What is El Niño? And La Niña, explained

El Niño, often written El Nino, is a natural, periodic warming of the surface of the central and eastern tropical Pacific Ocean, together with a weakening of the winds that blow across it. La Niña is the opposite: cooler water and stronger winds. The two are phases of one pattern, the El Niño–Southern Oscillation, or ENSO. Each phase returns every two to seven years and reshapes rain, temperature and storms far beyond the Pacific.

In short

• El Niño means warmer than usual water in the eastern tropical Pacific and weaker trade winds; La Niña means cooler water and stronger winds.
• Because the Pacific is so large, each phase shifts where tropical storms of rising air form, and that moves weather patterns around the world.
• NOAA counts a full El Niño or La Niña episode when a Pacific temperature index stays 0.5°C or more above or below normal for five overlapping three-month periods.

What is El Niño, exactly?

Normally, the trade winds near the equator blow from east to west across the Pacific. They push warm surface water toward Indonesia and Australia, where it piles up, and cold, nutrient-rich water rises from the deep along the coast of South America to replace it. That rising of cold water is called upwelling. So in a normal year the western Pacific is warm and wet, and the eastern Pacific is cool and dry.

In an El Niño, the trade winds weaken. Warm water sloshes back east, upwelling off Peru and Ecuador weakens or stops, and the warm, rainy zone of rising air shifts toward the central and eastern Pacific. The ocean and the air feed each other: warmer water weakens the winds further, and weaker winds let the water warm more. The name comes from South American fishermen, who noticed warm coastal water around Christmas and named it after the Christ child, El Niño, "the boy child" in Spanish.

The other half of the name, the Southern Oscillation, is the matching swing in air pressure across the Pacific, described by the British scientist Gilbert Walker in the 1920s. Only later did scientists connect his pressure see-saw to the ocean's warmings.

How is La Niña different?

La Niña is an exaggerated version of the normal state. The trade winds blow harder than usual, more warm water piles up in the west, and upwelling in the east strengthens, so the eastern Pacific runs colder than normal. A third state, neutral, is when neither has taken hold.

	El Niño	Neutral	La Niña
Eastern tropical Pacific	Warmer than normal	Near normal	Cooler than normal
Trade winds	Weaker	Normal	Stronger
Upwelling off South America	Weak or stopped	Normal	Stronger
Atlantic hurricanes	Usually suppressed	No push either way	Usually enhanced
Typical U.S. winter	Wetter South, milder North	Varies	Drier South, colder and wetter Northwest

According to NOAA's Ocean Service, El Niño is more common than La Niña, and episodes usually last nine to 12 months, though some persist for years.

How does a Pacific warming change weather elsewhere?

At the equator, the Pacific stretches almost halfway around the Earth. When its warmest water moves, the great columns of rising, rain-making air move with it, and that shifts the jet streams that steer storms over North America, South America, Asia and Africa. The effects are tendencies, not guarantees:

• Rain and drought. El Niño tends to bring heavy rain and flooding to coastal Peru and Ecuador and to the U.S. Gulf Coast and Southeast in winter, and drier conditions to Indonesia, Australia and parts of southern Africa. La Niña tends to bring drought to the southern United States and heavy rain to the Pacific Northwest.
• Hurricanes. El Niño strengthens the winds high over the Atlantic, which tears developing storms apart, so Atlantic seasons tend to be quieter, while the central and eastern Pacific see more storms. La Niña does the reverse. NOAA's Climate.gov explains that the key is wind shear, the change in wind with height. More on that in how hurricanes form.
• Fish. With less upwelling, fewer nutrients reach the surface off South America, plankton thin out and fisheries such as Peru's anchovy catch can collapse.
• Global temperature. El Niño releases heat from the ocean into the air, so El Niño years tend to set global temperature records and La Niña years run cooler. This is a swing on top of the long-term rise from climate change, not a cause of it.

