The key facts

  • The Met Office relies on absolute ocean surface temperatures, Japan on anomalies in the Nino3 zone, and the US on the RONI index in the Nino3.4 zone
  • NOAA admits the event could become a record one since 1950 if the three-month RONI anomaly reaches 2.5°C or higher
  • Scientists note that a final assessment of the phenomenon's scale can only be made after it ends

One Phenomenon — Three Different Verdicts

Assessments of the strength of the current El Niño differ among the world's leading weather agencies. The British Met Office calls the phenomenon 'unprecedented,' the Japan Meteorological Agency has declared it the strongest in the history of observations, while American forecasters believe it still falls short of the largest events in their archives. As AFP notes, the contradiction is explained by the fact that the agencies use different measurement methods, which makes direct comparisons difficult. University of Miami climatologist Emily Becker told AFP that 'by any measure, this is a very strong El Niño,' but its place in history can only be assessed after it ends.

What Is El Niño

The El Niño-Southern Oscillation (ENSO) is the planet's main natural climate driver. Normally, trade winds blow along the equator from east to west, carrying warm water from South America toward Asia, while cold water rises from the depths to replace it. El Niño, which occurs on average once every two to seven years and lasts nine to twelve months, disrupts this pattern: the trade winds weaken, warm water is pushed back toward the Americas, and the shift in the release of oceanic heat changes weather around the world — some regions become cooler and wetter, others hotter and drier.

Different Measurement Methods

The Met Office bases its 'unprecedented' assessment on absolute temperatures: according to its statement to AFP, the sea surface temperature in the central equatorial Pacific exceeded the levels of previous El Niño events. The Japanese agency last week declared the event a record one in terms of temperature anomalies in the eastern Pacific Nino3 zone: according to its calculations, the event surpassed 1997 in observations since 1949. The US monitors the neighboring Nino3.4 region in the east-central part of the ocean and declares an El Niño when the temperature there is at least 0.5°C above normal and the atmosphere responds — with weakening trade winds and a shift of tropical rainfall eastward (the so-called ocean-atmosphere coupling).

Dispute Over the Baseline and a New Index

The definition of 'normal' has become the subject of considerable debate. NOAA and other agencies, including the UN's World Meteorological Organization, long used the Oceanic Niño Index (ONI), which compares temperatures with a rolling 30-year average updated every five years. But amid accelerating climate change, NOAA has switched to the Relative Oceanic Niño Index (RONI), and the WMO is in the process of transitioning. RONI also relies on a 30-year average but additionally measures how much warmer or colder the east-central Pacific is than the rest of the tropical ocean, since it is this difference that determines the weather consequences. Becker said that with the old index there was a 'mismatch between the temperature of the tropical Pacific and the impacts': La Niña years produced stronger effects than their ranking suggested, while El Niño produced weaker ones.

Prospects for a Record According to NOAA

NOAA forecasts that this El Niño will become the strongest in its archives, which date back to 1950, if the three-month RONI anomaly reaches 2.5°C or higher — a level that would surpass the events of 1982-83, 2015-16, and 1997-98. The probability is assessed as 'high,' but not yet guaranteed. Becker added: 'However we rank or measure this event, it is an incredible opportunity to study and understand this very powerful phenomenon in the context of a warming world.'