El Niño / Southern Oscillation
World headed for an El Nino and it could be a big one, scientists say
8 May 2015
The world is headed into an El Nino event – potentially a big
one – which will lift global temperatures and likely exacerbate bushfires and
drought in eastern Australia, climate specialists say.
Fairfax Media understands that Australia's Bureau of Meteorology will announce next Tuesday that the El Nino event is all but certain.
Sea-surface temperatures in the central and eastern Pacific are recording anomalies of more than 1 degree, a combination that has not previously been seen in weekly data going back to 1991, according to a bureau climate forecaster.
Australia's measure of El Nino thresholds is sustained warmth of sea-surface temperatures of 0.8 degrees above average in the key regions surveyed, a higher bar to clear than set by the US and some other agencies.
"You can see a warming in the eastern Pacific, which looks to be a classic [El Nino] event," said Agus Santoso, an El Nino modeller at the University of NSW's Climate Change Research centre.
The most recent El Nino in 2009-10, with its characteristically warm sea-surface temperature anomalies in the central and eastern Pacific. Photo: NASA
Scientists, though, are surprised that the build-up of
unusual warmth in the eastern Pacific compared with the west is happening so
early in the year. "It's quite rare – this is an interesting one," Dr Santoso
In typical El Nino years, the usual easterly trade winds stall or even reverse in winter or later, dragging rainfall eastwards away from Australia and also south-east Asia. Droughts tend to deepen and spread and bushfire seasons are more active than normal.
A study by the bureau of 12 strong El Nino years since 1905 found rainfall declines were most evident in winter and spring – key agricultural seasons. The hardest hit areas cover most of NSW and parts of southern Queensland, while almost all of the eastern states have significantly reduced rain. (See below).
An El Nino event this year would be bad news for areas also
suffering serious or severe rainfall deficiency. A bureau drought report out
this week identified such areas over the past 30 months to include much of
inland Queensland, western Victoria and north-central NSW – some of which are
already receiving federal and state aid.
The bureau declined to say that its up-coming El Nino report will confirm the event.
"The tropical Pacific has continued to warm in the past week and all indices now exceed 1 degree," Andrew Watkins, head of the bureau's climate prediction services, said. "This warming has been fairly consistent since the start of the year, as noted in our fortnightly ENSO updates. Likewise, the models we survey suggest this warming will continue."
As the bureau notes, its survey of all most all model runs place the central NINO3.4 region clearly in El Nino territory:
Big El Nino possible
It's the early start to the process, though, that has climate scientists concerned the planet may be on course for a particularly strong El Nino event.
"If it peaks in winter then dies off it's interesting," Dr Santoso said. "But if it keeps going up and peaks in summer, that could potentially be a big El Nino."
Cai Wenju, a leading climate modeller at the CSIRO, said experts were predicting a strong El Nino a year ago but sustained westerly winds failed to eventuate. As a result, the atmosphere did not "couple" with, or reinforce, the warming trends in the oceans.
"Last year at this time, we didn't see the [westerly] winds," Dr Cai said. "This time, we see the strong westerly winds all along the equator."
If anything, forecasters have been overly cautious, he added. Some models generated by the US National Oceanic and Atmospheric Administration and not widely seen are pointing to "humungous anomalies" of as much as 5 degrees by October- November for parts of the eastern Pacific.
Even the ensemble of models is pointing to a 3-degree temperature anomaly by then, placing the departure from the norm in a similar league to previously powerful events such as the "super El Nino" of 1997 or 1982, Dr Cai said, cautioning that conditions could yet ease back.
Water storage levels in eastern Australia typically drop in El Nino years as little run-off makes it into reservoirs.
Sydney is currently well-placed to cope with a lengthy dry spell after last month's storms helped lift overall storage levels to 92.4 per cent as of Wednesday.
Melbourne's storages, though, continue to see a reduction, with recent below-average rain pulling dam levels below 70 per cent, slightly lower than a year ago.
The bigger impact is likely to be felt in towns in the Murray-Darling Basin, with Broken Hill's storage levels down to about 4 per cent.
NSW MPs such as the Greens Jeremy Buckingham say the state should investigate how much rainfall from good rain events earlier in 2015 that have been trapped by large agricultural dams further upstream, including the giant Cubbie Station cotton farm.
Graeme Anderson, a climate specialist at the Victorian Department of Economic Development, said El Nino tended to double the chances of a dry spring in Victoria, though did not guarantee it.
He said it would be important strong rains occurred this winter before any El Nino event kicked in, particular for farmers in large parts of Western Victoria who suffered a dry end to last year and start of 2015.
EFFECTS OF EL NINO
Eastern Australian rainfall is typically below average, particularly in winter and spring, the key agricultural seasons. During 12 strong El Nino events analysed by the Bureau of Meteorology, the biggest departures from long-term average rains were in most of inland NSW and parts of southern Queensland.
Drought, frost, snow
Relatively clear skies mean areas already dry – such as western Victoria and inland parts of NSW and Queensland – may not get much near-term relief, while others may be declared drought-hit. With diminished cloud cover, frosts can also be bad.
Heatwave and bushfires
With dry soils, evaporation is reduced, leading to potentially hot summers. Warm, dry springs make for worse bushfire seasons than usual.
El Nino years are also disruptive to our neighbours, with increased chance of forest fires and drought in South-East Asia and a reduced monsoon in south Asia. Across the Pacific, the reverse is true, with above-average rainfall.
El Nino conditions mean the Pacific absorbs less heat from the atmosphere and can even give some back. Global temperatures typically get a 0.1-0.2 degree kick in such years, meaning 2015 and 2016 are shaping up to exceed the warmest year on record, set only last year.
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