La Niña to El Niño: what it means for summer

After the warmest September in history, Australia headed into its first El Niño summer since 2015-16. So what does this mean and what are the risks? Tim McGrath reports

Spring is a beautiful time of the year. The flowers are blooming, the short, cold days of winter are disappearing into the rearview, and the sun is starting to show itself for the first time in months. 

For many, AFL grand final day exemplifies this change as we officially say farewell to winter. This year, however, it appeared to signify summer coming early.

Dr Sarah Perkins-Kirkpatrick, a senior lecturer at the University of New South Wales with a particular research focus on heatwaves, said that while the warmer weather could be pleasant at face value, this enjoyment may be distracting the general public from what is actually happening.

"Part of me like, I like warm weather, but I do agree that people can be led into a false sense of security when we have warmer conditions," she said. "We all get excited and pull out our summer clothes, but really that isn’t normal that time of year."

But what's driving this unnatural weather? If you answered ‘climate change’, you're definitely not wrong. 

However, the weather phenomenon known as El Niño (more on that later) is the central driving factor behind hotter and drier than normal temperatures. While summer is Australia’s favourite season, and many of us may be responding positively to these weather patterns (for now), it’s hard not to imagine what our actual summer may look like. If September is anything to go by, it could be unlike anything we’ve seen before.

The temperature reached 29.7 degrees Celsius in Melbourne on Saturday September 30, just 1.7 degrees shy of the recorded maximum for a September day. This came off the back of a record eight consecutive days reaching maximums above 20 degrees earlier in the month. While Melbourne weather is notoriously fickle, this unusually warm weather followed a global trend which saw September 2023 enter the record books as the warmest ever on record - and the largest anomaly of any month in over 150 years. 

In most countries the temperature is now beginning to chill. But for countries like Australia in the Southern Hemisphere, the temperature is only rising. 

"The data from the BoM (Bureau of Metereology) confirms what we already suspected," said Dr Simon Bradshaw, Research Director at the Climate Council of Australia. "Heat records are being broken with every passing week, and unfortunately they are just going to keep on tumbling this Spring and summer." 

El Niño (which means ‘little boy’ in Spanish), is declared when an upper-threshold is exceeded for certain factors including ocean surface temperatures, while La Niña (‘little girl’) is declared when lower thresholds are met. These two climate patterns make up the El Niño-Southern Oscillation (ENSO) cycle. 

"It’s a coupling between the ocean and the atmosphere," said Dr Perkins-Kirkpatrick. "The ocean develops changes in surface temperatures, and then the atmosphere has to respond. It’s a two way interaction."

The World Meteorological Organisation (WMO) declared the onset of an El Niño in July, while Australia’s BoM announced El Niño in September.  

This comes off the back of a rare three year La Niña which led to record rainfall, making the change to El Niño potentially jarring in comparison. 

El Niño weather events are closely linked with increased bushfire risk due to the likelihood of hotter and drier conditions. Dr Perkins-Kirkpatrick said that more heatwaves amplifies drying, causing droughts and increased wildfire risk.

As we are only a few years removed from the devastating Black Summer bushfires of 2019-2020, Dr Bradshaw said that this may understandably worry Australians.

"I'm sure many Australians are currently being reminded of the unseasonably hot and dry conditions that led up to Black Summer and are pretty anxious," said Dr Bradshaw. "Fire experts have indicated we are unlikely to see a bushfire catastrophe of a Black Summer-scale this summer, which is somewhat reassuring. But it will undoubtedly be a tough summer ahead, with potentially deadly heat and grass fires to contend with.”

Dr Perkins-Kirkatrick agreed that the conditions, while concerning, could be far worse. 

Dr Sarah Perkins-Kirkpatrick is an award-winning climate scientist and expert in heatwave research.

"This year it’s different because we’ve had three La Niñas, a lot of stuff is still reasonably wet,” she said. "[Before Black Summer] we certainly had that amplification of drying and heat in spring. The positive Indian Ocean Dipole,a three year drought, and then climate change on top of that were the main contributors to Black Summer."

While the lingering effects of three years of La Niñas can be viewed as somewhat of a saving grace, it has also provided the conditions for high vegetative growth. This, combined with the dry conditions of El Niño, have led to widespread bushfire warnings for large areas of eastern Australia, including areas that may have grown complacent in recent safer years. 

"This is the start of the drying cycle," said Dr Perkins-Kirkpatrick. "If we don’t have higher risk of bushfires by the end of summer, we’re certainly in a position to have that next summer."

She also said that, even if we don’t experience severe fires, heatwaves are associated with severe health risks affecting both mental and physical health, such as heat stroke. These are of particular concern among high risk or compromised groups, such as pregnant or elderly people.

The good news is El Niño climate patterns typically dissipate in autumn, and it is rare for El Niño to be declared again the following year. That doesn’t mean, however, that another La Niña pattern will occur next summer to counteract the drying effects of this El Niño. 

"It’s not like we have a La Niña, a break, an El Niño, break, it doesn’t oscillate so periodically, " said Dr Perkins-Kirkpatrick.

"Sometimes we don't have an El Niño or a La Niña, we have what we call neutral summer."

With the warming effects of climate change continuing to worsen, scientists warn that neutral summers, and even La Niñas, in the future will present increasing risks.

"An El Niño in the future is going to be a lot warmer than El Niño now, which is a lot warmer than El Niño in the 1800s," said Dr Perkins-Kirkpatrick. "We’ve actually reached a stage where today’s La Niñas are warmer than El Niños in the early 1900s."

Both Dr Perkins-Kirkpatrick and Dr Bradshaw agree that working to slow down climate change is the only thing that will counteract the devastating effects of climate variations such as El Niño and La Niña.

“If we don't hurry up and put solutions in place now, these types of summers will eventually become the norm. We may look back on this one as mild,” said Dr Bradshaw. “The only way to turn down the heat, is to turn off fossil fuels.”