El Niño explained
How a warmer Pacific changes the weather
El Niño · La Niña · Winter effects
A visual guide to the ocean pattern behind the headlines.
What is El Niño?
El Niño is the warm phase of a natural Pacific Ocean climate pattern. Surface water in the central and eastern tropical Pacific becomes unusually warm, and winds and tropical rainfall change with it. Those changes can shift weather far beyond the ocean.
The whole pattern is called ENSO: the El Niño–Southern Oscillation. Its cool phase is La Niña; between them are neutral conditions. Think of it as a change in the background conditions that steer weather over a season.
A change in the winds moves the warm water
Read each diagram from west (Asia) to east (the Americas). Orange represents warm surface water; blue represents cooler water beneath it. These are simplified cross-sections, not maps or temperature measurements.
Neutral
El Niño
La Niña
How does that reach land?
Warm ocean water helps air rise and clouds form. When that warm water and rainfall shift east, they change the atmosphere’s circulation. The jet stream is a fast-moving river of air high above us. It can change position, steering storms along different paths.
That is why a change in the tropical Pacific can influence a winter thousands of miles away. The effect is a shift in the odds of certain conditions, not a promise about any one storm.
El Niño and La Niña do not run on a schedule
Above zero: relatively warm conditions. Below zero: relatively cool conditions. The irregular spacing and different-sized peaks show why the next event cannot be predicted by simply counting years.
What does the number mean?
The Relative Oceanic Niño Index (RONI) compares a key patch of the Pacific with its usual conditions, accounting for temperature changes across the wider tropics. The index is expressed in temperature units. It does not represent the ocean’s actual temperature or the change to expect in your city.
NOAA uses thresholds of ±0.9°F. Its historical warm and cold episodes must meet the threshold for at least five overlapping three-month periods. Scientists also check that the atmosphere is responding. A single point above zero does not, by itself, mean an El Niño event.
NOAA’s RONI definition and complete record · Download all 432 plotted values
What does El Niño mean for U.S. winter weather?
ENSO’s U.S. influence is usually clearest in winter. These are broad historical tendencies compared with a region’s usual winter, not a local forecast. Event strength, other climate patterns and individual storms all matter.
Typical winter tendencies · December–February
| Region / pattern | El Niño | La Niña |
|---|---|---|
| Northern U.S. | Often warmer | Often colder |
| Southern U.S. | Often wetter | Often drier |
| Pacific Northwest | Tends drier | Tends wetter |
| Pacific jet stream | Shifts south / extends east | Often farther north |
“Wetter” does not mean rain every day, and “warmer” still allows cold snaps. There can also be large differences within these broad regions.
NOAA’s winter pattern maps and explanation
Start with your city’s normal climate
Seasonal tendencies make more sense with a baseline. Compare typical monthly temperatures and rainfall in Seattle, Chicago or Miami, or find your city. These climate averages are not ENSO-adjusted forecasts.
Common El Niño questions
How long does El Niño last?
An episode typically lasts about nine to twelve months, although some persist longer. El Niño and La Niña recur irregularly, roughly every two to seven years. They do not alternate on a fixed calendar.
Does El Niño mean a snowy winter?
Not everywhere. Snow requires both moisture and cold air. A wetter southern storm track can bring rain instead of snow, while milder northern conditions can limit snow. ENSO alone cannot tell you how much snow your town will get.
Is El Niño the same as climate change?
No. El Niño is a naturally recurring exchange of heat between the tropical Pacific Ocean and atmosphere. It can temporarily raise global surface temperatures. Human-caused climate change is a long-term warming trend; the two can occur together.
Is there an El Niño right now?
Check NOAA’s latest ENSO Diagnostic Discussion for the current status, issue date and outlook. The historical chart on this page ends in 2025 and is not a live status indicator. Read the latest NOAA ENSO discussion.
Sources, chart method and limitations
Written and illustrated by GraySky. Source review: . The Pacific diagrams summarize NOAA’s explanations; their arrow lengths and water depths are schematic, not measured data.
The chart uses NOAA CPC’s ERSSTv6 RONI, retrieved 2026-09-05, for every overlapping season from DJF 1990 through NDJ 2025. DJF means December–January–February; the year is the central month’s year, so NDJ 2025 includes January 2026. The baseline is 1991–2020. RONI removes the tropical-mean anomaly from Niño 3.4 and rescales the result to match the original index’s variability.
The line connects the published values without extra smoothing. Both sides of zero use the same scale. Fahrenheit differences are Celsius differences multiplied by 1.8, with no 32-degree offset. NOAA may revise its historical record; this page keeps a dated snapshot. No current-event forecast is inferred from it.
NOAA CPC data · NOAA ocean science · GraySky climate sources and methods