Understanding El Niño

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The 3 Phases of the El Niño-Southern Oscillation

I expect you have heard of El Niño. Huge parts of the world, if not all of us in some way, are affected by El Niño, or, more completely, the El Niño-Southern Oscillation (ENSO). ENSO is a weather pattern that varies over a period of years. From typical conditions, to El Niño (the boy), the warm phase, to La Niña (the girl), the cold phase.

ENSO includes changes in both the atmospheric and the tropical Pacific Ocean. It includes variations in sea surface temperatures, rainfall, air pressure and atmospheric circulation. El Niño refers mainly to the ocean warming pattern, while the Southern Oscillation describes accompanying shifts in atmospheric pressure and circulation.

Now, El Niño always seemed complicated to me, and I never really understood it. But with El Niño on the horizon, and wonderful content from the American Meteorological Society, I wanted to try to explain the basics.

El Niño is complex and varied, and the effects are widespread. I don’t profess to cover everything here. ENSO does not follow a perfectly regular cycle. Events vary in timing, strength and duration, and not every El Niño is followed by a La Niña. As George Box said: All models are wrong, but some are useful.

Because El Niño is a change in the normal conditions, to understand it, I think you need to understand how it’s different from normal. So here, and in the sketch, I focused on the three phases: Neutral, El Niño and La Niña.

The Neutral Phase - typical conditions

In typical patterns, between El Niños, trade winds push warm surface waters in the tropics of the Pacific Ocean to the west. All this movement of warm water leads to strong evaporation, low pressure and high rainfall in the western Pacific towards Indonesia and Australia. It also raises the sea level in the western Pacific compared to the east. These patterns of winds, rainfall and rising and sinking air across the Pacific are part of what is called the Walker Circulation.

At the same time, the western movement of surface waters creates an upwelling of deep water by South America, particularly along the coast of Peru. This upwelling provides nutrients that feed phytoplankton, which feed vast numbers of fish and, in turn, support major fisheries. The colder water is associated with higher pressure and drier conditions along the coast of Peru.

El Niño - The Warm Phase

El Niño is known as the warm phase because the warm surface waters spread east towards South America. This happens because the trade winds blowing westward weaken or even reverse. As the trade winds weaken, the slope in sea level across the Pacific relaxes, and warm water spreads east and suppresses the upwelling that supported the marine life near Peru.

Rainfall decreases in the western Pacific causing dry spells or drought. The fish stocks decline along the coast of Peru. And with the reduction of Peru’s typical high-pressure system, rainfall there often increases causing flooding.

Because the Pacific Ocean is so large, some El Niños cause weather changes in monsoon rains, the poles, Europe, Africa, and as far north as Alaska.

La Niña - The Cold Phase

Sometimes, after warm water spreads east during an El Niño event, conditions swing back strongly the other way—with stronger trade winds and increased cold upwelling. With the difference in sea level across the Pacific, I think of it a bit like a giant slosh in a bathtub.

In La Niña conditions, the trade winds strengthen, pushing warm waters further west. High pressure often develops along South America, and the cold upwelling increases. Warmer water around Indonesia and Southeast Asia intensifies the low pressure, resulting in high rainfall and often flooding.

Because the El Niño phase spreads warmer sea surface temperatures across a larger area of the Pacific, it often coincides with warmer-than-normal global average temperatures. Strong El Niño events can increase the risk of drought, flooding, heatwaves and wildfires in different parts of the world.

Does this give you a better idea of it?

More about El Niño

The differences in low and high pressure for the Southern Oscillation are sometimes measured as the difference from normal between Darwin (Northern Australia) and Tahiti (in the central south Pacific) .

Why is it called El Niño?

El Niño, Spanish for the boy, actually corresponds to Christ, as it was first noticed by South American fishermen around Christmas.

The Walker Circulation

The trade winds and rainfall patterns behind ENSO are part of a larger atmospheric circulation called the Walker Circulation. I sketched that separately, too, as it varies during Neutral, El Niño and La Niña phases. You can see it in the newsletter for this sketch.

Related Ideas to El Niño

Also see:

Seasons
Know Your Clouds
Latitude and longitude: which is which?
Weather and Climate, an analogy
Thermocline
Thunder Clouds
Time hierarchy - fast and slow layers for robust, adaptable systems
Temperature scales: Celsius and Fahrenheit
All models are wrong, but some are useful

I learned about ENSO, El Niño, La Niña, the Walker Circulation and others from some oceanography textbooks. Find it in Chapter 7 of Introduction to Ocean Sciences 5th edition by Douglas Segar , the AMS Ocean Studies textbook, and the soon-to-be-released open-access Introduction to Ocean Sciences 6th Edition .

