What is ENSO?
El Niño and La Niña: how the tropical Pacific reshapes weather around the world.
This explanation is written conservatively from textbook-level science and has not yet been reviewed by a meteorologist. Treat it as a draft.
The mechanism
Follow the chain: each step causes the next.
01
Trade winds weaken
Normally, easterly trade winds pile warm surface water up in the western Pacific. At the start of an El Niño they slacken.
02
Warm water spreads east
The pool of warm water drifts toward the central and eastern Pacific, and the ocean surface there warms above normal.
03
Rain follows the warm water
Deep thunderstorm activity shifts east with the warmest water, rearranging the tropical Walker circulation.
04
Feedback locks it in
A weaker east–west temperature contrast weakens the trade winds further — the Bjerknes feedback — so the pattern can persist for months.
05
Weather shifts worldwide
Through changes in atmospheric circulation, rainfall and temperature patterns shift far from the Pacific — for example, El Niño tends to bring drier conditions to much of Indonesia and eastern Australia.
Explore it
Step through the mechanism one stage at a time.
Step 1 of 5
Trade winds weaken
Normally, easterly trade winds pile warm surface water up in the western Pacific. At the start of an El Niño they slacken.
In short
Three phases
ENSO — the El Niño–Southern Oscillation — is a naturally recurring swing in the coupled ocean and atmosphere of the tropical Pacific. It has a warm phase (El Niño), a cool phase (La Niña) and a neutral state in between. Episodes typically recur every two to seven years and often last around a year.
Deeper science
Open a section for more detail.
How it is measured
A common index is the Oceanic Niño Index (ONI): the three-month running mean of sea surface temperature anomalies in the Niño 3.4 region of the central-eastern equatorial Pacific. NOAA classifies El Niño conditions when the ONI stays at or above +0.5 °C, and La Niña at or below −0.5 °C, over several consecutive overlapping seasons. Other agencies use related but different thresholds and indices, so declarations can differ.
The atmospheric half
The 'Southern Oscillation' is the atmospheric side: a see-saw in surface air pressure between the western and eastern tropical Pacific, often tracked with the Southern Oscillation Index. During El Niño, pressure tends to rise over the western Pacific and fall over the east. The ocean and atmosphere drive each other, which is why ENSO is treated as one coupled phenomenon.
Tendencies, not certainties
ENSO shifts the odds of regional outcomes; it does not guarantee them. Each event differs in strength, timing and location of the warmest water, and other influences — such as the Indian Ocean Dipole or the MJO — can reinforce or offset its effects.
Related topics
The Indian Ocean Dipole
A see-saw of sea surface temperature across the Indian Ocean.
4-step mechanismDraftRead topicThe Madden–Julian Oscillation
A travelling pulse of tropical rainfall that circles the globe in one to two months.
4-step mechanismDraftRead topic