Sea surface temperatures in the tropical Pacific Ocean have just passed Australia’s threshold for El Niño.
Meteorologists monitor a range of oceanic and atmospheric indices when tracking the development of El Niño. One of the main indices used to monitor El Niño is the Niño3.4 index, which measures sea surface temperature anomalies in the central tropical Pacific Ocean. El Niño occurs when the Niño3.4 index is positive and above a defined threshold for several months in a row.
Relative Niño Index
Several major meteorological agencies, including Australia's Bureau of Meteorology, have recently adopted a modernised version of the Niño3.4 index – called the relative Niño index – which aims to remove the background warming signal caused by climate change.
This new method provides a clearer picture of the status of El Niño and La Niña as the Pacific Ocean gradually becomes warmer. Without the relative index, El Niño events would appear to be happening more frequently, and La Niña would seem less common.
El Niño threshold reached
There is no internationally agreed threshold for El Niño. The United States Climate Prediction Center uses a relative Niño3.4 threshold of +0.5°C, while Australia’s Bureau of Meteorology uses a higher threshold of +0.8°C. In both cases, these warm anomalies must be sustained for several consecutive months to be classified as a proper El Niño.
The relative Niño3.4 index has been rapidly warming over the past few months, increasing by 1.2°C in just the last 10 weeks. The latest weekly relative Niño3.4 value of +0.81°C was the first time the index has exceeded Australia's El Niño threshold of +0.8°C since April 2024, at the tail end of the 2023-24 El Niño event.
Image: Weekly relative Niño3.4 index values since 2020, showing the index rising above Australia’s El Niño threshold this week. Source: Weatherzone.
The tropical Pacific Ocean will need to remain sufficiently warmer than average for a sustained period to be classified as an El Niño event. This looks likely to happen, thanks to a large slab of abnormally warm water currently lurking beneath the surface of the tropical Pacific, which will reinforce the warm surface water in the months ahead.
Computer models suggest that this El Niño signal is likely to strengthen further in the coming months and could persist through winter and spring in the Southern Hemisphere.
In addition to the recent ocean warmth, there are also signs that the atmosphere has started responding to the warming in the tropical Pacific Ocean. These early signs of ocean-atmosphere coupling are a clear sign that El Niño is getting underway.