El Niño and La Niña Oscillations

El Niño and La Niña are the warm and cool phases of the El Niño-Southern Oscillation (ENSO), a natural climate pattern in the tropical Pacific in which sea surface temperatures and atmospheric pressure shift every two to seven years, changing weather around the world. In brief: in an El Niño, the trade winds weaken and warm water spreads eastward toward South America, which brings heavy rain to Peru and drought to Indonesia and Australia. In a La Niña, the trade winds strengthen and the eastern Pacific becomes cooler than normal, with the opposite effects. Neutral conditions lie between the two phases.

ENSO is the strongest natural source of year-to-year variation in global climate, and forecasts of it help farmers, fishers and governments to prepare. This guide explains normal Pacific conditions, how El Niño and La Niña develop, how they are measured, what their impacts are, compares the phases in a table, works through calculations, outlines the history and corrects common mistakes.

What are normal conditions in the Pacific?

In normal years, strong trade winds blow from east to west along the equator and push warm surface water toward the western Pacific, so a deep pool of warm water collects near Indonesia and northern Australia. Sea level there is a few tens of centimeters higher, and air rises over the warm water, creating clouds and heavy rain. Off the coast of South America, the water pushed away from the shore is replaced by cold, nutrient-rich water from the depths (upwelling), which supports one of the world’s richest fisheries and keeps the coast of Peru dry. A boundary called the thermocline, between the warm surface layer and the cold deep water, is deep in the west and shallow in the east. High atmospheric pressure over the eastern Pacific and low pressure over the western Pacific complete a loop of air called the Walker circulation.

What happens in an El Niño?

The trade winds weaken or even reverse, and the pool of warm water moves eastward. The thermocline in the east becomes deeper, so upwelling weakens, and the sea surface along the equatorial central and eastern Pacific becomes warmer than normal. The pressure pattern also changes: pressure falls in the east and rises in the west, which is the Southern Oscillation. The rainfall zone moves east, causing heavy rain over the usually dry coasts of Peru and Ecuador, and drought in Indonesia, Australia, parts of South Asia and southern Africa. An El Niño often appears around December, and its name, Spanish for “the Christ child,” was given by Peruvian fishermen who noticed warm water around Christmas. Events typically last 9 to 12 months, and strong events add some warmth to global average temperatures, so the years after a strong El Niño, such as 1998 and 2016, have often been record-warm.

What happens in a La Niña?

La Niña is an intensification of the normal state. Trade winds are stronger than usual, more warm water piles up in the west, upwelling in the east is stronger, and the eastern and central Pacific are cooler than average. Rainfall increases in Indonesia, Australia and Southeast Asia, often causing floods, while drought can develop in the southwestern United States, parts of South America and East Africa. La Niña often lasts longer than El Niño and sometimes continues for two or three years, as in 2020–2023. It generally lowers global temperatures slightly.

What are the global impacts?

• Rainfall and drought: El Niño tends to cause floods in Peru, Ecuador and southern Brazil, and drought in Indonesia, Australia, India, southern Africa and northern South America. La Niña brings the reverse.
• Fisheries: warm water suppresses upwelling, so the anchovy catch off Peru collapsed in 1972–73, which affected the world market for fishmeal. Seabirds and marine mammals also suffer.
• Agriculture and food prices: harvests of rice, wheat, sugar and coffee can fall in affected regions.
• Storms: El Niño tends to reduce Atlantic hurricanes and increase Pacific typhoons and hurricanes, and La Niña does the opposite.
• Fires and health: drought increases the risk of wildfires in Indonesia and Australia, and floods and heat increase diseases such as dengue and cholera in some regions.
• Global temperature: El Niño years add warmth and La Niña years cool slightly, on top of long-term warming.

The effects of any single event are not exactly the same, since they depend on the strength of the event and on other climate patterns, but forecasts of ENSO give months of warning. Scientists are studying how climate change may alter the behavior of ENSO.

The phases compared

Worked example: measuring ENSO

The values are hypothetical and illustrate the methods.

1. Sea surface temperature anomaly: the usual October temperature in the central Pacific (Niño 3.4 region) is 27.3 °C, and the observed value is 28.2 °C. The anomaly is 28.2 − 27.3 = +0.9 °C.
2. Oceanic Niño Index (ONI): the ONI is the three-month running mean of the anomaly in the Niño 3.4 region. If the monthly anomalies are +0.6 (Oct), +0.9 (Nov) and +1.1 (Dec), the mean is (0.6 + 0.9 + 1.1) ÷ 3 = 0.87 °C.
3. Classification: NOAA declares an El Niño episode when the ONI is +0.5 °C or higher for at least five consecutive overlapping three-month periods, and a La Niña when it is −0.5 °C or lower for the same duration. Here, 0.87 is above the threshold, but the five-period requirement also needs to be met.
4. Southern Oscillation Index: in one month, the pressure at Tahiti is 1,010.0 hPa and at Darwin 1,013.0 hPa. The difference (Tahiti − Darwin) is −3.0 hPa. If the long-term mean difference is +1.0 and the standard deviation is 2.0, the index is 10 × (−3.0 − 1.0) ÷ 2.0 = −20. A sustained value below about −7 indicates El Niño, and above about +7 indicates La Niña.
5. Rainfall: a coastal town in northern Peru normally receives 100 mm of rain in a year, and in a strong El Niño receives 1,500 mm. The increase is 1,500 ÷ 100 = 15 times the normal rainfall.
6. Fish catch: if the anchovy catch falls from 10 million tonnes to 4 million tonnes, the fall is 6 million tonnes, or 6 ÷ 10 = 60%.

How did we learn about ENSO?

Peruvian fishermen had known for centuries about the warm water that appeared around Christmas. In the 1920s and 1930s, Sir Gilbert Walker, a British scientist working in India, searched for a way to forecast the monsoon and identified a “Southern Oscillation” in pressure between the eastern and western Pacific. In 1969, Jacob Bjerknes linked this oscillation to the ocean temperatures and proposed the interaction between the ocean and atmosphere that we now call ENSO. The severe El Niño events of 1982–83 and 1997–98 brought great public attention and led to the setting up of the TAO buoy array in the tropical Pacific. Today, satellites, buoys and computer models enable forecasts of ENSO several months ahead.

Which mistakes are common?

Mistake 1: Treating El Niño as a storm

It is a change in ocean and atmospheric conditions across the tropical Pacific that lasts months, and not a single weather event.

Mistake 2: Thinking that El Niño causes climate change

ENSO is a natural cycle. It adds short-term ups and downs to global temperatures on top of the long-term trend.

Mistake 3: Assuming that La Niña is just the absence of El Niño

La Niña is an active phase with its own temperature and rainfall patterns, and is not simply “normal.”

Mistake 4: Expecting identical effects in every event

The strength and location of the warming and other climate patterns change the effects from one event to the next.