El Niño and La Niña

Three states of one ocean

El Niño and La Niña are opposite phases of a single natural climate pattern called the El Niño-Southern Oscillation, or ENSO.⁴

• El Niño is a warming of the ocean surface in the central and eastern tropical Pacific.⁵
• La Niña is a cooling of the same waters.⁵
• Neutral is neither one. It is also the most common state: roughly half of all years.⁵⁶

The names come from South America. Fishermen there noticed periods of unusually warm water as far back as the 1600s, and called it El Niño, "the boy" or "the Christ Child," probably because it tended to show up around Christmas. La Niña means "the girl."⁷⁴

The "Southern Oscillation" half of the name is the atmosphere's part: a seesaw in air pressure between the western Pacific and the central and eastern Pacific. Scientists realized in the late 1960s that the ocean changes and the pressure seesaw were one linked system.⁸⁵

How it works

Pick a state to see what the ocean and the air above it are doing along the equator.

The pieces

• Trade winds and the warm pool. The trades pile water up in the west, so sea level is about half a meter higher near Indonesia than off Ecuador. The ocean north of Australia is typically 28 to 30 °C (82 to 86 °F) or warmer. Near South America it is close to 20 °C (68 °F).⁸⁹
• Upwelling. As the trades drag surface water west, cold water rises from below off South America to replace it.⁷
• The thermocline. The boundary between warm surface water and the cold deep ocean. It normally tilts up toward South America. In El Niño it flattens.¹⁰
• The Walker circulation. A loop of air along the equator: west along the surface, up over the warm western Pacific, east at high altitude, and down over the cooler east.¹¹⁹

What makes ENSO swing is that the wind and the water push each other in the same direction. When the trades weaken, the east warms. That shrinks the temperature contrast across the Pacific, which weakens the Walker circulation, which weakens the trades even more. This loop is called the Bjerknes feedback, and La Niña is the same loop running the other way.¹²⁹

How NOAA measures it

Forecasters watch one rectangle of ocean in particular: the Niño 3.4 region, from 5°N to 5°S and from 120°W to 170°W.¹³

• El Niño: an index of +0.5 °C or higher. La Niña: −0.5 °C or lower.³
• To count as a full episode in the historical record, the threshold has to be passed for at least five consecutive overlapping three-month seasons.³
• To declare conditions in real time, NOAA also needs to see the atmosphere responding, and a forecast that the anomaly will last three more months.³
• Strength comes in four classes, weak, moderate, strong and very strong, in half-degree steps. Above 2.0 °C is "very strong."¹⁶¹

A stronger event does not automatically mean stronger effects where you live. NOAA says so directly: strength "does not necessarily correspond with the strength of the influence or expected impact."¹⁶

How often, and for how long

ENSO is irregular. Events arrive every two to seven years, and NOAA's Climate Prediction Center says three to five is typical.⁷¹⁷ They tend to start developing between March and June and peak between November and February.⁴ El Niño typically lasts 9 to 12 months. La Niña can last one to three years.¹⁷

Winter in North America

ENSO reaches North America by moving the jet stream. These are the typical December to February patterns.

Why a jet that moves south warms the north

This can look backwards. Cold air comes south when the jet stream dips south. So why does a jet that moves south leave the north warmer, and a jet that moves north leave it colder? Because a shift and a dip are different things.

• A dip is a bend in the jet, called a trough. Air from the north flows down into it. That is what brings cold.
• A shift is the whole Pacific jet, with the storms that travel along it, reaching the coast farther south or farther north. That moves the rain and snow. Whether cold air comes with it depends on the shape the jet takes over the continent.

