El Nino is generally understood to mean blazing hot, uncomfortably dry conditions. So why have Australia’s eastern states suffered their worst inundation since the 2022 floods?

By Bernice Han

El Nino usually brings abnormally hot and dry weather. It can lead to reduced rainfall and intense heatwaves, worsen the severity of bushfires and contribute to drought conditions.

But we haven’t really seen that since the Bureau of Meteorology (BOM) finally declared in September last year that the climate event was under way, following weeks of deliberation after its counterparts in the northern hemisphere had already made the call.

Instead of sweltering heat, daytime temperatures from Brisbane to Sydney and Melbourne have often felt more like those of a Mediterranean winter, ranging from mild to even chilly on occasions – not the sort of weather Australians embrace with enthusiasm come December.

Parts of the eastern states have been hit with severe storms and downpours since November, leading to significant floods in Queensland, New South Wales and Victoria around Christmas. For insurers, the wet summer is proving rather costly, with insured losses topping $500 million to date.

The Insurance Council of Australia (ICA) declared the storms a catastrophe on December 29, two days after rating them a “significant event”. The catastrophe declaration meant an escalation in insurers’ response to policyholders and reflected the worsening situation on the ground.

The Christmas catastrophe declaration was soon extended to include the storms and flooding that struck the three eastern states in the first week of January. Insurers face a bill of at least $541 million from more than 70,000 claims lodged so far, according to ICA, and the figures are expected to rise.

The aftermath of cyclones Jasper and Kirrily has also resulted in significant flooding across some regions, with ex-tropical cyclone Jasper resulting in more than 8000 claims at a cost of $202 million so far.

All this un-El-Nino-like weather has puzzled insurers and everyone else who had been bracing for scorching conditions after the BOM declared the climate event was under way.

Australia last experienced an El Nino in 2015-16, and it was a “very strong” one. The BOM said it contributed to an early start to the southern fire season, a much drier than average northern wet season and prolonged heatwaves across the country during the autumn of 2016.

“There is still a fair amount of uncertainty as to what an El Nino will bring and the key point here is no two El Ninos are the same,” Aon Climate Risk Advisory Lead Tom Mortlock tells Insurance News. “So although we can say in general terms that El Nino tends to bring warmer than normal and drier than normal conditions, especially to the eastern states … that doesn’t necessarily hold true for every single event. This El Nino is doing a pretty good La Nina impression at the moment.”

Uncertain situation: Aon’s Tom Mortlock says no two El Ninos are the same

For this El Nino, the chances of extreme hot and dry weather were higher because another climate driver, the Indian Ocean Dipole, was in play.

The day the BOM declared El Nino – which means the boy in Spanish – had arrived, it also said the Indian Ocean Dipole was in a positive phase, with cool water to the north-east of Australia and warm water near the Horn of Africa. A positive Indian Ocean Dipole usually contributes to dry weather in eastern Australia.

Warmer and drier conditions, the BOM declaration of September 19 stated, would be “more likely over spring and summer for parts of Australia, under the influence of these two climate drivers”.

The positive Indian Ocean Dipole was a short one, reaching its peak in December, but the country remains in an El Nino state, climate modelling shows.

Mark Howden, Director of the Institute for Climate, Energy & Disaster Solutions at the Australian National University, says it has been a weak El Nino because warm water in the western Pacific, left over from the three-year rain-inducing La Nina, did not dissipate quickly, reducing the temperature gradient across the Pacific.

“This limited the atmospheric response part of an El Nino, which usually results in a positive feedback, strengthening the overall system,” he tells Insurance News.

Another point to remember, climate scientists say, is El Nino does not necessarily translate into abnormally hot and dry conditions. When one is declared, it simply means there’s a higher chance of the conditions occurring over eastern Australia. In the past, Australia has experienced significant rainfall during an El Nino.

And El Nino is not the only climate driver influencing our weather, scientists say. Many other climate systems such as the Indian Ocean Dipole, the East Australian Current, the 40-Day Wave and Southern Ocean circulation systems influence the weather when El Nino is here.

“Normally in an El Nino, we can get anything from very dry to reasonably wet conditions,” Professor Howden says.

“While El Nino tends to bring drier and consequently hotter conditions, it isn’t a black and white thing. It just pushes the probabilities towards those conditions.

“The same goes with La Nina and the other climate drivers, they shift the probabilities. They shift the risk. So it’s not really unprecedented to have the level of rainfall and flooding we have experienced since November in an El Nino year. We have got other things happening in our weather that are linked with the observed level of flooding and wet conditions.”

For example, he says, unusually warm water off the south-east Australian coast has probably boosted rainfall in that region.

Mark Howden

Not unprecedented: ANU’s Mark Howden says floods do happen in El Nino years.
Credit: Salty Dingo

Another contributor to the wet weather has been some “very strong” episodes of the 40-Day Wave, also known as the Madden-Julian Oscillation, Professor Howden says.

The oscillation is an important driver of tropical weather. It can be characterised as an eastward-moving “pulse” of cloud and rainfall near the equator that typically recurs every 30 to 60 days, according to the BOM.

In Australia, a strong Madden-Julian Oscillation raises the chances of above-average rainfall across the north and east.

“In a dry El Nino, the 40-Day Wave is often not very effective in bringing rain,” Professor Howden says. “But in a La Nina or a wet year like we’re experiencing in eastern Australia, those 40-Day Wave events tend to be quite effective. They bring a lot of moisture from the tropics down to south-east Australia. We’ve had a couple of really wet, strong 40-Day Wave events and that’s when we had a lot of heavy rainfall and floods.”

He says the rainy season in northern Australia often starts when the 40-Day Wave hits, and this can bring tropical moisture to the south-east that is often observed as the north-west cloud bands.

At the same time, he says, the Southern Ocean has had strong winds, and such conditions “tend to pull the pressure systems southwards, dragging more of the tropical systems southwards, which again delivers extra rain in eastern Australia”.

Aon’s Dr Mortlock, who is also Adjunct Fellow at the University of New South Wales Climate Change Research Centre, says the weather conversation often overlooks the Pacific Decadal Oscillation – a fluctuation of the Pacific Ocean that can be in El Nino or La Nina states for decades.

PSC

This oscillation can “really impact the strength and persistence of individual El Nino and La Nina events that occur”, he says.

La Nina – Spanish for girl – tends to bring wetter seasons. Australia was in a La Nina state, enduring record floods in 2022, for three years before El Nino developed last year.

Dr Mortlock says the Pacific Decadal Oscillation has been in a La Nina-like state since the mid-1990s, meaning “it may act to weaken the El Nino”.

Kimberley Reid, a postdoctoral research associate in atmospheric sciences at Monash University, says there has been a lot of education in the past five to 10 years about El Ninos and La Ninas, and the public is now more aware of what they mean.

“There’s probably been too much emphasis [on El Nino and La Nina] and perhaps not enough emphasis on the fact there are these other climate drivers as well that influence the weather,” Dr Reid tells Insurance News.

El Nino tends to peak around spring in Australia, and climate modelling suggests it did so when conditions were dry and warm towards the end of last year.

“So the impacts, I think, will be weaker as we move into autumn,” Dr Reid says.