El Niño Has Arrived: The Pacific Crosses the Threshold

For the first time since the 2025–26 La Niña unwound, the central Pacific is officially running warm. Weekly readings in the Niño 3.4 region crossed the +0.5°C anomaly threshold in early May and have held above it since — the working definition of El Niño-level ocean conditions. The dashboard now shows the warm anomaly spreading east along the equator.

The turnaround has been quick. La Niña faded in February–March; April's readings sat near zero — a neutral ocean at the surface. But neutral was never the full picture. Beneath the surface, ocean-heat surveys through the spring showed a substantial pool of warmer-than-normal water sliding eastward along the thermocline — the classic loaded spring that precedes El Niño onsets. Downwelling Kelvin waves have been delivering that heat to the surface of the central and eastern Pacific through April and May.

What has to happen before a declaration

Warm water alone does not make an El Niño — 2014 taught everyone that lesson, when a promising warm spring fizzled after the atmosphere declined to participate. NOAA's checklist for an official El Niño Advisory has three parts.

First, the ocean: monthly-scale Niño 3.4 anomalies at or beyond +0.5°C. That box is effectively ticking now.

Second, the atmosphere: evidence the tropical circulation has coupled to the warm water. Forecasters look for slackening trade winds, thunderstorm activity migrating from Indonesia toward the dateline, and the Southern Oscillation Index — the Tahiti–Darwin pressure seesaw — turning negative. Early signs of each are appearing, but forecasters will want another month of consistency.

Third, persistence: an expectation that the pattern lasts. Model guidance is unusually united on this point, with dynamical models strengthening the event through summer and autumn.

If the atmospheric signals hold through the next few weeks, a formal Advisory would be the natural outcome of the June diagnostic cycle. We will cover it the day it lands.

What the models see for autumn

The May model suite points one direction: up. Dynamical forecasts favor continued strengthening through the June–August season, with the usual autumn ramp toward a Northern Hemisphere winter peak. It is too early for responsible numbers on peak intensity — May forecasts still carry the residue of the spring predictability barrier — but the combination of observed surface warming, abundant subsurface heat and early atmospheric cooperation is the strongest onset setup the Pacific has offered in years.

The historical company this setup keeps is notable. Springs that flipped rapidly from La Niña toward El Niño with a loaded subsurface — 1997 is the canonical case, 2015 another — went on to produce major events. That is a base-rate observation about past analogs, not a forecast; plenty of moderate events also started briskly. The event page tracks how 2026 compares as the record fills in.

For readers planning around the pattern, the practical takeaway this early is about direction, not magnitude: assumptions calibrated to the past two years of La Niña — active Atlantic hurricane seasons, dry US Southwest winters, wet eastern Australia — should be re-examined now rather than after a declaration. The phase comparison lays out which planning defaults flip, and our regional guides cover what a warm-phase winter has historically meant market by market and coast by coast.

What to watch in June

Three markers will define the next update. Whether weekly Niño 3.4 anomalies push toward +1.0°C, which would signal an unusually fast intensification. Whether the SOI locks in negative and the rainfall shift consolidates — the atmosphere's committed vote. And what the June official outlook says about intensity odds for winter, the first set of numbers issued substantially clear of the spring barrier.

For a primer on the difference between the phase we just left and the one we appear to be entering, see El Niño vs La Niña. The weekly numbers, as always, run on the live dashboard.

Bottom line

The ocean has crossed the El Niño line, the atmosphere is showing early signs of following, and the models want to strengthen it from here. May forecasts deserve humility — but this is what the start of a significant El Niño looks like.