Down in the tropical Pacific Ocean, meanwhile, in late 2014 an El Niño began to brew. It pooled warm water off the coast of South America, and resulted in one of the strongest El Niños on record by 2015. The phenomenon persisted into early 2016.

Until now most researchers believed that the two events were not connected, but a new study suggests that both are part of a larger Pacific-wide ocean-atmosphere coupling. And whilst the blob didn't directly cause the development of the strong El Niño, it was a precursor, and could help predict El Niño variability in the future.

Over the last three decades there have been two other "super" El Niños (1982/83 and 1997/98) in addition to that in 2015/16. But neither of these previous "super" El Niño events was preceded by an extremely warm blob. So was the overlap of the 2014/15 warm blob and the 2015/16 super El Niño just pure coincidence?

To find out, Yu-Heng Tseng from National Taiwan University and colleagues looked at Pacific monthly sea surface temperature data going back to 1958 along with atmospheric sea level pressure data. Their aim was to understand the coupling between the ocean and atmosphere over the entire Pacific region. The results showed a tight connection between sea surface temperature and atmospheric sea level pressure in the North Pacific.

From early 2014 the North Pacific developed a temperature gradient, with negative sea surface temperature anomalies in the mid-latitude western Pacific, surrounded by positive anomalies in the eastern Pacific (extending from off California to the western Bering Sea). Previous work has shown that such a pattern is known as the "Victoria Mode" and is forced by a basin-wide atmospheric ocean coupling.

"The development of the warm blob in late 2013 was mainly driven by positive sea level pressure anomalies over the Gulf of Alaska, which suppressed the local ocean heat loss to the atmosphere," said Tseng. "The positive sea level pressure anomalies are also associated with negative sea level pressure anomalies further south, which together are called the North Pacific Oscillation."

In this case, the warm blob sea surface temperature anomalies spread equatorwards down the west coast of the US and to the central tropical Pacific. When they met the warm water that had accumulated in the tropical Pacific, it triggered the development of the exceptional El Niño of 2015/16.

So how did the super El Niño events of 1982/83 and 1997/98 arise if they weren't preceded by a warm blob in the North Pacific? Earlier work had shown that both these super El Niños were also preceded by positive sea surface temperature anomalies in the eastern North Pacific, but these anomalies were not as strong as the most recent ones. Nonetheless, the additional warmth is likely to have enhanced both these previous El Niño events too.

"It is a bit like starting a fire," said Tseng. "If you just light a match it won't necessarily cause a fire, but if you throw the match into an oilfield [in this case the pre-warmed tropical Pacific] it definitely creates a blaze."

Positive Victoria Mode events don't always lead to El Niños; the findings show that from1950 to 2011 around one third of the positive Victoria Mode events didn't result in an El Niño. "The surface patterns associated with these Victoria Mode events did not extend far enough towards the equator, compared with other events," Tseng and his colleagues write in Environmental Research Letters (ERL) .

Nonetheless, Tseng’s study shows that the warm blob was an intermediate step leading to the 2015/16 El Niño. Such events, more commonly observed after 2000, favour the development of El Niño but don't guarantee its occurrence, showing that the events are linked, but that one does not directly cause the other. The linkages are complicated and related to other processes, but this enhanced understanding should help to predict El Niño variability better in the future.

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