Warming oceans prompt major overhaul of how we monitor El Niño and La Niña

Global warming is changing how we monitor one of the world’s biggest climate drivers, says Earth Sciences New Zealand.

NIWA
1 October 2025
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El Niño–Southern Oscillation (ENSO is a fluctuation of ocean temperatures and atmospheric pressure in the Pacific. It is made up of opposite phases - El Niño and La Niña - and has a considerable impact on global weather, including here in New Zealand.

Earth Sciences NZ meteorologist Chris Brandolino says as our climate and seas warm, they’re interfering with the long-standing method used to monitor the ocean component of ENSO.

“Traditionally, ENSO is tracked by measuring how unusually warm or cool sea surface temperatures are in certain areas of the Pacific. However, as the climate warms, so do our oceans, which can muddy the ENSO waters. As ocean temperatures increase, we can overstate the frequency of the warm phase of ENSO, or El Niño, and understate the cool La Niña phase,” said Chris.

For example, during summer 2024–25, the traditional Niño index (TONI) did not reach the threshold of –0.7°C to classify La Niña. However, La Niña-like conditions were still observed across New Zealand, with above average air temperatures, widespread marine heatwave conditions, and rainfall totals that were above normal in eastern areas and below normal in western areas.

A graph showing the sea temperature thresholds of the traditional method vs the new relative method for defining El Niño and La Niña.

A graph showing the sea temperature thresholds of the traditional method vs the new relative method for defining El Niño and La Niña.

“In response, we’re updating the way we calculate the sea surface temperatures anomalies, or difference from average, that help us track and forecast ENSO” says Chris.

The move follows a similar announcement by the Australian Bureau of Meteorology last month.

The new approach uses a method that assesses how unusually warm or cool ocean temperatures are in specific locations compared to the current tropical mean temperature (averaged between 20° south and 20° north).

“Had this been applied during summer 2024-25, the La Niña classification threshold would have been met, demonstrating how the updated methodology better reflects ENSO impacts in a warming climate,” says Chris.

A graphic showing the sea temperature thresholds of the traditional method vs the new relative method for defining El Niño and La Niña.

A graphic showing the sea temperature thresholds of the traditional method vs the new relative method for defining El Niño and La Niña.

While ENSO remains a key driver of climate variability in New Zealand, it is only one part of a complex system.

For the most reliable guidance on expected rainfall and temperature patterns in the coming season, refer to our Seasonal Climate Outlook.

FAQs

No, and nor is the underlying data used to monitor these events. What is changing is the method of calculating the sea surface temperatures anomalies used as oceanic indicators of ENSO. This ensures a more consistent reflection of the ENSO status that is not biased by the long-term warming trend in our oceans.

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. Climate Scientists at Earth Sciences New Zealand have been engaged in several technical discussions with international institutions such as National Oceanic and Atmospheric Administration (NOAA), the Australian Bureau of Meteorology and the Centre for Climate Research (Singapore).

References:

This change is not anticipated to make ENSO forecasts themselves significantly more or less accurate. However retrospective analysis of the relationship between the observed relative Oceanic Nino index and regional circulation and climate anomalies over Aotearoa New Zealand will help forecasters better contextualise the expected ENSO impacts over the country.

There is currently no clear evidence that climate change is tipping the balance toward either El Niño or La Niña, though this remains an active area of research. While global ocean temperatures have risen significantly since the onset of reliable measurements, the eastern tropical Pacific has exhibited a cooling trend, suggestive of a more La Niña-like mean state. Climate projections remain uncertain regarding future changes in the frequency, intensity, or characteristics of ENSO events.

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