by Michael Slezak, NewScientist, February 10, 2014
Hold onto your ice lollies. Long-term weather forecasts are suggesting 2014 might be the hottest year since records began. That's because climate bad-boy El Niño seems to be getting ready to spew heat into the atmosphere.
An El Niño occurs when warm water buried below the surface of the Pacific rises up and spreads along the equator towards America. For nine months or more it brings rain and flooding to areas around Peru and Ecuador, and drought and fires to Indonesia and Australia. It is part of a cycle called the El Niño-Southern Oscillation.
It is notoriously hard to make a prediction before the "spring barrier" as to whether there will be an El Niño in a given year. "The El Niño-Southern Oscillation cycle more or less reboots around April-May-June each calendar year," says Scott Power from the Bureau of Meteorology in Melbourne, Australia.
The problem is that there is so much background variability in the atmosphere and ocean that it is hard to see any signal amidst the noise, says Wenju Cai from the CSIRO, Australia's national research agency in Melbourne. "Even if there is a developing El Niño, it is hard to predict."
Links in the air
But now a model aimed specifically at predicting El Niño seems to be able to sift through the noise by examining a previously-unexplored feature of Pacific weather.
Previous predictions have relied on full climate models. Rather than using this traditional approach, Armin Bunde of Justus Liebig University in Giessen, Germany, and his colleagues looked at the strength of the link between air temperature over the equator and air temperature in the rest of the Pacific. The records showed that, in the year before each El Niño, the two regions became more closely linked, meaning their temperatures became more similar than at other times.
The team also found that, once these atmospheric links reached a critical strength, around 75% of the time an El Niño developed within a year (PNAS, doi.org/rdn). "There is certainly a correlation between the cooperative mode in the atmosphere that we measure and the onset of an El Niño event," says Bunde. Nobody knows why.
Now they say the threshold was crossed in September 2013. "Therefore, the probability is 0.76 that El Niño will occur in 2014," says Bunde. In other words, there is a 76% chance of an El Niño this year.
As a result of climate change 2014 is likely to be one of the hottest years on record. If El Niño does develop this year, it will make 2014 even hotter – maybe the hottest ever, says Cai. But since El Niño normally straddles two calendar years, it might give 2015 that title. "It is possible, but not a sure thing. It can be tipped over either way by other variability."
An increasing number of climate models are now predicting El Niño this year too. It is unclear whether it will be an extreme El Niño like the 1998 event, which is thought to have killed tens of thousands. But Cai thinks an extreme El Niño is unlikely because longer-term variability in the Pacific's weather is suppressing it.
Journal reference: PNAS, DOI: 10.1073/pnas.1323058111
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