Supersized El Niño: how a warming Pacific reshapes weather around the world
This El Niño could become stronger than any since 1950. The ocean is already showing exceptional warming, and the forecasts are no longer routine.
This El Niño could become stronger than any since 1950. The ocean is already showing exceptional warming, and the forecasts are no longer routine.
In August the Met Office called the event developing in the Pacific likely the strongest in living memory. On 8 October the World Meteorological Organization reported that sea-surface temperatures were still rising, with changes in tropical rainfall bringing increased risks of drought in some regions and flooding in others.
El Niño is the warm phase of ENSO, the El Niño–Southern Oscillation. Normally, easterly trade winds push warm surface water towards Indonesia, where warm, moist air rises and produces heavy tropical rainfall.
During El Niño, those winds weaken. Warm surface water spreads back towards the central and eastern Pacific, the thermocline (the boundary between warmer surface water and colder water below) deepens in the east, and the main region of rising air and heavy rainfall shifts eastwards too. As that happens, the circulation of rising and sinking air across the tropical Pacific changes, often leaving Indonesia drier than usual.
La Niña is the opposite phase: the central and eastern equatorial Pacific become cooler than average, the trade winds strengthen, and the main region of tropical rainfall shifts farther west towards Indonesia. Between El Niño and La Niña, the tropical Pacific also spends periods in a neutral state, when neither phase is sufficiently developed.
La Niña, neutral, and El Niño
Schematic of El Niño and La Niña, showing changes in trade winds, ocean temperatures and tropical rainfall. Not to scale, and not a forecast.
The Pacific has already made that shift. In December 2025 the Niño-3.4 region, the equatorial strip from 5°N to 5°S and from 170°W to 120°W, was still cooler than average. By July 2026 it was warmer than average. The dashed box on the globes marks that region, while the colours show sea-surface temperature anomalies relative to the same calendar month in 1991–2020.
December 2025 and July 2026
Sea-surface temperature anomaly relative to the 1991–2020 monthly climatology. December 2025 and July 2026. The dashed box is Niño 3.4. ERA5. Generated using Copernicus Climate Change Service information 2026.
The animation follows the same temperature anomalies month by month, from July 2025 to September 2026. The colour inside the box turns from cool to warm. October 2026 is not in the loop, because that month is not complete in the reanalysis used here.
July 2025 to September 2026
Sea-surface temperature anomaly relative to the 1991–2020 monthly climatology. Each frame is one month. The dashed box is Niño 3.4. ERA5. Generated using Copernicus Climate Change Service information 2026.
The maps show the box turning warm. The index in the next section asks how much of that warmth stands out as El Niño, once the warming shared by the tropical oceans as a whole is removed.
In its 8 October discussion, NOAA's Climate Prediction Center reported a relative Niño-3.4 index of 2.1°C for September.
This is a relative index, comparing warming in the Niño-3.4 region with warming across the tropical oceans more generally. The index takes the sea-surface anomaly in the box and subtracts the anomaly of the tropical oceans as a whole. What remains is multiplied by one fixed factor, taken from the historical record, so a typical rise or fall has the same size as on the usual Niño-3.4 index. The factor is the same every month.
For example, if the sea-surface temperature anomaly in the Niño-3.4 region were 1.6°C, and the anomaly across the tropical oceans were 0.6°C, subtraction would leave 1.0°C. That remainder is quieter than the usual index, because the part shared by the whole tropics has been removed. If the historical factor were 1.2, the index would read 1.2°C.
The subtraction is there because the tropical oceans are also warming as the climate warms. Left in, that general warming would be mixed with the extra warmth in the Niño-3.4 box, and a warmer tropical ocean could be mistaken for a stronger El Niño. The Niño-3.4 region covers roughly 6 million square kilometres of the equatorial Pacific, about 25 times the area of the UK. NOAA's relative index for that region reached +2.06°C in September, rounded to +2.1°C in its October discussion.
Farther east, on the same relative measure, Niño-3 reached +3.0°C, and Niño-1+2, closest to the South American coast, reached +3.9°C. Some parts of the eastern equatorial Pacific were above +4°C.
Monthly relative Niño-3.4
Monthly relative Niño-3.4 index, July 2025 to September 2026. September's value is +2.06°C in the underlying data, rounded to +2.1°C in NOAA's October discussion. This is a monthly index, not the three-month average used for the historic threshold. Reference period: 1991–2020. OISSTv2.1. Data: NOAA Climate Prediction Center.
The warmth also extends below the surface. Further east, in parts of the equatorial Pacific, water below the surface is more than 10°C warmer than average. That 10°C belongs to a layer of water at depth in the east. Across a much wider region, from the date line to 100°W, the subsurface is also unusually warm, and the thermocline sits deeper than normal. This unusually warm subsurface water acts as a reservoir of heat, helping sustain elevated sea-surface temperatures as the ocean and atmosphere continue to interact.
