ENSO 101

Some Basics

What are El Niño and La Niña?

El Niño is characterized by unusually warm ocean temperatures along the the Equatorial Pacific, whereas La Niña is characterized by unusually cold ocean temperatures in this region. These variations from "normal" temperatures influence the tropical Pacific ocean—atmosphere system, which in turn impacts weather and climate around the globe. The term El Niño Southern Oscillation (ENSO) basically refers to both the El Niño and La Niña phenomena together. See this page for a more detailed explanation of what happens in the ocean and atmosphere during ENSO.

2016 El Nino (Credit: NOAA/NESDIS)
A very strong El Niño in 2016 – large 'red tongue' in equatorial Pacific. (Credit: NOAA/NESDIS)
A very strong El Niño in 2016 – large 'red tongue' in equatorial Pacific. (Credit: NOAA/NESDIS)
La Nina, Credit: NOAA/NESDIS
A La Niña event – large blue area in equatorial Pacific. (Credit: NOAA/NESDIS)
La Niña event – large blue area in equatorial Pacific. (Credit: NOAA/NESDIS)

How often does ENSO occur?

While their frequency can be quite irregular, El Niño and La Niña events occur on average every 2—7 years. Typically, El Niño occurs more frequently than La Niña.


How long does ENSO last?

El Niño typically lasts 9–12 months, and La Niña typically lasts 1–3 years. Both tend to develop during March–June, reach peak intensity during December–April, and then weaken during May–July. However, prolonged El Niño episodes have lasted 2 years, and even as long as 3-4 years.

How will ENSO impact a particular region?

There has been a lot research investigating the effects of El Niño and La Niña on climate (temperature, rainfall, snowpack, climate extremes, etc.) around the world. However, these impacts can be different from year-to-year due to the varying nature of ENSO, and variations in the atmosphere–ocean system. The images below show typical El Niño and La Niña wintertime patterns for the U.S. Detailed information about worldwide impacts can be found on NOAA's El Niño Theme Page.

Typical wintertime El Niño pattern. (Credit: climate.gov)
Typical wintertime El Niño pattern. (Credit: climate.gov)
Typical wintertime El Niño pattern. (Credit: climate.gov)
Typical wintertime La Niña pattern. (Credit: climate.gov)
Typical wintertime La Niña pattern. (Credit: climate.gov)
Typical wintertime La Niña pattern. (Credit: climate.gov)

What is the Southern Oscillation Index (SOI)?

The cyclic warming and cooling of the eastern and central Pacific can be seen in the sea level pressure in the region. For example, when the pressure measured at Darwin, Australia is compared with that measured at Tahiti, the differences between the two can be used to generate an "index" number. A positive number indicates La Niña, and a negative number indicates El Niño. Historical and recent values can be found at NOAA's Climate Prediction Center .


Learn More

For Students and Teachers

Data In the Classroom (NOAA/NESDIS)
Investigating El Niño Online Activity

El Niño Student Activity (NOAA/OAR)
Student Booklet
Teacher Information on El Niño
Activity Key