Explaining the ‘unusually extreme’ rain and weather that caused Sydney’s fourth major flood in two years

·6 min read
<span>Photograph: Loren Elliott/Reuters</span>
Photograph: Loren Elliott/Reuters

The fourth major flood to hit greater Sydney in less than two years had no single cause. It was triggered by a confluence of events that combined over the weekend to dump as much rain in some areas as Melbourne or London typically receive in a year.

The extent of the flooding was exacerbated by circumstances on the ground – a saturated landscape that was unable to absorb much more water after the wettest start to the year on record, and dams being close to full and unable to hold the downpour.

Global heating may have exacerbated the intensity of the storm, though scientists say the jury is out on the role it played. One notable point: the waters off the Illawarra on Sunday were more than 2C hotter than average for this time of year – and warmer oceans tend to fuel heavier storms.

Related: Australia is woefully unprepared for this climate reality of consecutive disasters | Greg Mullins

So what happened?

The rain wasn’t a surprise – a deluge was forecast mid-last week – but the amount of water dropped on the city on Saturday was greater than expected.

Ben Domensino, a meteorologist with Weatherzone, said it was caused by a tropical moisture that came down from the country’s north and fed into a low pressure trough off the New South Wales coast. The low pressure trough had an east coast low – an intense low pressure system, sometimes called an extratropical cyclone – embedded within it.

It resulted in more intense rain across Friday night and Saturday than some weather forecasting models had predicted, particularly for outer suburban and regional areas south of Sydney. The ferocity of the storm had subsided by Sunday morning – rain was lighter than models had suggested on Sunday and Monday – but by then much of the damage had been set in motion.

Parts of the Illawarra, around Wollongong, received more than 700mm over three days. To put this into context, Melbourne and Canberra each average less than 650mm a year.

In Sydney’s west, Warragamba dam received 244mm over three days to 9am Monday, mostly on Saturday. The intense early burst of rain led to it spilling from about 2am Sunday when forecasts had suggested that would not happen until that afternoon.

Scientists said this was not necessarily a failure of forecasting. East coast lows always cause heavy rain and damaging winds, but the precise timing of their impact is difficult to forecast.

Kimberley Reid, an atmospheric scientist at Monash University, said the weather models showed about five days ahead “that something big was going to happen” over eastern Australia, and it was not unusual for forecasts to be a few hours out.

Domensino said this east coast low was likely to have carried more rain than most because it had more water to draw on. He said the ocean temperatures off the coast of the Illawarra were between 2C and 3C hotter than the long-term average.

Like almost everywhere else, the waters around much of Australia have been getting warmer due to global heating driven by the burning of fossil fuels. Scientists have established the atmosphere can hold roughly 7% more moisture for every additional degree of warming.

Domensino said it meant the east coast low on the weekend “had a lot more water to tap into, which is partly why we saw so much rain”. In short, heat may have amplified the impact.

Toni Guest works to salvage belongings from her flooded home in South Windsor in Sydney on Tuesday.
Toni Guest works to salvage belongings from her flooded home in South Windsor in Sydney on Tuesday. Photograph: Loren Elliott/Reuters

Untangling the climate influence

Dr Andrew King, a climate scientist at the University of Melbourne, agreed to a point. He said published science had demonstrated that the climate crisis intensified short duration rain events, but it was harder to assess what part it played in the type of multi-day deluge that hit NSW.

“It is quite hard to tell if there is climate change influence – there may well be, but it is hard to untangle the noise,” King said.

King said the backdrop to the current flooding was two La Niña events that had inflated rainfall over the past two years. La Niña involves strong trade winds blowing west across the Pacific, pushing warm surface water towards Asia and typically delivering increased rainfall across much of Australia, cooler daytime temperatures south of the tropics and warmer night-time temperatures in the country’s north.

The Bureau of Meteorology last month declared the 2021-22 La Niña over, but it warned winter was likely to be wetter than usual and there was a 50% chance the atmospheric and ocean pattern would return for a third straight summer.

Related: See the NSW floods’ record-breaking rainfall in two charts and a Sydney flood map

At the same time, the bureau said another influence on Australia’s rainfall, this time in the Indian Ocean, has been heading in a direction that tends to deliver more rain.

The Indian Ocean Dipole has been close to hitting a negative phase for weeks. When westerly winds push warmer water closer to Australia’s north-west, this makes more moisture available for rainfall in the winter and spring.

King said the current flood was still fuelled by colder waters in the tropical central and eastern Pacific and warmer water in the western Pacific. It had increased the likelihood of a low pressure system and meant there was “a bit more energy in the system around Australia”.

He said the current floods were breaking records in some places and “definitely unusually extreme”. But he also said Sydney had a history of heavy rain and the underlying weather systems were not unprecedented.

A kayaker paddles through a flooded residential area in South Windsor on Tuesday following heavy rain in Sydney.
A kayaker paddles through a flooded residential area in South Windsor on Tuesday following heavy rain in Sydney. Photograph: Loren Elliott/Reuters

At saturation point

The rain that fell during the back-to-back La Niña events has saturated the ground. It’s a bit like a sponge on a kitchen bench – once saturated it can’t hold any more water. What the land can’t absorb flows straight into river catchments.

King said there had been so much rain in Sydney this year that the landscape now had a reduced capacity to absorb moisture. “The earlier events and the latest flooding aren’t independent of each other,” he said. “The ground is just saturated and the dams are full so if you get more rain it floods very easily. It doesn’t go into the ground as it normally would.”

It raises questions about how well prepared the city was for another deluge. Stuart Khan, a professor of civil and environmental engineering at the University of NSW, is among those who argue Warragamba dam, in particular, should be kept at a lower capacity to reduce the impact of flooding when it comes.

He said rather than allow the dam to be 97% full, as it was before the flood, it would make sense to keep it about 60% full through strategic release of water when it was safe to do so. This change to using the dam for flood mitigation as well as water supply would be possible if Sydney doubled its capacity of desalinated water through a new plant in the Illawarra and boosted its use of recycled water, he said.

King said it was clear that greater planning for future extreme rain events was needed.

“We do know that areas of eastern Australia do suffer extreme floods from time to time. We’ve seen it over the last couple of years sometimes and also going back further,” he said. “We do need to be more resilient to that.”

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