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The reason scientists fail to forecast the occurrence of El Niño is they do not really understand the weather phenomenon, which is one of the most powerful drivers of climate variation and has widespread impact on ecology and economy. A recent study by scientists from University of Washington in the US and University of Montpellier in France reveals this knowledge gap by contradicting the current hypothesis about El Niño.
El Niño, which occurs along with another weather phenomenon Southern Oscillation in the tropical Pacific Ocean and hence is known as El Niño Southern Oscillation (ENSO), was widely thought to have been weaker in ancient times because of a different configuration of the Earth’s orbit. The recent study shows that El Niños occurring 10,000 years ago were as strong and frequent as the ones we experience today.
The study analysed fossil shells from mounds of Mesodesma donacium clams, which were eaten and then discarded over centuries into piles that archaeologists call middens, at seven sites along the coastal Peru. El Niño begins as an abnormal warming of tropical eastern Pacific Ocean along the coasts of Peru and Ecuador towards April.
Shells record ocean temperatures. So the researchers analysed the shell layers to know the sea surface temperature anomalies as the shell was forming. This revealed one to three-year-long records of monthly temperature of the Pacific Ocean along the coast of Peru. It turns out that 10,000 years ago, the El Niño cycles were strong. “Roughly 7,000 years ago the shells showed a shift to the central Pacific of the most severe El Niño impacts, followed by a lull in the strength and occurrence of El Niño from about 6,000 to 4,000 years ago,” according to a press release.
“Climate models and a variety of datasets had concluded that El Niños were essentially nonexistent, did not occur, before 6,000 to 8,000 years ago,” said Julian Sachs, professor of oceanography at University of Washington.
“Our data contradicts the hypothesis that El Niño activity was very reduced 10,000 years ago, and slowly increased since then,” said first author Matthieu Carré, who did the research as a postdoctoral researcher with University of Washington and now holds a faculty position at University of Montpellier. He says the new data is more reliable for three reasons: the Peruvian coast is strongly affected by El Niño; the shells record ocean temperature, which is the most important parameter for the El Niño cycles; and the ability to record seasonal changes, the timescale at which El Niño can be observed.
The study also questions how well computer models can reproduce historical El Niño cycles or predict how they could change under future climates. It was published online on August 7 in journal Science.