Environment

Summer heat builds across southern Australia as big El Nino starts to break down

Most of NSW will climb into the 40s on Wednesday, sending fire danger ratings to severe in places, with little relief until a southerly buster arrives late the following day.

Wilcannia in the state's west reached 45.3 degrees on Tuesday and may reach 46 on Wednesday, making it again one of the hottest places on Earth. It claimed that title on Tuesday, according to the WeatherUnderground website.

Another scorcher has been predicted for inland NSW.
Another scorcher has been predicted for inland NSW. Photo: Leigh Henningham

Sydney can expect much milder conditions with a top of 28 degrees while inland suburbs will reach 35 degrees. Thursday will be hot again with a top of 35 for the city and 41 forecast by the Bureau of Meteorology for Penrith.

Total fire bans have been declared for the northern Riverina and lower central west plains – where fire conditions will be severe on Wednesday. Conditions remain low-moderate for the Sydney region following recent heavy rains.

El Nino breaks down

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The latest burst of heat comes as scientists and climate experts say the giant El Nino – which drove global temperatures to record highs in 2015 – has started to break down in the Pacific.

The El Nino event, in which rainfall patterns shift eastwards away from Australia and south-east Asia, now appears to have passed its peak, the bureau said earlier this month

"Climate models suggest the 2015-16 El Nino will decline during the coming months, with a return to [neutral conditions] likely during the second quarter of 2016," the bureau said.

The current El Nino is one of the three largest such events recorded, challenging the intensity of the 1982-83 and 1997-98 events.

During El Ninos, the typical easterly winds blowing along the Pacific stall or even reverse, building up heat and even sea levels in the central and eastern Pacific compared with the west.

How the event breaks down will be closely watched around the world to see how long – and how severe – the impacts of the El Nino are likely to be.

While the most direct impacts of an El Nino on Australia are usually felt in the spring and early summer, other regions – such as the western US – are likely to see worsening floods as the event starts to terminate.

​Esteban Abellan and Shayne McGregor, two researchers at the ARC Centre of Excellence for Climate System Science, say they have found the trigger that leads to the break down of big El Ninos and that shift is now under way.

In a paper published last year in Climate Dynamics, the researchers found that near the end of the calendar year in which large El Ninos originate, the pattern of unusual winds shift from about the equator down to 5-7 degrees south. 

"It's lining up very well with the previous ones," Mr Abellan, a doctoral candidate at the University of NSW, said, noting the comparisons with the big 1982-83 and 1997-98 events.

The shift of the winds is a key mechanism for unwinding El Ninos, Dr McGregor, a lecturer at Monash University, said.

"When they move southwards, they remove that warmth" that had built up in the central and eastern Pacific, he said. "We are the first ones to show why [the event breaks down]."

The pattern, though, only appears to apply for large events, the researchers said.

At this stage, a neutral year or a La Nina are about equal odds following the demise of the current El Nino, the bureau said. Another El Nino following the current event is the least likely outcome.

A switch to a La Nina, when unusual heat builds up in the western Pacific, may lead to reverse conditions across the world, such as increased floods and cyclones across northern Australia.