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Why do La Nina events linger?

12 August 2014

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For millions of people, the onset of El Niño or La Niña in northern autumn indicates whether they're likely to face unusually warm, cold, wet, or dry conditions over the coming winter. A new modelling study pins down the process that apparently determines why La Niña events often last twice as long as typical El Niño events-a result with major implications for seasonal predictions extending more than a year out.

"Our work suggests that we can make significant progress on predicting La Niña events and determining how long they might last," said NCAR scientist Clara Deser. "Because of the disproportionate impact of La Niña on drought throughout the world, this could help lead to concrete social benefits." Deser recently coauthored a paper with Pedro DiNezio (University of Hawaii) that is now in early release in the Journal of Climate.

Although they're often portrayed as a pair of opposites, El Niño and La Niña aren't mirror images of each other. El Niño events-which involve warmer-than-usual water overspreading the eastern tropical Pacific-tend to last no longer than one year. However, in recent decades, almost half of La Niña events-which lead to strong trade winds and cooler-than-usual surface water-have returned for a second consecutive northern winter. This was the case during 2010-11 and 2011-12, two La Niña years that were both associated with devastating U.S. droughts.

Source: University Corporation for Atmospheric Research (UCAR)

Image credit: NASA - Data showing ocean height in 2010

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