The Australian summer has been a tale of two extremes: the west is baking hot, while the east is awash with devastating downpours. While Western Australia’s maximum temperature so far this summer is 1.5C above the long-term 29.5C average, Perth has melted through three heatwaves and smashed February records for the most days hotter than 40C. Last month, the Pilbara town of Marble Bar sweltered through its second-longest hot spell on record, with 23 consecutive days above 43C.
In stark contrast, storms have ravaged the east coast this summer. Ex-Tropical Cyclone Jasper caused flooding across vast areas of far north Queensland before Christmas, delivering Australia’s wettest December days on record. One weather station recorded 1.9 metres of rain during five days.
And this week storms brought heavy hail and strong winds to Victoria, causing the temperature to plummet 15C and leaving hundreds of thousands of people without power.
As the weather intensity has dialled up, so have the insurance claims. The Insurance Council of Australia says more than 46,000 extreme weather-related claims were lodged between 23 December and 3 January.
So, what’s causing this unusual polarisation? Scientists say it’s down to a rare combination of three climate systems hitting at once.
‘A perfect storm’
A shifting belt of westerly winds in the Southern Ocean is turning the heat dial up on WA while also filling rain gauges on the east coast, Hugh McDowell, a senior climatologist at the Bureau of Meteorology, says.
Meanwhile, the Indian Ocean is hotter than normal and El Niño arrived in the Pacific in September.
Climate scientists are calling it “a perfect storm” that is breaking weather records across the country.
“These three things that usually happen in isolation are happening together and they are all reinforcing each other,” the BoM meteorologist Jessica Lingard says.
Australia’s climate is complicated and influenced by many drivers. The three oceans bordering Australia each have their own sets of natural oscillations in currents and wind patterns, all of which affect the nation’s rainfall and temperatures.
‘Little boy’ and his cousin Sam
One of the best-known patterns is El Niño, Spanish for “little boy”, which weakens trade winds in the Pacific.
But the boy has a southern cousin – the Southern Annular Mode (Sam) – and when it is in a positive mode, it disrupts the strong westerly winds that blow continuously around the globe, pushing them further south and away from Australia.
In the summer, this drags moist tropical air over the eastern states, fuelling intense rainfall and flash flooding.
In the west, a positive Sam causes high-pressure systems and their associated anticlockwise winds to set up camp in the Great Australian Bight. This acts like a hairdryer that blows hot desert easterlies from Australia’s interior over WA.
McDowell says while the Sam has a big effect on opposite sides of the continent, it is normally short-lived and dissipates after a couple of weeks.
“It is not normally a long-term climate driver, but this year, we have had it positive for at least two months and it has been due to a stronger-than-average polar vortex,” McDowell says.
He says El Niño conditions, which occur in the Pacific Ocean, are normally associated with a neutral or negative Sam – not positive.
El Niño weakens trade winds and sloshes warm water from the east of Australia towards South America. La Niña has the opposite effect and piles warm water around northern Australia, according to the Australian Research Council.
“The last three La Niña years we saw positive Sam at play, we saw warmer-than-average sea surface temperatures all across the east, but we have those things and we have El Niño this year, which is very unusual,” McDowell says.
“That is probably why we have seen so much rainfall across the east this summer.”
The role of the dipole
The third and final player in the trio is the Indian Ocean dipole, which features fluctuations in sea surface temperatures between the Horn of Africa and Indonesia.
Right now, McDowell says, this dipole is also in positive mode, leading to less rainfall for north-west WA.
“It is unusual that we are seeing those three things together. It is a pretty usual situation, giving a bit extra to the heat and a twist with the wet east,” McDowell says.
“Sam has had an influence on Western Australia with the drier and warmer weather … that coupled with El Niño … [and] the positive Indian Ocean dipole, has given us every single climate driver going in the direction of heat and dryness across Western Australia,” McDowell says.
‘More extremes’ with climate change
The state’s fire and emergency services commissioner, Darren Klemm, says WA has experienced a 38% rise in bushfires compared with this time last year. His personnel attended 2,706 fires so far, many in the metropolitan area.
In late November, a blaze in Wanneroo on Perth’s north-eastern outskirts destroyed 18 homes after a pine forest caught fire.
Earlier this month, BoM released its January drought summary. It found that soil moisture was very much below average for large areas of WA and in parts of the Northern Territory and South Australia.
“The extent of areas with rainfall deficiencies, including those with record-low rainfall, expanded in Western Australia, particularly in the Pilbara and Gascoyne districts, but generally eased in eastern Australia,” the summary said.
In contrast, much of Victoria and large parts of NT experienced rainfall in the highest 10% since 1900. Victoria received double its average rainfall. January records were smashed in the state’s North Central district and the NT’s Gregory district.
The WAFarmers president, John Hassell, says the state’s extreme dry has meant some farmers were not able to plant crops this year.
“That was quite devastating for a major part of the north-eastern wheatbelt … It is reflected in the total tonnages that we are delivering – we are back down to 12m tonnes from 25m last year.” The average is 16m tonnes.
Lingard says the usual climate drivers occurring at once are most likely related to climate change and will happen more as the climate warms.
“With climate change, we expect to see more extremes in weather, both good and bad. We will see more extreme hot days, more extreme dry days, we will see more extreme flooding events and stronger cyclones.”
When will this end?
McDowell says it looks like it will continue to be warm in the west.
“I’m sorry to say it, but there is a very strong signal still of above-minimum and maximum temperatures [for the west coast] all the way out to June.”
The bureau says Sam should return to neutral in the next two weeks. Long-range forecaster Masoud Edraki says the above-average east coast rainfall is not likely to continue.
“There is no clear signal for March and April that rainfall will be above or below average for most of the east coast.
“North-east Queensland and parts of the Northern Territory are likely to be drier than average from March to May,” he says.