The atmosphere is responding too. NOAA reports enhanced convection and rainfall over the central and eastern Pacific, while rainfall-producing convection has been suppressed over Indonesia. The changes in ocean temperatures are already influencing where tropical air rises and where the heaviest rainfall develops, and this is something that has already been observed.
NOAA estimates an 83% chance that it will reach historically exceptional strength during October–December 2026, potentially exceeding every El Niño in its record going back to 1950. That forecast uses the Relative Oceanic Niño Index, or RONI, which accounts for warming across the tropical oceans. To exceed the previous events in NOAA's record, that index would need to reach at least +2.5°C as a three-month average. September's +2.1°C is a single month, so the event would still need to sustain exceptional conditions over a longer period before it can beat the record.
The neighbouring seasons have lower, but still significant, probabilities of reaching that same historic strength: 54% for September–November, and 70% for November–January. NOAA also assigns a high probability to strong or very strong El Niño conditions persisting into January–March 2027. That is a different forecast from the 83% chance of reaching historically exceptional strength during October–December. The Center expects the event to keep strengthening through the end of the year.
El Niño doesn't create new heat. It changes how heat is distributed between the ocean and atmosphere, allowing some of the energy stored in the tropical Pacific to be released into the atmosphere. Celeste Saulo, secretary-general of the World Meteorological Organization, described that release as the reason a strong event so often becomes one of the hottest years measured, as with 2024. El Niño was only part of the picture. The planet was already warming because of rising greenhouse gas concentrations.
Another potentially exceptional El Niño is developing against that warmer background. The tropical oceans have warmed considerably over recent decades, and unusually warm water is already stored beneath the equatorial Pacific. This can help sustain the event and temporarily push global temperatures higher still. Changes in tropical rainfall also influence atmospheric circulation far beyond the Pacific.
The World Meteorological Organization's outlook for October–December gives an overwhelming likelihood of above-average temperatures across large regions, relative to its 1993–2009 reference period. That is not every place, and not every day. It is a higher chance of heat extremes. It also indicates shifts in seasonal rainfall, including wetter conditions along parts of the equatorial Pacific and over the Greater Horn of Africa, and drier conditions in parts of the Indian subcontinent and western Pacific. A wetter season is where the flood risk rises. A drier one is where water supplies and the growing season come under more pressure. During the strong 1997–98 El Niño, Peru and Ecuador experienced serious flooding, while Indonesia suffered drought and widespread fires.
But the same impacts don't repeat in exactly the same places every time. Those places show what a strong event has done. They are not a map of the next one. El Niño can increase the likelihood of drought or heavy rainfall in particular regions and seasons, without telling us exactly when or where an extreme event will occur.
Hardiman and his colleagues at the Met Office investigated how ENSO influences North Atlantic atmospheric circulation. Their work suggests that strong El Niño events can favour a wavelike pressure pattern, while a strong La Niña, which they examined in Met Office ensembles rather than in the historical record, can favour stronger west-to-east circulation. Even moderate and strong El Niño events can produce different responses.
The position and strength of the North Atlantic jet stream help determine where storms travel, how frequently Atlantic weather systems reach us, and whether persistent pressure patterns can develop. Changes in those circulation patterns can influence the likelihood of wet, stormy, mild or cold conditions during winter.
The North Atlantic winter jet
How El Niño and La Niña can influence the North Atlantic winter jet stream. These are changes in the odds of particular circulation patterns, not a seasonal forecast.
BBC Weather has suggested that autumn and early winter in the UK could be more likely to be wet and stormy. A Government Office for Science roundtable note is more cautious, identifying a slightly increased chance of storminess towards the end of 2026 and a slightly increased chance of cold snaps early in 2027.
ENSO is only one influence on North Atlantic circulation. Even knowing how strong El Niño will become cannot tell us exactly where the jet stream will sit in a particular week. What we already know is that the equatorial Pacific is unusually warm, substantial heat is stored beneath its surface, and tropical rainfall patterns are responding. NOAA's forecast suggests the event could strengthen further through the end of the year.
But predicting the strength of El Niño and predicting its consequences are two different problems. Even a historically exceptional event cannot tell us which country will experience a flood, when a storm will arrive, or whether Britain will have a particularly cold winter.
The Atlantic hurricane season provides another example. Early in October, Isaias became the first hurricane of 2026, as BBC Weather reported. El Niño tends to increase vertical wind shear over the tropical Atlantic, meaning winds change with height in ways that can disrupt the organisation of developing storms.
Yet Isaias formed anyway. Conditions vary from place to place, and where wind shear is weaker, storms can still organise into hurricanes. Lower odds are not zero odds. How the season adds up is the next review.
NOAA's next update is due on 12 November. By then, we'll have a clearer picture of whether this El Niño is on course to break historical records. But even if it does, that won't tell us exactly what Britain's winter will look like.
El Niño changes the odds. It doesn't write the forecast.
The previous review is Europe's extraordinarily hot summer of 2026. That summer is not assigned to this El Niño. The LinkedIn version of this El Niño review covers the same argument in less detail.
The next review is El Niño and the Atlantic hurricane season: why fewer storms doesn't mean no storms.
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