El Nino record claims differ by agency

Meteorological agencies disagree on whether this year's El Nino is the strongest on record because they measure the climate phenomenon in different ways.

Britain's Met Office says the event is unprecedented, pointing to raw sea-surface temperatures in the central equatorial Pacific that it says have surpassed previous El Nino levels. Japan's weather agency has called it the strongest ever observed based on how far sea-surface temperatures rose above normal in the Nino3 region of the eastern Pacific.

US forecasters use a neighbouring region, Nino3.4, in the east-central Pacific, and require both warmer ocean temperatures and an atmospheric response, including weaker trade winds and shifted tropical rainfall. Under US records, the event has not yet clearly surpassed the biggest El Ninos.

El Nino is part of the Pacific Ocean's El Nino Southern Oscillation, the planet's main natural climate driver. During El Nino, trade winds weaken and warm water shifts east toward the Americas, changing where ocean heat enters the atmosphere and affecting weather around the world.

Another difference is how agencies define normal conditions as the planet warms. The US National Oceanic and Atmospheric Administration now uses the Relative Oceanic Nino Index, which compares the east-central Pacific with the rest of the tropical ocean as well as with a 30-year average.

NOAA projects this El Nino will be the strongest in its records, which go back to 1950, if the three-month average RONI anomaly reaches 2.5C or higher. The chances are rated high, but the outcome is not yet certain.

Climate scientist Emily Becker of the University of Miami said the event is very strong by any measure, but scientists will only know its final historical ranking after it ends.