In El Niño the jet shifts south and runs flat. It stretches east across the Pacific to the southwestern United States, then blows more nearly west to east across the country than usual. The storm track moves with it, from the northern states to the southern ones, so the South gets the rain. With fewer bends in the jet, less cold air flows south out of Canada and more mild ocean air flows into western North America. The northern states and western Canada get a milder winter.¹⁸

In La Niña the jet shifts north over the ocean, then turns wavy. Blocking is more common over the far northeastern Pacific, and the jet often meanders high into the North Pacific before it reaches land in the northwestern United States and southwestern Canada.¹⁸¹⁹ Across the continent its path is very wave-like, with more wind blowing north to south. Those dips bring the cold: central North America gets more storms and more significant cold-air outbreaks. The southern states are left with fewer storms and less rain.¹⁸

The official outlook for winter 2026-27

NOAA released its outlook for December 2026 through February 2027 on October 15, 2026. It favors warmer-than-average temperatures for northern California, the Pacific Northwest, the Midwest, the Mississippi Valley, much of the East, and Alaska. It favors wetter-than-average conditions across the southern tier, from the Southwest to the Southeast, and along the East Coast.²¹ The outlook does not forecast snowfall, which NOAA says is generally not predictable more than a week ahead. An update was due on November 19, 2026.²¹

Written November 10, 2026, and not updated automatically. NOAA's current seasonal outlooks are at the Climate Prediction Center.

One wild card is not in that outlook: the state of the polar vortex.

Around the world

El Niño's typical fingerprints, according to the World Meteorological Organization and national weather services:

• More rain and flooding in parts of South America, East Africa and the southern United States.⁴
• Drought in eastern and northern Australia, Indonesia, southern Africa and parts of South Asia.⁴ In Australia that also means more heat waves and fewer tropical cyclones.⁹
• East Africa: a wetter-than-normal "short rains" season from October to December.⁴
• India: El Niño generally suppresses the monsoon and La Niña generally boosts it, but not reliably. The 1997-98 El Niño, the strongest of the 20th century, brought above-average monsoon rain.²²
• Peru: warm water and weak upwelling hit the fisheries. In 1972-73 the anchovy population crashed.²³

La Niña tends to do the reverse, and has been linked to drought in the Horn of Africa and heavy rain in Southeast Asia and Australasia.²⁴ Not every region is affected, and no two events are the same.⁴

Hurricanes and tornadoes

Hurricanes work like a seesaw. El Niño suppresses hurricane activity in the Atlantic and boosts it in the central and eastern Pacific. La Niña does the opposite. The lever is wind shear: winds that change with height can rip a developing hurricane apart, and El Niño increases that shear over the Atlantic.²⁵

Tornadoes are murkier. The best-known study found that fewer tornadoes and hailstorms hit the central United States during El Niño and more during La Niña, because ENSO shifts the jet stream.²⁶²⁷ The historic outbreaks of 1974, 2008 and 2011 all began during La Niña.²⁸ But the signal is noisy, an earlier study of winter outbreaks found neutral winters the most active, and the link in summer is unclear.²⁹²⁷

The biggest events

Which El Niño was the strongest depends on which index you use.

Peak three-month values from NOAA's data files. Recent values can still be revised.³⁰³¹

On the La Niña side, the 2020 to 2023 event spanned three northern winters in a row, the first "triple-dip" La Niña of this century.²⁴

El Niño and global temperature

El Niño temporarily raises the global average temperature and La Niña lowers it, with the largest effect in the year after an event develops.⁴ NASA estimated that El Niño added 0.12 °C (0.2 °F) to 2016's global temperature.³² The WMO attributes 2024's record warmth to the 2023-24 El Niño on top of human-caused warming.⁴ La Niña's cooling is temporary too: it "will not halt or reverse the long-term warming trend."²⁴

El Niño and the polar vortex

The Pacific can reach all the way up into the stratosphere. El Niño tends to weaken and warm the polar vortex. It deepens the low-pressure area near Alaska's Aleutian Islands, and that sends more planetary-wave energy upward to batter the vortex. La Niña's effect is often the opposite.³³³⁴

Whether that translates into more full breakdowns of the vortex is less settled. In about 65 years of observations, sudden stratospheric warmings have been more common in both El Niño and La Niña winters than in neutral ones. Climate models show roughly a 30% increase during El Niño and no boost from La Niña, and NOAA's stratosphere scientists say the record may simply be too short to tell.³⁵³⁶ Either way, most sudden warmings are not caused by the tropical Pacific.³⁶

More on the polar vortex

Last updated November 12, 2026