Starting in September 2025 we updated the way we calculate sea surface temperature indices for the El Niño-Southern Oscillation (ENSO) to a new method called the relative Niño index.
ENSO refers to the natural climate pattern in the tropical Pacific Ocean that involves fluctuations in sea surface temperature patterns and the atmosphere above. It ranges from La Niña, to neutral, to El Niño. These patterns are tracked using indices that help us monitor ENSO phases.
These relative Niño indices provide better insights into the oceanic ENSO signal in a changing climate. We can also better forecast how it changes by accounting for the long-term warming trend in our oceans.
About relative Niño indices
Until August 2025 the Bureau used the traditional Niño index to track sea surface temperatures in the Pacific Ocean.
As the Pacific Ocean has gradually warmed, so have the temperatures in key regions. The traditional Niño indices show how warm certain regions are, but they don't account for the long-term warming trend.
That's where the relative Niño indices help by removing the warming effect, giving a clearer view of the ENSO status. If we don't account for the warming, El Niño may seem more common and La Niña may seem less common.
The relative Niño indices measure how unusually cool or warm the ocean is in areas of the Pacific Ocean. Temperature differences are compared to 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 thresholds we use to monitor for El Niño and La Niña events in the Australian region have not changed. While we no longer provide forecasts using the traditional Niño indices, both the relative Niño and traditional Niño index values are available on the Southern hemisphere monitoring page - from the latest weekly observed values, to historical values dating back to 2008.
The decision to implement relative Niño indices is based on the latest science, drawing on research published in the Journal of Climate in early 2024.
ENSO is only one part of a complex system that influences Australia's climate. The Bureau's long-range forecast is the best guide to likely rainfall and temperature for the season ahead.
