Researchers at the Bureau of Meteorology (BOM) are calling for the agency to change the way El Niño and La Niña are classified, to more accurately reflect how the major climate drivers behave.

While an El Niño has been declared, the process this year was long and at times confusing.

Sea surface temperatures in the Pacific rapidly rose to El Niño thresholds in June, prompting the bureau to move to “El Niño Alert”.

But it wasn’t until three months later, in September, that the event was officially declared underway.

International weather agencies in the US and Japan, as well as the World Meteorological Organisation, had already said it was underway months earlier.

BOM climate researcher Matthew Wheeler, who co-authored a recent report proposing changes to the index, said this year was the “perfect example” of why the system was no longer the best fit.

“The risk is confusion, and we’re trying to have less confusion,” he said.

“Global oceans are warming and it means that our indices for variability, like El Niño and La Niña, have to account for that.”


Flaws in the system

El Niño and La Niña are the world’s most consequential climate drivers, influencing the weather patterns of 60 per cent of the globe.

The event, which takes place over the Pacific Ocean, is characterised by a complex interaction between the ocean and the atmosphere which causes the two systems to effectively team up to start rearranging where rain falls.

The “coupling” between the two systems is a crucial part of the event, as it leads to the event being locked in for a prolonged period of time.

To determine whether there is an El Niño or a La Niña, the weather bureau looks at several different indices relating to the atmosphere and the ocean.

For the ocean component, the bureau compares temperatures in a certain part of the Pacific with the 1961-1990 average.

In the case of El Niño, the anomaly needs to be at least 0.8 degrees Celsius above the baseline, and 0.8C below the average for La Niña.

Dr Wheeler said the ocean and atmospheric indices tipped over the thresholds “about the same time”.

Climate researchers say rising ocean temperatures have impacted weather patterns.(ABC News: Chris Lewis)

But he said as the ocean warmed due to increasing greenhouse gas concentrations, it had become increasingly easy for the ocean to tip over its El Niño threshold, effectively skewing the view.

“If there’s a positive trend [in ocean temperatures], it becomes easier to fall into an El Niño category and harder to fall into the La Niña category,” he said.

According to Dr Wheeler’s report, it could reach a point where the part of the Pacific Ocean where El Niño measurements are taken in an “almost permanent El Nino state”, while reaching  La Niña thresholds “may not be possible”.

“Of course, this is an extreme example, but it highlights that a failure to directly address climate change in definitions will lead to an unworkable outcome,” the report said.

Proposed changes

To address the emerging issue, Dr Wheeler and other colleagues authored a report recommending the bureau switch to a different ocean index known as the Relative Niño 3.4 index.

The index doesn’t just take into account how hot the temperatures are in the eastern Pacific, but how hot they are compared to the anomalies being experienced by the entire planet’s tropical oceans.

The comparison to other parts of the ocean was considered important because the contrast in sea surface temperature across the rest of the tropics helped kick off El Niño and La Niña events.

Sea surface temperatures were unusually warm on the western side of the Pacific in 2023.(Supplied: NOAA)

When all of the oceans are warm everywhere, as was the case this year, temperatures in the eastern Pacific must become even warmer to trigger a change in the atmosphere.

In the case of this year’s El Niño, Dr Wheeler said it would have changed the way things played out considerably.

“This year was a good example, because we passed the 0.8C threshold back in June … but even though that had happened, the atmosphere hadn’t responded,” he said. 

“And with this new index, the relative one, the sea surface temperature didn’t pass the 0.8C mark until about September which is when we finally saw the atmosphere responding.”

“So 2023 is a very good case study of what would happen in real world situations.”

While it is only a recombination at this stage, the bureau’s head of climate monitoring, Karl Braganza, said it was likely the bureau would make the switch over coming years. 

While the report only made recommendations for the bureau’s operations, Dr Wheeler said other international agencies such as NOAA, in the US, were also considering making the change to the relative index.

Dr Wheeler said that would hopefully go some way to addressing the confusion created this year, when the bureau was at odds with the rest of the world in making its El Niño declaration.

Outlooks more important

What were once terms known only to meteorologists and weather nuts, El Niño and La Niña have increasingly become part of the Australian vernacular.

Dr Wheeler said the interest from the public about how and why the weather behaved the way it did was great to see. 

“I think it’s nice that the public is interested in the science behind it,” he said.

However, Dr Wheeler and Dr Braganza reiterated the bureau’s long-range seasonal outlooks were a better reflection of the likely conditions to come, rather than the declaration of an individual El Niño or La Niña event.

Dr Braganza said that was especially true in a changing climate where the behaviour of El Niño and La Niña events carried a lot of uncertainty.

He said the outlooks, on the other hand, encapsulated all the physics of the oceans and the atmosphere, including the climate drivers such as El Niño.