The US and Australia's chief forecasting agencies are projecting vastly different odds on the development of an El Niño climate pattern this year.
In their latest climate update, Australia's Bureau of Meteorology (BOM) has remained firm on their projection that there is a 50 per cent chance of the major climate driver, linked to warmer, drier weather, developing this winter or spring.
Meanwhile, US forecast agency National Oceanic and Atmospheric Administration (NOAA) has tipped El Niño an almost foregone conclusion now at a 90 per cent chance of developing this year.
The significantly different figures can seem perplexing. After all, all agencies are looking at the same part of the Pacific Ocean.
So why is there a difference, and who is right?
Australia monitors atmospheric changes
At its core, the reason for the differing odds is fairly simple – the US and Australia base their probability on different criteria, and the BOM's is stricter.
To understand the difference, you must first understand how El Niño works.
El Niño is known as a "coupled" phenomenon, where conditions in the ocean interact with the atmosphere and become self re-enforcing.
During the event, waters in the central and eastern part of the tropical Pacific become unusually warm, weakening the trade winds that blow from east to west around the globe.
The weaker trade winds keep the pool of warm water confined to the eastern Pacific, and the stronger pool of warm water in the eastern Pacific weakens the trade winds.
As a result, the warm water encourages the development of cloud and tropical rainfall closer to South America, while Australia misses out.
Atmospheric scientist Kimberley Reid, from Monash University, said this interaction between the ocean and the atmosphere was an important distinction between the NOAA and BOM's criteria.
Dr Reid said when determining the probability of an El Niño, NOAA was placing emphasis primarily on sea surface temperatures.
Whereas the BOM requires both the ocean and the atmosphere to be showing signs of change before they upgrade the likelihood to an El Niño "alert", which means a 70 per cent chance.
The BOM also requires sea surface temperatures to be warmer than NOAA does.
More specifically, NOAA's outlook is based on the probability that sea surface temperatures at NINO3.4 are more that 0.5 degrees Celsius warmer than normal for five consecutive, overlapping three-month periods. For example, May-June, June-August and July-September.
But, for the BOM to move from El Niño "watch" to El Niño "alert", three of the following four criteria must be met:
When it comes to actually declaring an El Niño underway, the criteria from both agencies become much closer, with both requiring observed changes in the atmosphere, although BOM still requires ocean temperatures to be warmer than NOAA.
In a statement, the BOM said the current conditions plus forecasts were "close" to those required for shifting from a "watch" to "alert".
But it said atmospheric conditions were keeping the odds at 50 per cent.
Careful balance between raising false alarm, and early warning
Dr Reid said the BOM's stricter criteria were about being confident there were going to be the impacts from El Niño before the alarm was sounded.
"People kind of know that El Niño equals hot and dry in Australia, farmers know that and they're constantly watching," she said.
"But if the atmospheric conditions aren't pointing to El Niño or, as is the case right now, we've got warmer ocean temperatures [on both sides] of the Pacific, then we're not necessarily going to get that hot and dry outcome.
"So the Bureau of Meteorology sort of has this stricter criteria because they don't want to be calling it an El Niño if we're not going to be getting these hot and dry conditions."
She said, in contrast, the US and South America were not as influenced by conditions on the western side of the Pacific, near Australia, so this was less of a consideration.
In a statement, the BOM said the different metrics between US, Australia and also Japan were tailored to their services around the impact of El Niño and La Niña.
"The bureau considers its service best suited for Australia," it said.
Dr Reid said there were benefits to the US criteria, too – namely early warning.
"I think there's lots of benefits for people being told that a potential El Niño is coming, especially, in Australia for example, when it's been so wet the last few years," she said.
"Vegetation has blossomed, and so if it becomes really hot and dry that means the next big fire season will be potentially bad.
"So early warning means we can prepare for that."
Dr Reid said the flip to that was losing trust should an event not eventuate.
"You don't want to tell everyone it's going to be hot and dry and then it doesn't happen, because you lose the public's confidence," she said.
"I don't really have an opinion on which is better, necessarily, because I can see the benefits of both."
There have been instances where the models were suggesting prolonged warming in the ocean, but an El Niño or La Niña event didn't eventuate.
This happened in 2014, when a strong El Niño was projected but did not occur that year, instead kicking in the following year.
