A multinational team of researchers has cracked the mystery behind the unusually strong El Niño events that occurred in 1982-1983 and 1997-1998.

El Niño is a large-scale warming of surface water which takes place every 3 – 6 years in the equatorial zone of the central and eastern Pacific Ocean.

Extreme El Niños of 1982 and 1997 differ from the common kind in that sea surface temperatures start warming in the west of the Pacific Basin and spread eastwards.

“These unusual El Niños appeared for the first time in the available record sometime after the mid 1970s,” said Dr Agus Santoso from the University of New South Wales, the lead author of the study published in the journal Nature.

Climate scientists have struggled to explain why these events occurred and if the frequency would change in the future.

“The most common theory used to explain these unusual El Niños was that competing air and ocean feedbacks drove the direction of the warming,” Dr Santoso said.

“But if this was true, La Niñas would have propagated in the same direction. Observations show they do not.”

In a world first, the researchers found the key to the mystery was the weakening of westward flowing currents along the Equator in the Pacific Ocean.

As these currents weakened and even reversed, it allowed the heat during these unusual El Niño events to spread more easily into the eastern Pacific.

La Niña events didn’t behave in a similar way, because the currents are strong and flow to the west.

The scientists also determined what this could mean for the future frequency of these unusual El Niños.

“Using observations we demonstrated the likely role of the weaker currents in the unusual behavior,” Dr Santoso said.

“These currents are well represented in a number of climate models. Using these models we confirmed, even under modest global warming scenarios, these unusual El Niño events doubled in frequency.”

Past experience shows that these super El Niño events bring more than just unusual weather conditions – they matter for people and economies.

______

Bibliographic information: Agus Santoso et al. Late-twentieth-century emergence of the El Niño propagation asymmetry and future projections. Nature, published online November 17, 2013; doi: 10.1038/nature12683