When Was the Last El Niño? The 2023–24 Event and What Comes Next

The most recent El Niño was the 2023–24 event. It officially began in June 2023, peaked as a strong episode between November 2023 and January 2024, and ended by late April or May 2024, when the tropical Pacific returned to neutral conditions. By June 2024, the Pacific had swung toward La Niña, which persisted into 2025. That sequence — a strong El Niño followed quickly by La Niña — is unusual but not unprecedented, and it matters for anyone trying to understand current weather patterns.

This explainer goes deeper than the usual "El Niño is warm water" summary. It covers the physical mechanism, the numbers that define strength, the edge cases that break simple rules, and how forecasters actually decide when an event starts and stops.

The mechanism: why the Pacific flips

El Niño is not simply "warm water in the Pacific." It is a coupled ocean-atmosphere oscillation. The ocean and atmosphere feed each other, which is why the event can persist for months instead of fading in weeks.

In neutral conditions, trade winds blow west across the equatorial Pacific. They pile warm surface water in the western basin near Indonesia and Australia. That warm water fuels rising air and heavy rain there. Meanwhile, cold water upwells along the South American coast, and the east-west temperature contrast drives the atmospheric circulation.

During El Niño, the trade winds weaken. Sometimes they reverse. The warm water pool sloshes eastward, and the thermocline — the boundary between warm surface water and cold deep water — flattens or deepens in the east. Upwelling weakens off Peru and Ecuador.

The atmosphere responds within weeks. Convection shifts east, and the Walker circulation — the east-west atmospheric loop over the Pacific — weakens or reverses. The result is a downstream chain of weather changes: wetter conditions in the central and eastern equatorial Pacific, drier conditions in Indonesia and Australia, and shifted storm tracks that affect North America, South America, and Africa.

Why does one event end? The warm water eventually discharges eastward, and the system overcorrects. Cold water surges back along the equator, sometimes producing La Niña. This recharge-discharge cycle is why El Niño and La Niña often alternate.

What "last El Niño" means by different thresholds

The word "last" depends on the threshold you use. That is the single biggest source of confusion in thin explainers.

NOAA's official definition uses the Oceanic Niño Index, or ONI. The ONI is a three-month running average of sea-surface temperature anomalies in a specific central-Pacific region (the Niño-3.4 box). An El Niño requires ONI values of at least +0.5°C in overlapping three-month seasons, typically for five consecutive overlapping seasons.

Here is where it gets slippery. The 2023–24 event crossed that threshold in June 2023 and stayed above it through roughly April 2024. But some events briefly touch +0.5°C without qualifying as a full episode. Weak events can be borderline. And the 2018–19 event, for instance, was a weak El Niño that many people barely noticed.

There is also a distinction between ONI and a separate index, the relative ONI (RONI), which subtracts the tropical-mean warming trend. Some recent borderline events look stronger on ONI than on RONI. Forecasters increasingly discuss this, though there is no single official switch to RONI.

The practical takeaway: when someone asks "when was the last El Niño," the defensible answer is the 2023–24 event. A pedant might say "the last one that met NOAA's full criteria." Both are correct.

The 2023–24 event by the numbers

The 2023–24 El Niño was strong but not record-breaking on most measures, and its timing was unusual.

Two things stand out. First, the event developed quickly out of a long La Niña. Second, the eastern Pacific (Niño-3 region) warmed more than the central Pacific at times, which is more typical of a "canonical" eastern-Pacific El Niño. That distinction affects which regions see which impacts.

A counterintuitive detail: a strong El Niño does not guarantee strong impacts everywhere. The 2023–24 event produced notable global effects, but the atmospheric response lagged the ocean in places, and some expected regional signals were muted. Forecasters track the atmosphere, not just the ocean, for that reason.

Timeline: how the 2023–24 event unfolded

That table is the honest picture. The event was not a single moment. It was a roughly year-long arc with a defined peak.

Misconceptions that keep circulating

Misconception one: "El Niño means a warm winter everywhere." False. El Niño shifts probabilities, not certainties. It tends to favor warmer-than-average winters in parts of northern North America and wetter conditions in the southern tier, but individual winters vary widely.

Misconception two: "Every El Niño is the same." Wrong. Eastern-Pacific events and central-Pacific (Modoki) events have different impact patterns. The 2023–24 event leaned eastern-Pacific at its peak, which changes the downstream teleconnections.

Misconception three: "The last El Niño was 2015–16." That was the last very strong one before 2023–24, but it is not the last. The 2018–19 event was weak, and the 2023–24 event was the most recent qualifying episode.

Misconception four: "El Niño causes a specific storm." No single storm can be attributed cleanly to El Niño. It changes the odds of certain patterns over a season.

Red flags to watch for in coverage:

Claims that a single event "proves" El Niño or La Niña is over — look for the three-month ONI and the official status. Forecasts that confuse ocean temperature with atmospheric response. Maps that generalize one El Niño's impacts to all El Niños. Stories that call every warm patch "El Niño" without the threshold.

Practical actions for reading El Niño news

You do not need to be a climatologist to interpret coverage well. These steps help.

Check the source's definition. NOAA and Australia's Bureau of Meteorology use different thresholds, and that explains many apparent contradictions. Look for the ONI value and the season it covers, not just a headline. Distinguish ocean state from atmospheric response — forecasters watch both. Treat seasonal outlooks as probabilities, not predictions. For the current state, check the latest official ENSO discussion rather than a dated article. Do not attribute a single storm or a single hot month to El Niño without context.

A concrete threshold to remember: +0.5°C on the ONI for five overlapping seasons is the common bar. Anything below that is not an official El Niño, even if the water looks warm.

What "next" looks like and the limits of prediction

As of the most recent official assessments available for this explainer, the Pacific has been in a La Niña-leaning or neutral state following the 2023–24 El Niño. ENSO forecasts typically extend about 9 to 12 months with meaningful skill, and skill drops sharply beyond that.

Forecasters use ocean heat content, trade wind anomalies, and coupled model ensembles. Even so, spring is a notoriously difficult time to forecast ENSO — the "spring predictability barrier" — because the ocean-atmosphere coupling is weakest then. A forecast made in March or April deserves more caution than one made in autumn.

The honest limitation: no article can tell you with certainty when the next El Niño will begin. What it can tell you is what to watch for — sustained ONI values above +0.5°C, weakened trade winds, and a eastward-shifting warm pool. Those are the ingredients, and they are measurable.

This article is general educational information about climate patterns, not a forecast or a basis for financial, agricultural, or safety decisions. For decisions that depend on seasonal outlooks, consult your national meteorological service and a qualified local professional.