This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Britain's Met Office says this year's El Niño is "unprecedented." Japan's weather agency has called it the strongest ever observed. U.S. forecasters say it still trails the biggest events in its record books.
The answer lies in how they measure El Niño's strength, with meteorological agencies using different methods to assess the climate phenomenon that make direct comparisons difficult. "By any measure, we are looking at a very strong El Niño," climate scientist Emily Becker of the University of Miami told AFP, adding that only once it is over will scientists be able to assess how big it is and "where it fits in the context of history." The Pacific Ocean's El Niño Southern Oscillation (ENSO) is the planet's main natural climate driver. Normally, trade winds blow east to west along the equator, pushing warm water from South America toward Asia, while cold water rises from the depths to replace it.
El Niños, which occur every two to seven years on average and last nine to 12 months, disrupt this pattern. The trade winds weaken and warm water is pushed back east toward the Americas. That shift in where ocean heat is released into the atmosphere ripples across global weather, making some regions cooler and wetter and others hotter and drier.
In describing the El Niño as unprecedented, the Met Office points to raw temperatures in the Pacific. "Our observations confirm that sea-surface temperatures in the central equatorial Pacific have now surpassed levels recorded during previous El Niño events," it said in a statement to AFP. But absolute temperatures are just one way of measuring El Niño's strength.
Agencies also look at anomalies—how far temperatures rise above a historic baseline—to assess the event. The Japan Meteorological Agency last week declared this El Niño the strongest on record based on how far sea surface temperatures were above normal in the Nino3 region of the eastern Pacific. By its count, the event beat 1997 in observations going back to 1949.
The U.S. watches a neighboring box, Nino3.4, in the east-central Pacific. It declares an El Niño when temperatures there run at least 0.5C above normal and the atmosphere responds, weakening the trade winds and shifting tropical rainfall eastward. This process is known as "ocean-atmosphere coupling." How "normal" is defined has been a matter of significant debate.
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