Pacific Heat

How it compares with the strongest events

Satellites have measured Pacific temperatures weekly since 1981. At the end of September 2026, Niño 3.4 was 3.2°C above the 1991–2020 average, the highest reading in that series. The previous record, +3.0°C, was set on November 18, 2015, during that El Niño. In 2026 the Pacific matched it by mid-September, two months earlier in the season.

But the comparison with the average of 1991–2020 is not entirely fair: the whole tropical ocean is warmer today than it was 30 years ago. Hence, NOAA has been using relative values since February 2026, from which the average deviation of the entire tropical zone has been subtracted. Transfer the graph to a relative scale: 2026 is also headed for this time of year, but only just before 1982 and 1997.

How El Niño develops

El Niño is a shift in which changes in the ocean and atmosphere reinforce each other. In an ordinary year, easterly trade winds push warm surface water towards Indonesia. Sea level there is about half a metre higher than off Ecuador, while cold, nutrient-rich water rises from depth along the South American coast.

When the water falls, the warm water goes back to the east. The cold water no longer springs, the east warms, the pressure difference across the ocean decreases and grazes even more. Here the feedback 1969. described Norwegian meteorologist Jacob Bjerknes. The name is much older: Peruvian fishermen called El Niño, a child Jesus, a warm current that appeared along the coast around Christmas and with which the fish disappeared.

A new measure for a warmer ocean

For decades NOAA tracked El Niño using the Oceanic Niño Index (ONI), the three-month average temperature anomaly in the Niño 3.4 region relative to a 30-year baseline. As the tropical ocean warms, that baseline lags behind. El Niño looks stronger and La Niña weaker than the atmosphere’s response suggests.

As of February 2026, the official indicator is the relative index of RONI. It subtracts from the derogation in Niño 3.4 the average deviation of the whole tropical zone, between 20° N and 20° N, and the difference scales so that it has the same variability as ONI. The thresholds are the same: El Niño starts at +0.5 °C and is very strong from +2.0 °C.

According to RONI, the strongest El Niño from 1950 was the one from 1982–83, with +2,40 °C. El Niño 2023–24, which by ONI reached +2,0 °C, has +1,4 °C, because much of the heat then was the heating of the whole ocean, not El Niño himself.

How much stronger could it get?

The forecasts point to a stronger El Niño by the end of the year. NOAA puts the chance of the Relative Oceanic Niño Index (RONI) reaching at least +2.5°C for October–December at 83%. That would exceed every El Niño since 1950. NOAA’s median forecast for the period is +2.74°C.

The IRI model set, published on September 21st, is even more determined. All models keep El Niño very strong at least until February, and their average reaches +3.4 °C in the classic index without subtracting the tropical average. After the peak follows a rapid weakening: by summer 2027, the most likely are neutral conditions, 70 percent for May – July. After El Niño 1982 – 83, 1997–98, 2015–16 and 2023–24 followed La Niña, but current forecasts do not reach that far.

Global impacts in 2026

El Niño shifts jet streams and major rain belts, bringing drought to some regions and flooding to others. Some effects have already emerged. The largest impacts are expected from November to March, when El Niño is strongest.

The record year is still ahead

El Niño redistributes heat. As warm water spreads across the Pacific surface, it releases heat into the atmosphere. Global temperatures tend to rise several months after El Niño peaks, with Carbon Brief estimating a lag of around three months.

It is already hot. August 2026 evened July 2023 as the hottest month in the history of measurement, 1.65 °C above pre-industrial level, and the temperature of the world's oceans on August 22nd reached a record 21.1 °C.

The main effect is still to follow. Carbon Brief from July, 2027. will be around 1.71 °C warmer than pre-industrial period, with 92 as a percentage of the chance to become the hottest year in the history of measurement. The current record holds 2024., a year after El Niño 2023.–24., with 1.60 °C.

What about Croatia?

El Niño’s effect on European weather is weak and variable. Ivan Güttler, head of Croatia’s meteorological service (DHMZ), told Novi list in May that the influence is less pronounced in Croatia and the Mediterranean than in other parts of the world. In winter it could bring more moisture and storms to southern Europe and the Mediterranean, while northern Europe could see more outbreaks of cold air.

The review by Stefan Brönnimann of 2007 shows that the strong El Niñi in the late winter is often followed by the negative phase of the North Atlantic oscillation, along with the colder north and weter south of Europe. The signal is weak and easily overcome by other factors, so it cannot be taken as a forecast for this winter.

Gütler warned that the strong El Niño could temporarily raise global temperature above 1.5 °C and make 2026. and 2027. the hottest in the history of measurement. The European Centre for Medium-term Forecasts (ECMWF) season forecast from August for Croatia gives a very likely warmer autumn and the beginning of winter than average.

From La Niña to record warmth in nine months

The Pacific started 2026 in the opposite phase, La Niña. As late as February, NOAA still expected a quiet summer. Here is how forecasts and measurements changed.

Common misconceptions

Sources

Maps and graphs are made from publicly available NOAA, Copernicus and IRI data. RONI values for the last months of NOAA can still be corrected.