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Hot water battle
Original painting from: "A Sea Battle" by Johannes Lingelbach, 17th century

POSTED 11 JANUARY 2007


What causes El Niño , and how does it affect our weather?
Every few years, water temperatures along the equator in the eastern Pacific Ocean cycle from cool to warm -- from La Niña to El Niño. Two graphs show difference in blue layers and amount of greens and reds presentClimatologists call the change in water temperature, and the resulting change in wind and clouds, the "El Niño-Southern Oscillation." (Curious about current conditions in the Pacific?)

Between El Niños, the trade winds blow towards the west across the tropical Pacific. The major updraft of warm, wet air occurs around Indonesia. During El Niño, the convection cell moves to the east, changing weather around the globe. Image: NOAA

To understand this cycle, you've got to grapple with the idea of a "heat engine." Heat engines -- there's one in every conventional car -- use heat to create kinetic energy. In a car, heat from burning gasoline creates motion that allows you to cruise for burgers. Earth's oceans and atmosphere are a slightly larger heat engine, where solar heat warms mainly the equatorial regions, driving the distribution of air, water and energy toward the poles via air and water.

Without Earth's heat engine, the tropics would be even hotter and the poles would be even colder than normal.

One key driver of atmospheric movement is heat release by tropical rainfall, which drives the circulation in giant loops called "Hadley cells." This overturning circulation moves heat away from the equator, ultimately determining the location and intensity of the mid-latitude jet streams. And those jet streams control the weather. In winter, the northern United States and Europe are exposed to warm air from the south or frigid air from the north.

 Global view with red arrows swirling every which wayHadley cells transport warm, wet air from the tropics up, up and away. When El Niño moves the warm ocean water that feeds the convection cell, it causes a change in major winds and weather systems around the globe. Courtesy Kevin Trenberth, National Center for Atmospheric Research

During normal conditions, a giant convection cell around Indonesia and Australia raises warm, wet air high into the atmosphere. As you can see from the Hadley cell diagram, this air moves poleward, delivering heat and water to locations hundreds or thousands of miles distant from the origin of the convection cell. During El Niño, the convection cell shifts east, hovering over the hot spot in the ocean. And, just like throwing a rock into a pond will generate ripples that move outward, the shift in convection generates ripples that move across the Pacific into the Northern Hemisphere, which shove the jet streams circling the North Pole. And it is that change that has allowed some New Yorkers to frolic in spring-like conditions in January.

Two maps, orange cutting across equator on both, blue covers the poles
During an El Niño, warm water stretches across the tropical Pacific; during the opposite La Niña phase, a "cold tongue" of water (arrow) appears along the eastern equator. The cyclical difference in water temperatures changes where rainfall occurs in the tropical Pacific, which alters the global circulation of moisture and energy. Courtesy Kevin Trenberth, National Center for Atmospheric Research

Windy waters
How does El Niño get started? Usually, trade winds in the Pacific push water west from South America toward Australia and Indonesia, where the sea surface may be two feet above its level near South America. The westbound current allows the rise of deep, cold, nutrient-laden water along South America, cooling the surface and feeding a vibrant fishery along Peru and Ecuador. Hot ocean water near Indonesia fuels a strong convection cell, where damp, hot air rises, helping to drive a Hadley cell.

During normal conditions, water is actually colder along the Equator in the Eastern tropical Pacific than north or south of the Equator, says Daniel Vimont, assistant professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison. "That's called the cold tongue."

Blue waves  crash into sandy seashore A periodic change in temperatures in the North Atlantic Ocean may account for some of the extremely warm weather in Europe and North America. Photo: UCAR

If the westward winds weaken, eastward winds can take over, allowing the ocean to slosh back toward South America, warming the eastern tropical Pacific and interrupting the upwelling of cold water along South America. That's El Niño -- simplified so we Why-Filers can understand it.

"When the cold tongue starts to warm [at the start of an El Niño], a chain of events starts to occur that tends to enhance the warming," says Vimont, whose research focuses on El Niño. "The warmed sea surface helps weaken the winds, and the weakened winds help further warm the sea surface." This positive-feedback system, "is why El Niño can be predicted and why it tends to last for such a long time," typically about nine months.

But this isn't the whole story. The weakened winds of El Niño, "also set up a lagged response in the ocean that will shut off the El Niño several months later," Vimont adds. This cyclical change is normal in the Pacific, Vimont stresses. "If you ring a bell, no matter what way you hit it, you tend to hear the same sound. If you think of the tropical Pacific as a bell ... no matter what way the system is bounced around, conditions will develop for El Niño to occur. Oscillation is intrinsic to the system."

El Niño: Is it the whole explanation for the warm weather?
In fact, the explanation for our weird winter weather is a lot more complicated and incomplete than we have suggested.

Consider the North Atlantic Oscillation, a cycle of pressure changes in the North Atlantic that may also affect the Northern Hemisphere jet stream, and therefore its weather. "El Niño tends to give us warm conditions, but the current phase of the North Atlantic Oscillation also tends to enhance the warm weather in the East and Midwest," Vimont says. "Both in concert have been giving us pretty crazy weather."

Red and blue spikes on a graph, a globe with red arrows showing weather system over the Atlantic
Left: A map of the North Atlantic Oscillation. Changes in this ocean-weather system affect winds and temperatures in North America and Europe. Right: Oscillation intensity, measured by an arbitrary index; black line indicates trend. Courtesy Kevin Trenberth, National Center for Atmospheric Research

A second factor -- warming in the Indian Ocean -- also reflects the planetary nature of the ocean-atmospheric system. Trenberth says the Indian Ocean has been warming and becoming a larger source of atmospheric moisture, and as a result, "The evolution of El Niño has been completely different" since 1976.

What goes up must come down, so any large updraft in one place must be counterbalanced by downdraft someplace else, which explains how the Indian-Ocean warming could, like El Niño itself, impact the global atmospheric circulation. "Now that you have the Indian Ocean competing with the Pacific, the downward motion has to occur somewhere else," says Trenberth. "That affects the whole nature of the monsoon and weather in the tropics, and the teleconnections [long-distance impacts] around the world."

Earth's climate is a system, Trenberth insists. "Any time you try to split anything out in a linear fashion and say this is responsible for this portion, this is responsible for that portion, it's to some extent artificial." Computer weather models could suggest how a change in Indian Ocean temperature would affect weather in the Northern Hemisphere, but they are models, not reality, and they have not been run, Trenberth says.

Boat floats along on the open sea as a metal spire bobs along atop an orange flotation device
This buoy monitors Pacific Ocean conditions, helping detect and study El Niño. Photo: NOAA/PMEL/TAO

But here's another complication: We have said that global warming has played only a small role in directly raising temperatures this winter. But 1976, the year that Trenberth says El Niño began to stray from its historic course, happens to be about when global warming started to accelerate. So even if global warming is not the immediate cause of most of the winter warming, it could be working behind the scenes to warm the oceans, change El Niño, and thereby change global weather in the short term.

Here's way more on the Southern Oscillation.

The best explanation for today's abnormally warm winter weather is a "mixture of El Niño and other stuff going on in the tropics, especially the Indian Ocean," Trenberth says.

When was El Niño first recognized?

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