Links to other weather are weaker. Researchers have studied whether ENSO affects U.S. tornado seasons, and any connection is modest and inconsistent. No single storm or flood can be pinned on El Niño alone; it changes the odds over a season.

How do forecasters track it?

Forecasters watch sea surface temperatures in a patch of the equatorial Pacific called the Niño-3.4 region, along with winds, air pressure, rainfall and the temperature of water below the surface, measured by buoys and satellites. NOAA's Climate Prediction Center publishes a diagnostic discussion every month and runs an alert system: an El Niño Watch or La Niña Watch when one is likely to form, and an Advisory once it has.

Two rules apply. In real time, NOAA issues an Advisory once the monthly anomaly, the departure from normal, has reached +0.5°C for El Niño or −0.5°C for La Niña, the atmosphere is responding, and the change is expected to last. To count in the historical record as a full episode, the three-month average must stay past that threshold for five overlapping three-month periods. In 2026 NOAA changed how that anomaly is measured, from the Oceanic Niño Index (ONI) to the Relative Oceanic Niño Index (RONI), because the whole tropical ocean is warming and the old baseline kept falling behind.

A worked example shows the idea, leaving out RONI's final rescaling step. Suppose the Niño-3.4 region is 1.2°C warmer than its 30-year average:

1. Under the old ONI, the anomaly is simply +1.2°C, a moderate El Niño.
2. But suppose the whole tropical belt, from 20°S to 20°N, is also running 0.4°C warm.
3. RONI asks how much warmer Niño-3.4 is than the tropics around it: 1.2 − 0.4 = +0.8°C.
4. That still passes the +0.5°C threshold, but it is a weaker El Niño than the old index suggested.

What drives the weather effects is the contrast between the Pacific and the rest of the tropics, so the relative measure tracks impacts better. NOAA reclassified past events with it, and some shifted: the 2023–24 El Niño peaked at +1.5°C on RONI against +2.1°C on the old index, and three weak El Niño winters, including 2014–15 and 2019–20, dropped off the list.

Is it El Niño or La Niña right now?

As of NOAA's diagnostic discussion of 8 October 2026, an El Niño Advisory was in effect and El Niño was strengthening, with the monthly Niño-3.4 anomaly at +2.1°C in September. NOAA put the chance of a strong to very strong El Niño lasting through January–March 2027 at more than 83%, and gave an 83% chance that October–December would pass +2.5°C on RONI, stronger than any event in its record back to 1950. The next update was scheduled for 12 November 2026. A phase can shift within months, so check the latest discussion before relying on this.

Past strong El Niños give a sense of scale. The events of 1982–83, 1997–98 and 2015–16 each brought severe floods to parts of South America, drought to parts of Southeast Asia and Australia, and record or near-record global temperatures.

Still argued

How climate change will alter ENSO is not settled. Some climate models project more frequent extreme El Niños as the planet warms; others show little change, and models disagree on whether the Pacific's long-term warming pattern looks more like El Niño or La Niña. The record of reliable observations, about 75 years, is short for judging a pattern that returns every few years, so researchers also use corals and other natural records to reconstruct older events.

Check yourself

• Without looking back: what happens to the trade winds and to upwelling off South America in an El Niño, and why does each make the other stronger?
• Why does El Niño usually mean a quieter Atlantic hurricane season?
• Why did NOAA switch to measuring the Pacific against the rest of the tropics instead of against its own average?

Sources

• What are El Niño and La Niña? — NOAA National Ocean Service
• What is the El Niño–Southern Oscillation (ENSO) in a nutshell? — NOAA Climate.gov, 2014
• Impacts of El Niño and La Niña on the hurricane season — NOAA Climate.gov, 2014
• ENSO diagnostic discussion, 8 October 2026 — NOAA Climate Prediction Center, 2026
• Relative Oceanic Niño Index (RONI) information circular — NOAA Climate Prediction Center, 2026