Relief for Australia's drought-hit regions could be a long way off, with climate influences in the Pacific and Indian oceans tilting towards drier conditions and a "monster" El Nino a possibility by year's end.
Climate scientists said the conditions in the Pacific were particularly concerning given an unusual build-up of equatorial heat below the surface that could provide the fuel for a significant El Nino.
Sea surface warmth
El Ninos are marked by unusually warm sea surface temperatures that typically result in rainfall patterns shifting eastwards away from south-east Asia and the Australian continent.
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| Source: NOAA |
If such an event transpires, the Great Barrier Reef would face another bout of mass coral bleaching while the drought gripping southern and eastern Australia could intensify.
Agus Santoso, a senior scientist at the ARC Centre of Excellence for Climate Extremes, said there were two likely outcomes from the developments in the Pacific.
"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.
What lies beneath
Scientists have identified particularly warm waters just below the surface in a section of the El Nino 3.4 region. This pool of warmth could provide the fuel for a powerful El Nino re-forming by the end of 2019.
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| Source: NOAA |
'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.
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.
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| Farmers have been already been hit hard by drought - with the Bureau of Meteorology saying rainfall totals in Australia in 2018 were the lowest since 2005. Credit: Joe Armao |
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