Extreme weather has buffeted Australia’s east coast over the past few days, with six people dying in floods in Queensland and New South Wales.
Records show that Sydney has experienced its wettest summer in 30 years, and the most humid in a decade.
The current La Niña weather pattern is typically associated with increased rainfall across northern and eastern Australia. But in NSW, La Niña isn’t the only reason for the continuing heavy rainfall – so why has summer been so dreary and muggy?
Wettest in decades
As of 8pm on Friday, Sydney had recorded 570mm of rain over December, January and February, according to Weatherzone data.
It is the highest seasonal total in three decades, since the summer of 1991–92, which had a total rainfall of 665mm.
Ben Domensino, a meteorologist at Weatherzone, says the monthly average for February is typically in the vicinity of 120mm. “We’ve had more than a month’s rain just in the last 48 hours,” he said on Thursday.
In 164 years of records, Domensino said there were only 21 summers when Sydney had recorded more than half a metre of rain.
Preliminary data showed the city was also experiencing its most humid summer since 2011-12, Domensino said.
The finding was based on the differences between two measures – “dry-bulb temperature” (the true air temperature) and “wet-bulb temperature” (which is measured by a thermometer covered in a water-moistened cloth).
Relative humidity – the amount of moisture in the air at a given temperature – has also been higher.
The average relative humidity at 9am in Sydney in February this year has been 82%, compared with a historical average of 74%. The same reading at 3pm has been 68%, compared with a historical average of 64%.
“We are talking five to 10 percentage points above average,” Domensino said, emphasising that relative humidity does not take temperature into account.
In cooler air, the same amount of water vapour results in a higher relative humidity than at warmer temperatures.
Warmer sea temperatures
The wet and humid summer is partly attributable to the current La Niña.
But Domensino said every La Niña is different in terms of how it impacts Australia.
“Even weak La Niñas can cause very wet summers in Australia and strong La Niñas don’t necessarily correlate with rainfall,” he said.
“We have had stronger La Niñas in the past, including the back-to-back La Niñas that occurred in 2010 to 2012.
“The rainfall in Sydney from this summer is heavier than we saw during the 2010 to 2012 La Niña event.”
For Sydney and other parts of NSW, there’s also a separate phenomenon at play: unusually warm sea surface temperatures in the Tasman Sea, off Australia’s east coast.
“When you’ve got warm surface temperatures it causes more evaporation – it puts more moisture in the atmosphere and then that fuels rainfall,” Domensino said.
How long will the wet weather last?
For NSW, the wet conditions are likely to continue into autumn, as the marine heatwave in the Tasman Sea persists. “The models are suggesting that we are more likely than not going to see above-average rain in March for eastern NSW. In April, that signal is still there as well,” Domensino said.
He added that in Queensland, where sea surface temperatures off the central Queensland coast and over the southern Coral Sea have been closer to average, La Niña was likely to be the dominant climate driver.
According to the Bureau of Meteorology’s latest climate outlook, released on Thursday: “March to May rainfall is likely to be above median for large areas of Australia.”
Is this the new normal?
Dr Nina Ridder, a research associate at the University of NSW’s Climate Change Research Centre, said “already with climate change, we see more heavy rainfall events, and the rainfall events themselves become more intense”.
The centre’s climate models show global warming will lead to more heavy rainfall events, even in areas that are on average drier than normal.
“That has a lot to do with the ability of the atmosphere to hold more water when it is warmer,” Ridder said. “For each degree that the atmosphere warms, it can hold about 7% more water vapour.”
The extent of flooding this summer has been “a lot higher than what we would have expected”, she added. “That is because we already had a La Niña last year … all the soils [are saturated] and water reservoirs are full, basically.”
