Earth Sciences New Zealand has introduced a new approach to monitoring the El Niño-Southern Oscillation (ENSO), responding to the challenges posed by rising ocean temperatures.
ENSO, which alternates between El Niño and La Niña phases, is a key factor influencing weather patterns across New Zealand and the wider Pacific region.
Chris Brandolino, meteorologist at Earth Sciences New Zealand, explained that the traditional method of tracking ENSO – based on sea surface temperature anomalies in specific Pacific regions – has become less reliable as global ocean temperatures rise.
“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,” he said.
Recent observations during the 2024-25 summer illustrated this issue. The conventional Niño index did not meet the threshold for a La Niña classification, yet New Zealand experienced conditions typically associated with La Niña, such as higher air temperatures, widespread marine heatwaves, and regional rainfall imbalances.
In response, Earth Sciences New Zealand is moving to a relative method for calculating sea surface temperature anomalies. This new system compares current temperatures to the present-day tropical mean, rather than a fixed historical average.
“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,” Brandolino said.
The change is limited to the calculation method; the underlying data and event thresholds remain the same. This adjustment is designed to provide a more consistent and unbiased view of ENSO status, unaffected by the long-term warming trend.
The update follows similar moves by the Australian Bureau of Meteorology (BoM) and is grounded in recent scientific research and international collaboration.
For New Zealand’s insurance professionals, the revised ENSO monitoring approach has direct implications for weather risk modelling and event forecasting.
Accurate classification of ENSO phases is critical for anticipating seasonal climate impacts, which influence underwriting, pricing, and claims management.
Brandolino noted that the change is not expected to significantly alter the accuracy of ENSO forecasts themselves.
However, retrospective analysis using the new index may help forecasters better interpret the relationship between ENSO and local climate anomalies.
In a related development, a collaborative study between Earth Sciences New Zealand and the University of Waikato projects a substantial increase in extreme rainfall from tropical cyclones affecting New Zealand if high emissions persist.
Using high-resolution climate simulations, researchers found that while cyclone frequency is not expected to rise, the most intense storms – Category 4 or higher – could become more common, and associated rainfall could increase by up to 35% by century’s end.
The study attributes this trend to the atmosphere’s greater capacity to retain moisture as it warms, leading to heavier rainfall during storms.
