The tropical Pacific Ocean is rapidly warming towards a ‘very strong’ El Niño event, with a key monitoring index this week crossing a threshold not reached for more than 10 years.
El Niño was declared by the Bureau of Meteorology in the middle of June this year following four months of persistent warming in the tropical Pacific Ocean, along with a response from the overlying atmosphere. Since then, El Niño has been rapidly gaining strength.
Ocean warming – Relative Niño 3.4 index rising rapidly
The main oceanic index used to monitor El Niño is the relative Niño 3.4 index, which is a measure of sea surface temperature anomalies in the central equatorial Pacific Ocean. This index factors in the background warming influence of climate change to make sure the current water temperature anomalies can be accurately compared to those from past years.
According to the Bureau of Meteorology, El Niño occurs when sustained monthly values of the relative Niño 3.4 index are above 0.8°C. The U.S. Climate Prediction Center (CPC) also uses the following thresholds to identify the strength of El Niño based on the relative Niño 3.4 index:
Moderate El Niño: +1.0°C to +1.49°C
Strong El Niño: +1.5°C to +1.99°C
Very Strong El Niño: +2.0°C and above
According to data from the Bureau, the relative Niño 3.4 index reached +2.02°C during the week ending on August 2, 2026. This is the first time the Bureau’s weekly index has exceeded +2.0°C since February 2016.
While the weekly Niño 3.4 index exceeding +2.0°C is a milestone, it will need to remain above this threshold for several months to be recognised as a very strong El Niño event.
Atmosphere responding
El Niño is a coupled ocean-atmosphere phenomenon, meaning it occurs when there are changes in both the ocean and atmosphere that interact and reinforce each other. This feedback loop between the ocean and atmosphere helps sustain and amplify El Niño events.
One of the main atmospheric indices used to monitor El Niño is the Southern Oscillation Index (SOI), which measures the mean sea level pressure difference between Darwin, in northern Australia, and Tahiti, in the central Pacific Ocean.
When El Niño is underway, air pressure over Darwin becomes higher than the pressure over Tahiti, which causes the SOI to become negative. Sustained negative values below -7.0 indicate the presence of El Niño. Over the past 90 days, the average SOI has been around -21.4.
Other atmospheric indicators are also showing signs of a well-established and strengthening El Niño, including weakened or reversed trade winds over parts of the tropical Pacific Ocean and reduced cloudiness near the international date line.
How strong will this El Niño get?
Global forecast models predict that El Niño will continue to gain strength in the coming months, likely reaching its peak towards the end of the year. Given its current and projected strength, it will likely persist into early 2027.
The Bureau of Meteorology’s ACCESS-S model predicts the monthly relative Niño 3.4 index will peak at around +3.5°C in November 2026. According to Bureau data, this would easily beat the previous monthly relative Niño 3.4 record of +2.65°C from November 1902.
Image: Monthly Niño 3.4 index outlook, according to the ACCESS-S model. Source: Bureau of Meteorology.
What will this record-challenging El Niño mean for our weather?
Past El Niño events have shown that the strength of El Niño does not necessarily determine the strength of its impacts. However, there is no precedent to which we can compare an El Niño of this magnitude, so we will be wading into uncharted climate territory later this year.
A very strong El Niño will make certain global temperature and rainfall patterns more likely.
According to the World Meteorological Organization (WMO), forecast modelling “predicts an overwhelming likelihood of above average temperatures across most land areas.”
Rainfall outlooks also show clear influence from the strengthening El Niño.
“Wetter-than-normal conditions are expected across the Greater Horn of Africa; parts of Central Asia; Southern Europe; Western North America (south of 45°N); and Southeastern South America. In contrast, drier-than-normal conditions are more likely over the Indian subcontinent; Southern and eastern Australia; Southern Central America and parts of the Caribbean; Northwestern South America; and Northern Europe,” says the WMO.
Image: Typical global rainfall impacts from El Niño. Source: WMO.
In Australia, El Niño typically enhances the likelihood of below average rain and above average daytime temperatures over parts of the country. El Niño events also increase the risk of a below average snow season in the Australian Alps and boost the likelihood of droughts and bushfires.
The latest seasonal outlooks from the Bureau of Meteorology predict below average rain over most of the eastern half of Australia during the August-to-October period. Temperatures are also predicted to be higher than normal across most of the country during this period.