El Niño and La Niña are two opposing climate patterns that develop in the tropical Pacific Ocean every few years, shifting weather around the globe. Together with a third, neutral state, they form a cycle called the El Niño-Southern Oscillation, or ENSO. El Niño brings warmer-than-normal ocean waters to the eastern Pacific, while La Niña brings cooler-than-normal waters. These temperature shifts, though centered in one stretch of ocean, alter rainfall, drought patterns, and storm tracks across multiple continents.
How the Cycle Works
Under normal conditions, steady trade winds blow from east to west along the equator, pushing warm surface water toward Australia and Southeast Asia. This leaves cooler, nutrient-rich water to rise up along the western coast of South America. A large-scale loop of air called the Walker Circulation reinforces this setup: warm, moist air rises over the western Pacific, travels eastward at high altitude, sinks over the eastern Pacific, and flows back west at the surface. It’s a self-reinforcing engine that keeps the western Pacific warm and wet while the eastern Pacific stays relatively cool and dry.
El Niño and La Niña are what happens when that engine speeds up or breaks down.
What Happens During El Niño
During El Niño, the trade winds weaken. Without that steady push, warm surface water that normally piles up near Australia and Indonesia spreads eastward across the Pacific. The pool of warm water flattens out the boundary between warm surface water and the cold deep ocean (called the thermocline), and upwelling of cold, nutrient-rich water off South America slows or stops. Sea surface temperatures in the central and eastern tropical Pacific climb at least 0.5°C (0.9°F) above average, and in strong events they can rise far higher.
That shift in ocean heat drags the atmosphere along with it. The rising branch of the Walker Circulation follows the warm water eastward, bringing heavy rainfall to the central Pacific and sometimes the west coast of South America. Meanwhile, the western Pacific and Maritime Continent experience weaker rising air, which can trigger severe drought in Australia, Indonesia, the Philippines, and Thailand. The warmer ocean surface also heats the air above it, lowering atmospheric pressure and further disrupting the normal east-to-west pressure pattern between Tahiti and Darwin, Australia.
El Niño events typically last 9 to 12 months and recur on average every 2 to 7 years, though the timing is irregular.
What Happens During La Niña
La Niña is essentially the normal pattern turned up to full volume. Trade winds blow even harder than usual, pushing more warm water toward Asia and intensifying the upwelling of cold water off the Americas. Sea surface temperatures in the central and eastern Pacific drop at least 0.5°C below average. The cold surface water lowers sea level in the eastern Pacific and fuels an even stronger Walker Circulation, with more vigorous rising air and rainfall over the Maritime Continent and South America, and enhanced sinking and dryness over the central and eastern Pacific.
La Niña events tend to last longer than El Niño, typically persisting for one to three years. Some La Niña episodes stretch across consecutive winters, which is why forecasters sometimes refer to “double-dip” or “triple-dip” La Niña events.
Effects on North American Weather
Both phases reshape the jet stream over North America, steering storms and temperature patterns in predictable ways. During El Niño winters, the southern tier of the United States tends to be wetter and cooler, while the northern states and Canada often see milder, drier conditions. The Gulf Coast and Southeast typically get more rain, and the Atlantic hurricane season tends to be less active because of increased wind shear over the tropical Atlantic.
La Niña pushes the jet stream northward. This brings wetter, colder conditions to the Pacific Northwest and Canada, while the southern U.S. trends warmer and drier. Winter temperatures run cooler than normal across the northern states and warmer than normal in the South. La Niña winters also tend to produce more active Atlantic hurricane seasons. Notably, there are no consistent ENSO-related impacts on North American weather during the summer months; the effects are strongest from late fall through spring.
Worldwide Impacts
The reach of ENSO extends well beyond the Pacific and North America. El Niño commonly brings drought to Australia, Indonesia, and parts of southern Africa, while increasing rainfall in East Africa and along the Pacific coast of South America. Flooding can hit Peru and Ecuador hard during strong El Niño years, while Central American and Caribbean nations often face drought conditions that damage crops.
La Niña generally flips these patterns: heavier monsoon rains across Southeast Asia and northern Australia, drier conditions in East Africa, and increased rainfall in northern South America. Both phases affect the Indian monsoon, though the relationship is complex and varies by event.
Effects on Fisheries and Agriculture
One of the most dramatic consequences of El Niño plays out off the coast of Peru. Normally, upwelling there brings cold, nutrient-rich water to the surface, supporting one of the world’s most productive fishing grounds. This region produces 12 to 20 percent of the global fish catch. When El Niño shuts down that upwelling, the food web collapses. The Peruvian anchovy fishery, which once landed over 12 million metric tons per year, suffered a major stock collapse during the 1972-73 El Niño and hit its lowest level on record during the 1982-83 event. Recovery can take years.
Agricultural impacts are harder to pin down with precision, but the patterns are real. During the 1997-98 El Niño, Indonesia, the Philippines, and Thailand experienced serious drought, while Central American countries lost roughly 15 percent of their crop output on average, with some areas hit much harder. In South America, unusually heavy rains disrupted wheat planting in Argentina and Brazil. La Niña, by contrast, can threaten crops in the southern U.S. through drought while boosting rainfall in already wet tropical regions.
The Neutral Phase
Between El Niño and La Niña events, the tropical Pacific often settles into a neutral state. Ocean temperatures, rainfall patterns, and wind strength hover near their long-term averages. Neutral periods frequently serve as the transition between events, and they can last anywhere from a few months to a couple of years. During neutral conditions, other climate drivers (like the North Atlantic Oscillation or the Indian Ocean Dipole) play a larger role in shaping regional weather, since the powerful ENSO signal isn’t dominating the picture.
How Scientists Classify Each Phase
NOAA uses the Oceanic Niño Index, or ONI, to officially determine whether El Niño or La Niña conditions are present. The index tracks sea surface temperature anomalies in a specific patch of the central Pacific (called the Niño 3.4 region, spanning from 170°W to 120°W longitude). When the three-month running average of that temperature is 0.5°C or more above the historical baseline, the period is classified as El Niño. When it’s 0.5°C or more below, it’s La Niña. Anything in between is neutral.
These baselines are updated every five years using 30-year averages, which helps account for the gradual warming trend in ocean temperatures.
Current ENSO Status
As of early 2025, La Niña conditions are in place. NOAA’s Climate Prediction Center forecasts a transition to neutral conditions by February through April 2026, with about a 60 percent probability. Neutral conditions are expected to persist through the Northern Hemisphere summer of 2026. Beyond that, there is a 50 to 60 percent chance of El Niño forming by late summer 2026, though forecasts made this far in advance carry considerable uncertainty.