The study, which examined Pacific El Niños over nearly two decades, revealed that El Niños generating sea-surface temperature anomalies over the eastern Pacific are more likely to result in fires than those generating anomalies over the central Pacific.

"We suggest that projecting the location of El Niño events might be more important than projecting their strength for fire management in southern Borneo," said Min-Hui Lo of the National Taiwan University in Taipei.

Fires are a big problem in Indonesia, as events in 2015 showed. According to an estimate by the Global Fire Emissions Database, that year saw Indonesian fires generate more carbon dioxide than the entire US economy. The 2015 fires are also thought to have caused billions of dollars of damage, and inflicted respiratory problems on hundreds of thousands of people.

Indonesian fires are known to be linked to El Niños – weather events that result in warm water shifting eastwards in the tropical Pacific, towards Indonesia. El Niño is thought to weaken the Walker circulation, in which low winds blow westwards across the Pacific, in turn inducing dry, fire-prone conditions over land.

One might assume that stronger El Niños would weaken the Walker circulation more, leading to drier conditions and more fires. But now Lo, together with Chu-Chun Chen at National Taiwan University and colleagues, has challenged this assumption. Using data taken between 1997 and 2015, the researchers compared two types of El Niño: those with sea-surface temperature (SST) anomalies centred over the eastern Pacific, and those with SST anomalies centred over the western Pacific. They analyzed precipitation, fire records and atmospheric circulations, and searched for links using climate simulations.

Fire activity did not increase with the El Niño strength, the team found. Instead, greater fire activity seemed to be linked with El Niños over the eastern Pacific, and lesser fire activity was associated with El Niños over the central Pacific. Though the 2015 El Niño was indeed strong, the researchers believe it was the event’s eastern location that led to the particularly damaging fires that year. Even in the case of a weak El Nino, "if the warm SST anomalies are on the eastern Pacific, it could still be a very active fire year," says Lo.

The researchers believe that their study could help officials to forecast fire activity, and so better prepare them. They reported their work in Environmental Research Letters (ERL).

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