The famed and feared El Niño climate system is often linked to hotter and drier springs and summers for Australia, but the torrential and damaging storms and floods on the east coast in recent weeks have been anything but dry.
So what’s happened to the El Niño that experts warned earlier could mean warmer and drier weather and increased bushfire risk for much of the country?
The Bureau of Meteorology declared the El Niño in September, three months after US weather agency NOAA and two months after the UN’s World Meteorological Agency.
One bureau climate expert noted that this El Niño was developing when global ocean surface temperatures had been at record high levels for months, and that the changing climate was making forecasting more challenging.
What is El Niño?
El Niño is the phase of the El Niño Southern Oscillation (Enso) – the shifting of warm waters in the tropical Pacific – that usually delivers warmer and drier conditions for many parts of Australia. In the La Niña phase of Enso, the opposite is true.
But Dr Andrew King, a climate scientist at the University of Melbourne, says the link between Enso and how it influences rainfall is much stronger in La Niña cycles than it is during El Niño.
“El Niño events are typically a bit drier than average, mainly in spring,” he said. “But you only need a few weather systems coming through and you can get wetter than usual. People are forgetting how dry early spring was.”
According to bureau data, the spring months of September, October and November were only slightly drier than average across most of New South Wales, Northern Territory, Queensland, Victoria and Western Australia and particularly dry in South Australia and Tasmania. September and October were particularly dry.
Those months coincided with a shift in the Indian Ocean that took warmer waters away from the north-west of the continent and that can also mean less rain.
This phenomenon – known as the positive phase of the Indian Ocean Dipole – is expected to fade in the coming weeks.
But the first month of summer, December, was generally average or slightly wetter than usual in parts of NSW, QLD, SA and Victoria and drier than usual for the NT, Tasmania and WA.
What about temperatures?
Globally, 2023 was the warmest year on record. Experts say El Niños tend to have a greater influence on temperatures the year after they are declared.
For Australia, the El Niño has coincided with above average temperatures almost everywhere in spring.
Overall, 2023 tied with 2016 as Australia’s eighth warmest year on record, but spring of 2023 was the fifth warmest on record.
Many areas were under heatwave conditions throughout December, including Sydney and parts of Queensland. The middle of December saw bushfires in New South Wales.
For December, bureau data suggests almost all states were on average at least 1C warmer than usual, with South Australia only slightly warmer than normal.
Why has it not been dry?
Last year, a study led by the CSIRO of Enso cycles going back to 1950 suggested that El Niños don’t necessarily guarantee dry periods.
“But they do heavily slant your chances away from it being very wet,” said Dr James Risbey, a climate scientist and co-author of the study.
The study also suggested that in coastal areas east of the Great Dividing Range from south-east Queensland to Gippsland in Victoria, the link between drier periods and El Niños is quite weak. The link was stronger in inland areas, including the Murray-Darling Basin.
Risbey says WA tended not to be influenced much by ENSO.
Dr Andrea Taschetto, an expert on Enso at the University of New South Wales, says each El Niño is different and that they are just one of many factors that influence Australia’s weather.
She points to another important influence that is likely influencing rainfall right now: the position of the east-to-west winds to the south of Australia.
If those winds are closer to Antarctica in summer – which they currently are in what’s known as the positive phase of the Southern Annular Mode – this tends to increase winds blowing from the Tasman Sea on to the eastern seaboard, translating to a higher chance of rain for large parts of south-east Australia, including south-east Queensland.
Taschetto says the area of the Tasman Sea that supplies moisture to the east of the country has been unusually warm through December.
She says: “If you combine [those warmer sea temperatures] with the positive SAM, then you have a perfect combination for increased rain over the eastern part of Australia.”
Before Christmas, the bureau had suggested the unusually warm waters in the Tasman Sea “may also be contributing to a chance of above median summer rainfall over parts of Australia”.
Extremes hard to predict
In late November, the bureau’s long-term forecast suggested the chances were about even of there being above or below average rainfall in December across much of the country. The bureau’s model suggested northern parts would probably be dry.
Cyclone Jasper dumped huge amounts of rain in far north Queensland last month, and the south-east of the state and northern NSW have been hit by violent storms and flash flooding since Christmas.
Risbey says the weather systems that have delivered storms have been unusually “well organised” and persistent over large areas in recent weeks, “but it’s too early to say why”.
Andrew King says in any case, extreme events are much harder for forecasters to predict more than a few days in advance.
The bureau’s long-term weather outlooks that stretch out over three months can’t account for storms and cyclones so far ahead.
“Seasonal outlooks will always struggle with those big systems,” King says. “In summer the seasonal outlooks are less skilful because rain is mostly falling from storm systems.”
The Bureau of Meteorology declined to be interviewed.
What is the role of climate change?
According to the most recent State of the Climate report, there is evidence that storms in Australia are becoming more intense – that is, delivering more rain. This trend is expected to increase as the planet keeps heating up.
Dr Jaci Brown, of CSIRO’s Climate Science Centre, says one major concern from global heating was the effect of “compounding events” on societies – the times when multiple extremes happen almost simultaneously.
“We’ve had heatwaves, bushfires and floods happening together [in recent weeks],” she says. “Even if these events are not due to climate change, it is a glimpse of what we should be preparing for in the future.”
