When Was the Last Super El Niño? The 2023–24 Event and the Physics Behind It

The last super El Niño peaked in late 2023 and faded by mid-2024. NOAA's Oceanic Niño Index (ONI) reached 2.0 °C for November–January 2023–24, tied with 2015–16 as the strongest event since 1950 by that measure. On the relative Niño 3.4 index, which corrects for the basin-wide warming trend, 2023–24 ranks below 1997–98 and 2015–16. That correction is why scientists still debate whether 2023–24 was truly "super."

What "super" actually measures

There is no official "super El Niño" category. The term entered circulation after the 1982–83 and 1997–98 events and is now loosely applied to events with ONI (or relative ONI) at or above roughly 2.0 °C in the Niño 3.4 region.

Niño 3.4 spans 5°N–5°S, 170°W–120°W. It is the canonical monitoring box because it captures the eastern-central Pacific where coupled ocean-atmosphere anomalies are most coherent. ONI is the three-month running mean of ERSSTv5 sea surface temperature (SST) anomalies in that box, relative to a rolling 30-year baseline.

The relative ONI subtracts the tropical-mean SST anomaly, removing the background warming signal that has raised baseline temperatures. That matters: an ONI of 2.0 °C in 2023–24 is not the same physical anomaly as 2.0 °C in 1982. NOAA's operational El Niño definitions use ONI ≥ 0.5 °C for five overlapping seasons; that threshold is for "El Niño," not "super."

The mechanism, one level deeper

El Niño is a disruption of the Walker circulation, not merely warm water. The tropical Pacific normally sustains a west-to-east pressure gradient: high over the western warm pool, low over the cooler east. Easterly trade winds pile warm water west and drive upwelling of cold, nutrient-rich water off Peru.

During El Niño, weakened trades let the warm pool surge east. Convective heating follows the warm water, shifting the atmospheric rising branch from the Maritime Continent toward the central Pacific. The resulting Rossby wave train alters jet streams and storm tracks well beyond the tropics.

Bjerknes feedback amplifies the shift. Weakened trades reduce east-Pacific upwelling, which warms the east, which further weakens the trades. Westerly wind bursts and oceanic Kelvin waves carry the signal eastward, deepening the thermocline and sustaining the anomaly across seasons.

The event terminates when warm water discharges poleward and the thermocline rebounds, often via a recharge-discharge cycle. The 2023–24 event decayed through boreal spring 2024 as equatorial heat content fell and easterlies returned.

A counterintuitive detail: 2023–24's relative ONI was lower than its raw ONI suggests, because the tropical mean was itself anomalously warm. Raw ONI exaggerates event strength in a warming climate; relative indices are more comparable across decades.

How the big events compare

Different indices rank events differently, so "strongest" depends on the metric. The table compares the major post-1980 events.

Values are approximate and vary by dataset and averaging window. The table's point: 2015–16 and 1997–98 remain the benchmark super events on most raw measures, while 2023–24 was comparable in ONI but weaker in relative terms.

Timeline of the 2023–24 event

The sequence below traces the event from onset to decay, illustrating how El Niño develops and ends.

The event's rapid intensification from spring to fall 2023 was unusually fast. Forecasters noted the speed exceeded typical onset rates, complicating seasonal outlooks.

Where the "super" label creates confusion

Several misconceptions persist. The strongest is conflating El Niño strength with local impacts. A stronger ONI does not guarantee stronger California rainfall or weaker Atlantic hurricanes; atmospheric teleconnections depend on event flavor, background state, and timing.

Central-Pacific ("Modoki") events warm the central Pacific more than the east, producing different rainfall and storm-track responses than canonical east-Pacific events. The 2009–10 event was Modoki-flavored, which is why its impacts diverged from 1997–98 despite overlapping SST magnitudes.

Red flags when reading claims about "super" events: A single index quoted without naming the dataset. Raw ONI presented as if comparable across decades. "Strongest ever" claims without specifying which metric. Impact claims generalized from a different event's flavor. Seasonal forecasts framed as deterministic rather than probabilistic.

Another misconception is that a super event guarantees record global temperatures the same year. The lag is real but variable; 2023–24 and 2015–16 both contributed to record or near-record annual warmth, but aerosol, volcanic, and internal variability also matter.

Practical use of El Niño information

For planners, farmers, water managers, and emergency officials, the useful question is not the label but the probability shift in regional outcomes. Concrete steps:

Treat seasonal outlooks as probability distributions, not predictions; NOAA CPC issues them with confidence intervals. For any given region, check the historical base rate for the relevant season before applying an El Niño multiplier. Distinguish onset, peak, and decay timing; impacts often persist a season beyond the SST peak. Use multiple indices (ONI, relative ONI, Niño 1+2, SOI) to triangulate event flavor. Reconcile thresholds explicitly: ONI ≥ 0.5 °C defines El Niño; "strong" is conventionally ≥ 1.5 °C; "very strong" or "super" is loosely ≥ 2.0 °C. Build contingency plans against the historical range of super-event impacts, not the average.

The thresholds are conventions, not physical boundaries. A 1.9 °C event is not categorically weaker than a 2.0 °C event. The convention exists to standardize communication, and different agencies apply it slightly differently.

Limitations of the record and forecast

The instrumental record is short. Reliable SST coverage of the tropical Pacific spans roughly 1950 onward, with sparser pre-satellite data. The 1982–83 event was the first observed with modern tools, and reanalysis products differ in how they reconstruct older events.

Paleoclimate proxies, such as corals and lake sediments, suggest super events occurred before 1950, but their magnitudes and frequencies carry substantial uncertainty. Claims about "the strongest El Niño in 1,000 years" rest on proxy calibrations that remain debated.

Forecast skill also matters. El Niño onset is more predictable than its peak amplitude and regional impacts. The 2023–24 event's rapid intensification illustrates that forecasting the exact peak remains difficult even with modern models.

A final caveat: the 2023–24 event coincided with an unusual background state, including record global SSTs and stratospheric water vapor from the 2022 Hunga Tonga eruption. Attributing specific outcomes to El Niño alone requires separating those influences, which is an active research problem. The label "super" describes one index, not a full causal story.

For the practical question that opened this article: the last super El Niño, on raw ONI, peaked in November 2023–January 2024 at about 2.0 °C, tied with 2015–16 by that measure and below 1997–98 on relative indices. The next such event cannot be predicted with confidence years ahead; forecasts gain skill only as ocean heat content and wind anomalies accumulate in the months before onset.