Published 24 May 2026

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These little openings—through vulnerability, cooperation, kindness, or shared experience—form a little social bridge. Often they relax social barriers—don’t talk in the elevator, keep yourself to yourself, don’t make eye contact. I stop short of deliberately dropping my books and papers on the floor, but I don’t hesitate to accept help when offered, give it when it might be appreciated or exchange a glance of shared understanding. These small disruptions make my everyday life a little warmer and kinder. What Little Openings have you experienced? Brendan Leonard wrote an illustrated story about “When we hold the door” Related Ideas to Little Openings Also see: Be nice to tourists Learn kind words first in new countries Clear is kind Hanlon’s Razor The Fundamental Attribution Error Compliments are gifts Calm like a duck Equality and equity Front stage, back stage - life as theatreI’ve long been intrigued by the innocuous power of Little Openings. Say you are walking down a corridor with your hands full of papers and books. Before long, they start to slip. As you try to stabilise the falling papers, the rest go down in a clatter and flutter. However, a passerby in the corridor kneels down to help you pick everything up. They say hello. You say thank you. You each introduce yourselves, maybe exchange a laugh. You go on your way, books and papers in hand, feeling good about things with a new acquaintance. They continue on their way, happier to have helped someone out, and pleased to have met someone new they otherwise would have passed by. The fall of books and papers, though it would at first seem unhelpful, actually created a little opening that enabled two people to connect who would have passed each other by. So that’s a Little Opening: a small moment that invites connection. Small imperfections in daily life often create opportunities for connection that wouldn’t otherwise exist. If everything worked smoothly and efficiently all the time, we’d have fewer opportunities to connect. A bit of friction can be a blessing. In a Little Opening, you don’t have to help somebody, you don’t have to lend eggs to a neighbour, you don’t have to stop and give directions. But there’s an opportunity there, a little opening where you can. Dogs (and small children) are masters of Little Openings: being inappropriately friendly, running across someone’s path, waiting for them while they snuffle around, or simply walking dogs together. I’m sure you can think of a host of situations like this. Reflecting on them, I think these openings are of at least four different types. Moments of vulnerability When there’s an opportunity to help in a tough spot When you can’t work the parking machine Reaching something off a high shelf Getting a push chair with a baby up a flight of stairs Looking lost in a foreign city, holding a map upside down Helping with a car breakdown Letting someone know what stop this is on a train Picking up dropped shopping Offering your parking spot in a busy parking lot Borrowing eggs from your neighbour Helping work the printer Enforced cooperation When people have to interact to go on their way Waiting to let someone pass in a narrow space or on the stairs Pressing the buttons for others in an elevator Passing around a leaflet or sign-in sheet Moving down in a packed commuter train Clearing your things from a desk as someone sits down Sometimes connection comes not from kindness or vulnerability, but because people briefly have to coordinate with one another. Acts of kindness When an opportunity for generosity presents itself Holding the door for someone Offering a snack Sharing your umbrella Moving seats for someone at a theatre Giving away excess food at a picnic or camping Lending a tool to make someone’s task easier Micro-connections Connecting through a moment of shared understanding or shared inconvenience An old-fashioned cigarette break Catching someone's eye when a speaker says something you both think is silly/clever/funny Waiting for a bus or an appointment together Watching a beautiful sunset Joining in a round of happy birthday for another table at a restaurant A mutual groan as a train delay comes through the speaker Coordinating in a power cut Saying “Bless you” when someone sneezes Travel in a foreign country opens up a host of Little Openings. It’s much easier to connect with fellow travellers trying to figure things out abroad than it would be to connect with them at home. These little openings—through vulnerability, cooperation, kindness, or shared experience—form a little social bridge. Often they relax social barriers—don’t talk in the elevator, keep yourself to yourself, don’t make eye contact. I stop short of deliberately dropping my books and papers on the floor, but I don’t hesitate to accept help when offered, give it when it might be appreciated or exchange a glance of shared understanding. These small disruptions make my everyday life a little warmer and kinder. What Little Openings have you experienced? Brendan Leonard wrote an illustrated story about “When we hold the door” Related Ideas to Little Openings Also see: Be nice to tourists Learn kind words first in new countries Clear is kind Hanlon’s Razor The Fundamental Attribution Error Compliments are gifts Calm like a duck Equality and equity Front stage, back stage - life as theatre

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