Record El Nino? Why scientists can’t agree on how strong it is

Scientists are divided over the intensity of this year's El Nino, with varying assessments from different meteorological agencies. The United Kingdom's Met Office declares it "unprecedented," while Japan's agency claims it is the strongest on record. The United States, on the other hand, finds it still falls short of the most intense events in its historical records.

The discrepancy arises from the diverse methods employed by these organizations to evaluate the strength of El Nino, a climate phenomenon that is challenging to compare directly.

El Nino, a natural climate driver, is characterized by the disruption of the usual trade winds along the equator, causing warm water to shift from South America to Asia and cold water to rise from the depths. This shift impacts global weather patterns, causing certain regions to become cooler and wetter or hotter and drier. The intensity of El Nino is often gauged through raw temperatures in the Pacific and temperature anomalies, which measure how far temperatures deviate from historical averages.

The Met Office focuses on the sea-surface temperatures in the central equatorial Pacific, noting that they have surpassed levels seen during previous El Nino events. However, agencies also assess the event by examining how much above the historical baseline the temperatures have risen. The Japan Meteorological Agency determined this El Nino to be the strongest in their records based on the rise in sea surface temperatures in the Nino3 region of the eastern Pacific. This region's temperatures beat 1997 levels, which occurred in the past.

The US, meanwhile, watches the neighboring Nino3.4 region in the east-central Pacific. An El Nino is declared when temperatures in this area are at least 0.5°C above the norm, and the atmosphere responds, weakening the trade winds and shifting tropical rainfall eastward—a process known as "ocean-atmosphere coupling." This metric is determined by organizations such as the NOAA, World Meteorological Organization, and UN.

The challenge lies in defining what constitutes "normal" temperatures. For decades, NOAA and other agencies used the Oceanic Nino Index (ONI), which compares temperatures to a 30-year average that is periodically updated to account for climate change. However, NOAA has now transitioned to the Relative Oceanic Nino Index (RONI), which measures how temperatures in the east-central Pacific compare to the rest of the tropical ocean. This new approach is more reflective of the actual climate impacts that El Nino causes.

Climate scientist Emily Becker of the University of Miami expresses confidence that this El Nino is among the strongest on record, regardless of the measurement method. She emphasizes that only after the phenomenon concludes will scientists be able to accurately assess its magnitude and position within the broader historical context. This El Nino, says Becker, presents a unique opportunity to study the powerful climate phenomenon in the context of a warming world.