A large pool of exceptionally warm water lying beneath the surface of the tropical Pacific Ocean could push El Niño to record-breaking strength in the coming months.

El Niño has been rapidly gaining strength in the Pacific Ocean since becoming established in the middle of 2026. Earlier this month, a key oceanic index used to monitor the strength of El Niño – the weekly relative Niño 3.4 index – reached its highest value on record.

What is the relative Niño 3.4 index?

El Niño is monitored using a range of oceanic and atmospheric indices that allow meteorologists to keep track of how each El Niño event is evolving. The primary oceanic index is called the Niño 3.4 index, which measures sea surface temperature anomalies in an area of the central tropical Pacific Ocean called the Niño 3.4 region.

In recent years, some organisations – including Australia’s Bureau of Meteorology (BOM) and the U.S. National Oceanic and Atmospheric Administration (NOAA) – have adopted a relative Niño 3.4 index.

While the traditional Niño 3.4 index just looked at sea surface temperature anomalies inside the Niño 3.4 region, the new relative index attempts to filter out the background warming influence of climate change by subtracting the current average temperature of the world’s tropical oceans from the current sea surface temperature anomaly in the Niño 3.4 region.

Without the relative Niño 3.4 index, climate change would be making El Niño events appear stronger and more frequent, while La Niña events would appear to be weaker and occurring less frequently.

Weekly relative Niño 3.4 index record

According to the Bureau of Meteorology, the weekly relative Niño 3.4 index reached +2.81°C during the week ending on October 11, 2026. This was the same value from the previous week ending on October 4.

These latest weekly values are more than 2°C above the Bureau’s El Niño threshold of +0.8°C. They are also the highest values on record in the BoM’s weekly relative Niño 3.4 index, which dates back to 2008.

Image: Weekly relative Niño 3.4 index values from 2008 to 2026. Source: Bureau of Meteorology.

It’s important to note that while the weekly relative Niño 3.4 index is useful to track the ongoing evolution of El Niño from week-to-week, there is also a monthly index – which is based on a different underlying dataset – that is better for long-term comparisons of individual El Niño events.

According to the Bureau, the latest monthly relative Niño 3.4 index value was +1.96°C in September 2026. This is the highest monthly value since 2015.

El Niño getting stronger

Forecasts from the Bureau suggest the relative Niño 3.4 index could peak around +3.7°C in November or December this year. If realised, this would be more than 1°C above the current record, which was +2.56°C from December 1902*.

One reason meteorologists expect El Niño to continue strengthening in the coming weeks is a large pool of abnormally warm water lying beneath the surface of the tropical Pacific.

According to data from NOAA, water temperature around 100 m below the surface in parts of the eastern tropical Pacific reached 12°C above average on October 11, 2026. This warm sub-surface water is likely to help El Niño strengthen further in the coming weeks.

Image: Subsurface water temperature (top) and anomalies (bottom) in the tropical Pacific Ocean for the five-day period to October 11, 2026. The bottom chart shows temperature anomalies exceeding 12°C around 100 m below the eastern tropical Pacific. Source: Bureau of Meteorology.

This El Niño event is rapidly approaching a strength never previously recorded, even with the background influence of climate change removed. The exceptional strength of this event means El Niño is likely to persist through the Southern Hemisphere’s summer and could linger into autumn 2027.

* Note: Weatherzone had previously reported that the highest monthly relative Niño 3.4 value on record was +2.65°C from November 1902. While this was accurate at that time, the Bureau is now using an updated version of the dataset underlying the relative Niño 3.4 calculation, which has changed the historical record from +2.65°C in November 1902 to +2.56°C in December 1902. More information about the Bureau’s historical monthly sea surface temperature data is available here.