What El Nino actually is
El Nino gets talked about like a storm or a single weather event, but it isn't one. It's a months-long shift in ocean temperature and wind patterns across the tropical Pacific — specifically, warmer-than-average sea-surface water paired with weaker-than-average trade winds. NOAA declared El Nino conditions underway this year, issuing an El Nino Advisory on June 11, 2026 (upgraded from a Watch), and conditions strengthened through the summer. For the current status and index readings, see the live dashboard; this page is about the mechanics — what El Nino is, how it's measured, and why it matters.
Normal conditions, then the flip
In a typical year, trade winds blow steadily east to west along the equator, from South America toward Indonesia. Those winds push warm surface water westward, piling it up in the western tropical Pacific, while cooler, nutrient-rich water wells up from the deep ocean off the coast of South America to replace it. That's the baseline state the tropical Pacific spends most of its time in.
During El Nino, that pattern weakens. The trade winds relax, and without them holding the warm water in the west, it sloshes back eastward — spreading across the central and eastern tropical Pacific instead of staying pinned near Asia. The upwelling of cool water off South America weakens too, since the winds that drive it aren't blowing as hard. The net result is a big stretch of warmer-than-average ocean sitting where cooler water usually is, and an atmosphere that responds to it: rainfall patterns, tropical thunderstorm clusters, and wind belts all shift with the warm water. That ocean-atmosphere pair acting together is what climate scientists call ENSO (El Nino-Southern Oscillation) — El Nino is its warm phase. See the glossary for more on the individual terms.
How NOAA measures and classifies it
The patch of ocean NOAA watches most closely is called the Nino 3.4 region — roughly 5°N-5°S and 170°W-120°W, a strip of the central-to-eastern tropical Pacific. Sea-surface temperature there, compared to what's typical for that time of year, is the basis for NOAA's two main ENSO indices:
- ONI (Oceanic Nino Index) — a rolling 3-month average of the Nino 3.4 anomaly against a fixed historical baseline. NOAA's classic index, published continuously back to 1950.
- RONI (Relative Oceanic Nino Index) — the Nino 3.4 anomaly minus the tropical-mean (20°N-20°S) SST anomaly for the same period. NOAA adopted RONI as its operational ENSO index on February 1, 2026, specifically to correct for a long-term basin-wide ocean-warming trend that had been gradually inflating raw ONI values decade over decade. ONI is still published alongside it.
For the full story on why NOAA runs both, and what it means when they don't match for the same season, see ONI vs. RONI, explained. What matters here is the threshold both indices share: NOAA calls El Nino when the anomaly reaches +0.5°C or higher, and grades its strength from there.
| Band | ONI / RONI anomaly |
|---|---|
| Weak | +0.5°C to +0.99°C |
| Moderate | +1.0°C to +1.49°C |
| Strong | +1.5°C to +1.99°C |
| Very strong | +2.0°C or higher |
A single warm season isn't enough to call an "official" El Nino event in NOAA's historical record — that requires the anomaly to stay at or above +0.5°C for at least five consecutive overlapping 3-month seasons (the raw index table is published by CPC as the ONI series). NOAA's Climate Prediction Center (CPC) also publishes a monthly ENSO Diagnostic Discussion, issued on the second Thursday of each month, which is where the official Watch/Advisory alert level and the outlook for coming seasons gets updated. See the glossary for what "Watch" and "Advisory" mean.
El Nino, La Nina, and the state in between
El Nino is one of three ENSO phases. La Nina is the mirror-image cool phase — stronger trade winds, cooler-than-average Nino 3.4 water — and ENSO-neutral is what NOAA calls it when the anomaly sits inside that +/-0.5°C band, neither warm nor cool enough to count as either phase. We cover the direct comparison, including a side-by-side table of how the two phases tend to affect US weather, on El Nino vs. La Nina.
Why it matters for US weather
El Nino doesn't cause any single storm, but it loads the dice on the broader pattern for an entire winter. The composite tendency — based on how past El Nino winters have behaved on average, not a guarantee for any one year — is a Pacific jet stream that shifts south and strengthens, steering more storms toward the southern tier of the country (California, the Southwest, the Gulf Coast) while the northern tier (the Pacific Northwest, Northern Rockies, Northern Plains) tends to see a warmer, drier signal. Every event is still its own event: 2015-16 was one of the strongest El Ninos on record and famously underdelivered on the wet signal in Southern California, a reminder that "tendency" is not "certainty."
NOAA's seasonal winter outlook — the temperature and precipitation probability maps most people actually want — typically comes out around mid-October, the first official "what will my winter look like" product of the season. We track it, along with region-by-region detail, on the Winter Forecast 2026-27 page and the regional impacts hub, which breaks out expected tendencies for all ten regions this site covers, from California to the Northeast.
A gap we're trying to fill
NOAA's own plain-language ENSO explainer, the long-running ENSO Blog on climate.gov, lost its content team in May 2025 and has largely sat still since. That's a real loss — it was a great resource. Nino34 is an attempt to fill some of that gap with a live-data-driven approach: current index readings pulled straight from CPC, plain-language explainers like this one, and a policy of never printing a stale number without saying so.
FAQ
Is El Nino good or bad?
Neither, universally. El Nino shifts the odds, and which way those odds tilt depends entirely on where you live. A wetter tendency that eases drought and refills reservoirs in the Southwest can also mean a higher risk of flooding and debris flows in the same places. A quieter Atlantic hurricane season is good news for the Gulf Coast and Southeast, but the flip side is often reduced mountain snowpack across the northern tier. There's no single verdict — see your region's page for what the tendency actually looks like locally.
How long does El Nino last?
Most El Nino events run somewhere in the neighborhood of 9 to 12 months, typically peaking in Northern Hemisphere winter (December-February) and fading through spring. Some are shorter, and some stretch on longer. NOAA doesn't count a stretch as an official "event" in its historical record until the anomaly has held at or above the +0.5°C threshold for at least five consecutive overlapping 3-month seasons.
What causes El Nino?
It starts with a weakening of the Pacific trade winds. Normally those winds pile warm water in the western Pacific and let cooler water rise off South America. When the winds relax, that warm water spreads back east across the central and eastern Pacific instead of staying pinned in the west. The warmer water then further weakens the winds that would otherwise push it back — a reinforcing feedback loop between ocean and atmosphere that can sustain itself for months.
How is El Nino different from La Nina?
They're opposite phases of the same cycle: El Nino is warmer-than-average Nino 3.4 water and weaker trade winds; La Nina is cooler-than-average water and stronger trade winds. Their US weather tendencies run roughly opposite too. See El Nino vs. La Nina for the full comparison, including a side-by-side table.
Related: