El Niño and La Niña Explained: How the Pacific Shapes Global Weather
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You have probably heard forecasters mention El Niño or La Niña when explaining a wet winter, a drought or a quiet hurricane season. These two patterns are opposite phases of a natural climate cycle centered in the tropical Pacific Ocean. They are not caused by climate change, but they have a big influence on weather around the world and on year-to-year swings in global temperature.
Key points
- El Niño and La Niña are the warm and cool phases of the El Niño–Southern Oscillation, or ENSO.
- During El Niño, surface waters in the central and eastern tropical Pacific are warmer than average. During La Niña, they are cooler than average.
- Episodes typically recur every two to seven years and usually last about 9 to 12 months, though La Niña sometimes persists for two or three years.
- ENSO shifts rainfall, temperature and storm patterns across much of the globe, but its effects are tendencies, not guarantees.
- NOAA's Climate Prediction Center issues official US ENSO updates every month.
What is ENSO?
ENSO stands for El Niño–Southern Oscillation. The name joins two discoveries. "El Niño" was the name fishermen in Peru and Ecuador gave to warm ocean currents that sometimes appeared around Christmas. The "Southern Oscillation" is a seesaw in air pressure between the eastern and western tropical Pacific, identified by scientists in the early 20th century. Later research showed that the ocean and atmosphere changes are two sides of the same phenomenon.
ENSO has three phases:
- El Niño, the warm phase
- La Niña, the cool phase
- Neutral, when conditions are close to average
What "normal" looks like in the Pacific
In typical conditions, steady trade winds blow from east to west along the equator in the Pacific. They push warm surface water toward Indonesia and Australia, where it piles up. Off the coast of South America, cooler water rises from the depths to replace it, a process called upwelling. That cold, nutrient-rich water supports some of the richest fisheries in the world.
The warm water in the west heats the air above it, which rises and produces heavy rainfall over Indonesia and nearby regions. The eastern Pacific stays relatively cool and dry.
El Niño: the warm phase
During El Niño, the trade winds weaken, and in strong events they can even reverse in parts of the Pacific. Warm water that normally sits in the west spreads eastward across the Pacific. Upwelling off South America weakens, and surface waters there become warmer than usual.
Because the warmest water shifts east, the area of heavy rainfall and rising air shifts east too. That rearranges atmospheric circulation, including the jet streams, which in turn affects weather far from the Pacific.
La Niña: the cool phase
La Niña is roughly the opposite. The trade winds strengthen, more warm water piles up in the western Pacific, and upwelling of cold water in the east intensifies. Surface waters in the central and eastern Pacific become cooler than normal. The normal pattern becomes exaggerated rather than reversed.
How scientists define an event
NOAA tracks ENSO mainly using sea surface temperatures in a region of the central equatorial Pacific called Niño 3.4. Its main index, the Oceanic Niño Index, is a three-month running average of temperature differences from normal in that region.
- El Niño conditions: about 0.5 °C or more above average, along with matching changes in the atmosphere
- La Niña conditions: about 0.5 °C or more below average, along with matching atmospheric changes
Scientists look for the atmosphere to respond as well, for example through changes in winds, rainfall and pressure, because it is the ocean and atmosphere working together that produces global effects.
Effects around the world
ENSO effects are strongest in certain regions and seasons. The table below shows common tendencies, especially during the Northern Hemisphere winter, based on patterns described by NOAA's Climate Prediction Center and the World Meteorological Organization.
| Region | Typical El Niño tendency | Typical La Niña tendency |
|---|---|---|
| Southern United States | Wetter and cooler winters | Drier and warmer winters |
| Northern United States and Canada | Milder winters | Colder winters in some areas |
| Indonesia and Australia | Drier, higher drought and wildfire risk | Wetter, higher flood risk |
| Peru and Ecuador coast | Heavy rain and flooding; poorer fishing | Drier; strong upwelling |
| Southern Africa | Drier summers | Wetter summers |
| Atlantic hurricanes | Usually fewer | Usually more |
| Eastern and central Pacific hurricanes | Usually more | Usually fewer |
India's summer monsoon also tends to be weaker during El Niño years, although the relationship is not perfect. Atlantic hurricane effects come from changes in upper-level winds: El Niño tends to increase wind shear over the Atlantic, which disrupts developing storms. For more on that topic, see Hurricanes and climate change.
Tendencies, not guarantees. ENSO tilts the odds. Not every El Niño brings a wet winter to California, and not every La Niña brings drought to Texas. Other patterns in the atmosphere and oceans also play a role, and individual events differ in strength and character.
ENSO and global temperature
El Niño releases heat from the Pacific Ocean into the atmosphere, so global average surface temperatures tend to be higher during and just after El Niño years. La Niña tends to make years slightly cooler. The effect is typically around a tenth or two of a degree Celsius on the global average.
This is part of why scientists look at long-term trends rather than single years when studying climate change. The long-term warming trend continues regardless of ENSO, and today's La Niña years are generally warmer than El Niño years were several decades ago. You can read more about this distinction in Climate vs weather.
ENSO also affects marine life. Strong El Niño events have been linked to widespread coral bleaching, as ocean heat waves spread across the tropics.
How ENSO forecasts work
In the United States, NOAA's Climate Prediction Center publishes an ENSO diagnostic discussion every month, usually on the second Thursday. It reports current conditions and gives the probability of El Niño, La Niña or neutral conditions for the coming seasons.
Forecasters base these outlooks on:
- Observations from satellites, ocean buoys moored across the tropical Pacific, and Argo floats that measure temperatures below the surface
- Computer models, both physics-based models that simulate the ocean and atmosphere and statistical models based on past patterns
- Expert judgment that weighs the different model results
NOAA may issue an "El Niño Watch" or "La Niña Watch" when conditions are favorable for an event to develop within the next six months, and an advisory when an event is underway.
ENSO forecasts made in early spring are notably less reliable than those made later in the year, a challenge known as the "spring predictability barrier." Forecasts issued in summer and fall about the coming winter are generally much more skillful.
Does climate change affect ENSO?
This is an active area of research. Some studies suggest climate change could make strong El Niño and La Niña events more frequent or change their effects, but climate models do not yet agree on how ENSO itself will change. What is clearer is that ENSO's effects play out on a warmer background, so El Niño-related heat and the heavy rainfall linked to both phases may become more intense.
The bottom line
El Niño and La Niña are natural, recurring shifts in the tropical Pacific that reshape weather patterns worldwide for months at a time. They make some outcomes, like a wet or dry winter, more likely, but they do not determine weather on their own. Monthly forecasts from NOAA's Climate Prediction Center are the best place to follow what the Pacific is doing.