Australia’s Bureau of Meteorology has updated the way it calculates sea surface temperature indices for the El Niño-Southern Oscillation (ENSO). The new method is called the relative Niño index.
The relative Niño indices provide better insights into the oceanic ENSO signal in a changing climate. The new method also enables the Bureau to better forecast changes by accounting for the long-term warming trend in the oceans.
About relative Niño indices
Until August 2025, the Bureau used the traditional Niño indices to track sea surface temperatures in the Pacific Ocean. This method worked by measuring how much warmer or cooler specific regions of the Pacific are compared with the usual temperature against a baseline period.
Although they showed how warm certain regions are, they didn’t account for the long-term warming trend.
The relative Niño indices remove the warming effect, which gives a clearer view of ENSO status. By not accounting for this, El Niño might seem more common and La Niña less common.
The new method measures how unusually cool or warm the ocean is in areas of the Pacific. Temperature differences are compared with the average sea surface temperature for the broader global tropical region.
They also better reflect the shifts in cloud and rainfall patterns in the Pacific. This helps to more clearly identify how the ocean and atmosphere interact to reinforce El Niño or La Niña states in a varying climate.
The criteria the Bureau uses to assess ENSO events will not change, as the thresholds remain the same using relative Niño indices.
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