorded, leaving the region in severe drought. Fast forward to last month, the city is a complete contrast with high temperatures being experienced this week even though so far summer has had some wet and cold weather. Today, Canberra looks green and not dry-yellow as it was in 2019.
So, why is this occurring in a warming globe? And of course it continues to warm with 2020 going down in books as the fourth hottest year on record.
The answer is thousands of kilometres away in the Pacific Ocean.
The changing weather patterns should be considered as climate change—it’s simply the regular, but unpredictable flow and ebb of the La Niña and the El Niño phenomena.
As at now, eastern Australia is undergoing its highly intense La Niña event in the past ten years. Ideally, this means the weather is wetter and cooler than usual.
However, this specific event won’t last longer and could probably be at its peak, and neither is it as severe as we think it is.
What is La Niña and where does it emanate from?
La Niña and El Niño show up when the tropical waters off South America’s western coast become unusually cool (La Niña) or very warm (El Niño).
Their arrival is normally evident by the Southern Oscillation Index. This index measures the difference in air pressure between Tahiti and Darwin—since barometric records in these areas are considered accurate, dating back to the 19th century.
La Niña takes place when air pressure in Tahiti becomes significantly higher than air pressure in Darwin, for a sustained period of time.
According to the Bureau of Meteorology (BoM) senior climatologist Blair Trewin, the change in water temperature carries “broader effects 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.”
Shifts that clearly defy prediction
On average, approximately two La Niña and two El Niño events occur each decade. Nevertheless, these events do not happen cyclically in that a La Niña does not always follow an El Niño or vice-versa.
Forecasts regarding the arrival of a La Niña or El Niño’s, their intensity and duration is extremely important to many sectors particularly agriculture.
However, regardless of the innovative modelling and over 140 years of data, meteorologists are still unable to make more accurate, long-term forecasts.
Dr Trewin noted that modern predictions often focus on sea temperatures instead of air pressure (though the two are intertwined) and continue to improve.
“We are pretty reasonably skilled at predicting three to six months ahead,” he said.
This comes with limits though. One of them is the “autumn predictability barrier” which means it’s more challenging to make a forecast in March than it is, say, in July, since these weather events tend to breakdown (or sometimes not) during the autumn period in the southern hemisphere.
La Niña expected to fade after summer
In spite of this volatility, Dr Trewin stated that he was “relatively confident” of the medium-term outlook in Canberra.
“We’ve been predicting a high probability of La Niña development until about the middle of the year,” he said.
So yes, showers and cooler temperatures are set to continue over the next couple of months at least.
However Dr Trewin notes that the ongoing event could have peaked and could pass away sooner than expected.
Thankfully, this is nothing close to the La Niña that hit a decade ago, causing precipitation of the distressful Queensland floods of 2010 and 2011 that left 33 people dead.
“The 2010-11 events 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.”
Do you think this summer is cold? That’s not it
Canberrans are concerned that the recent weather has not been as usual for the second summer in a row, after bushfires and smoke ravaged through the city denying the residents of some pleasures.
Nevertheless, the official records reveal a different story—that last month was a bit cooler than normal, but that’s only just.
Dr Trewin added that the average maximum temperature in the ACT for the month of December 2020 was a degree Celsius less than the existing record 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.”
This could just be another case of the continuing warming climate shifting our beliefs of what is “normal” and as such the ‘unbearably hot’ weather this week could evolve into the ‘pleasantly mild’ days for the next generation.
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