"We could have an El Nino fully formed by the end of May and then it could dissipate," Dr Santoso said.

"The other is that by May it’s already formed and it still keeps building up... and by the end of the year we could have a monster El Nino."

During El Ninos, the normal easterly winds blowing along the equator slow and even reverse. Rainfall patterns tend to shift eastwards away from south-east Asia and Australia, setting up conditions favourable for below-average rainfall and bushfires.

'Very exciting'- but not in a good way

The prospect of a big El Nino later this year was raised at an international conference of climate scientists in Chile earlier this month.

They considered parallel years, such as 2014 when a near-El Nino was reached before conditions revived a year later, creating one of the three most powerful such events in the past half century.

"There is more heat now below the surface waiting to be tapped than there was in early 2015," said Michael McPhaden, a senior scientist with US National Atmospheric and Oceanic Administration who attended the Chilean event.

"If westerly wind bursts of sufficient amplitude, duration and zonal extent develop along the equator in the next couple of months, 2019-20 could be very exciting," he said.

The scientists stress that a "predictability barrier" that falls during the southern hemisphere autumn means model reliability is lower than at other times of the year.

"While it's not a slam dunk that El Nino is going to persist, I think that the odds have certainly increased over one to two months ago," Phil Klotzbach, a research scientist at Colorado State University, said. " We've had a big build up of heat in the eastern and central Pacific."

'Not a great starting place'

Andrew Watkins, head of long-range forecasting at the Bureau of Meteorology, said "all models are suggesting an El Nino the over next month", with several having it peaking in autumn and dropping off.

The bureau has a higher threshold than NOAA - which has already declared an El Nino - but may announce one at its update next week.

Dr Watkins said "it's too early to say there'll be an extreme event", but he noted other influences - especially from the Indian Ocean - may also favour below-average rainfall during the spring for southern Australia.

Loading

The so-called Indian Ocean Dipole - which gauges relative warmth between the east and western Indian Ocean - is forecast to become positive by mid-winter, the bureau said.

“The odds are [favouring] on the drivers that create dry conditions for eastern and southern Australia," Dr Watkins said.

“We’re not starting from a great place," Dr Watkins said, noting inland reservoirs are dropping and near-term stream-flow forecasts are for below-average flows for much of the country.

Cai Wenju, a senior CSIRO scientist who has published widely on the El Nino Southern Oscillation climate pattern, said the chance of El Nino returning is high.

A return of westerlies by about June to halt the easterly tradewinds “could spark the fire and there’s a lot of fuel", Dr Cai said.

“If it’s similar to 2015, the impact this time will be big," he said. "We are already so dry.”

Climate change and big events

Dr Santoso's research, including a paper published late last year, has found the frequency of big El Ninos will increase with climate change.

That result is "quite concerning", particularly for ecosystems sensitive to heat spikes such as coral reefs that suffered mass bleaching during the 2015-16 big El Nino.

"If we get one or two bleaching events, [the Great Barrier Reef] can recover, but if we keep having these events coming up then maybe the corals are not going to be able to adapt," Dr Santoso said.

During El Ninos, the Pacific Ocean takes less heat from the atmosphere and even gives some up, giving global surface temperatures a bump up.

The trialling years of big El Ninos, especially 1998 and 2016 - the current holder of the world's hottest year on record - are particularly warm.

An event later this year would likely see temperatures next year "spike up, and that's not very helpful for global warming", Dr Santoso said.