El Niño

Pacific Wind and Current Changes Bring Warm, Wild Weather

By Mike Carlowicz Design by Joshua Stevens February 14, 2017

If you want to understand how interconnected our planet is—how patterns and events in one place can affect life half a world away—study El Niño.

Episodic shifts in winds and water currents across the equatorial Pacific can cause floods in the South American desert while stalling and drying up the monsoon in Indonesia and India. Atmospheric circulation patterns that promote hurricanes and typhoons in the Pacific can also knock them down over the Atlantic. Fish populations in one part of the ocean might crash, while others thrive and spread well beyond their usual territory.

GOES-West satellite image of tropical cyclones.

The GOES-West satellite observed four tropical cyclones roiling the Pacific on September 1, 2015, during an El Niño event. (Image courtesy of the NASA/NOAA GOES Project.)

During an El Niño event, the surface waters in the central and eastern Pacific Ocean become significantly warmer than usual. That change is intimately tied to the atmosphere and to the winds blowing over the vast Pacific. Easterly trade winds (which blow from the Americas toward Asia) falter and can even turn around into westerlies. This allows great masses of warm water to slosh from the western Pacific toward the Americas. It also reduces the upwelling of cooler, nutrient-rich waters from the deep—shutting down or reversing ocean currents along the equator and along the west coast of South and Central America.

The circulation of the air above the tropical Pacific Ocean responds to this tremendous redistribution of ocean heat. The typically strong high-pressure systems of the eastern Pacific weaken, thus changing the balance of atmospheric pressure across the eastern, central, and western Pacific. While easterly winds tend to be dry and steady, Pacific westerlies tend to come in bursts of warmer, moister air.

Illustration of atmospheric circulation.

Atmospheric circulation over the equator—the Walker circulation—changes substantially with the arrival of El Niño. (Illustration by NOAA/Climate.gov)

Because of the vastness of the Pacific basin—covering one-third of the planet—these wind and humidity changes get transmitted around the world, disrupting circulation patterns such as jet streams (strong upper-level winds). We know these large-scale shifts in Pacific winds and waters initiate El Niño. What we don't know is what triggers the shift. This remains a scientific mystery.

Illustration of the Pacific jet stream.

El Niño usually alters the Pacific jet stream, stretching it eastward, making it more persistent, and bringing wetter conditions to the western U.S. and Mexico. (NASA Earth Observatory illustration by Joshua Stevens.)

What is not a mystery is that El Niño is one of the most important weather-producing phenomena on Earth, a "master weather-maker," as author Madeleine Nash once called it. The changing ocean conditions disrupt weather patterns and marine fisheries along the west coasts of the Americas. Dry regions of Peru, Chile, Mexico, and the southwestern United States are often deluged with rain and snow, and barren deserts have been known to explode in flowers. Meanwhile, wetter regions of the Brazilian Amazon and the northeastern United States often plunge into months-long droughts.

Chart showing increased rainfall during El Niño years.

Typically dry regions can experience nearly two times as much rain during a strong El Niño. (NASA Earth Observatory chart by Joshua Stevens, using data from the California-Nevada Climate Applications Program.)

El Niño events occur roughly every two to seven years, as the warm cycle alternates irregularly with its sibling La Niña—a cooling pattern in the eastern Pacific—and with neutral conditions. El Niño typically peaks between November and January, though the buildup can be spotted months in advance and its effects can take months to propagate around the world.

Though El Niño is not caused by climate change, it often produces some of the hottest years on record because of the vast amount of heat that rises from Pacific waters into the overlying atmosphere. Major El Niño events—such as 1972-73, 1982-83, 1997-98, and 2015-16—have provoked some of the great floods, droughts, forest fires, and coral bleaching events of the past half-century.

Chart showing temperatures over time.

El Niño years tend to be warmer than other years. (NASA Earth Observatory chart by Joshua Stevens, using data from the Goddard Institute for Space Studies.)

NASA, the National Oceanic and Atmospheric Administration (NOAA), and other scientific institutions track and study El Niño in many ways. From underwater floats that measure conditions in the depths of the Pacific to satellites that observe sea surface heights and the winds high above it, scientists now have many tools to dissect this l'enfant terrible of weather. The data visualizations on the next page show most of the key ways that we observe El Niño before, during, and after its visits.

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