What
is El Nino?
  El Nino Globe

by David Herring
April 27, 1999

Fishermen who ply the waters of the Pacific off the coast of Peru and Ecuador have known for centuries about the El Niño. Every three to seven years during the months of December and January, fish in the coastal waters off of these countries virtually vanish, causing the fishing business to come to a standstill. South American fishermen have given this phenomenon the name El Niño, which is Spanish for "the Boy Child," because it comes about the time of the celebration of the birth of the Christ Child. During an El Niño, the physical relationships between wind, ocean currents, oceanic and atmospheric temperature, and biosphere break down into destructive patterns that are second only to the march of the seasons in their impacts to weather conditions around the world.

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Kermond Talking Head
John Kermond, of NOAA, discusses the significance of El Niño. (Courtesy of NOAA) (click to view 2.3MB)

       
  El Niño: A temperature anomaly
The vast tropical Pacific Ocean receives more sunlight than any other region on Earth. Much of this sunlight is stored in the ocean in the form of heat. Typically, the Pacific trade winds blow from east to west, dragging the warm surface waters westward, where they accumulate into a large, deep pool just east of Indonesia, and northeast of Australia. Meanwhile, the deeper, colder waters in the eastern Pacific are allowed to rise to the surface, creating an east-west temperature gradient along the equator known as the thermocline tilt.
  What is El Niño?
El Niño: a temperature anomaly
El Niño's impacts on the biosphere
       
  Pacific trade winds and El Nino The trade winds tend to lose strength with the onset of springtime in the northern hemisphere. Less water is pushed westward and, consequently, waters in the central and eastern Pacific begin to heat up (usually several degrees Fahrenheit) and the thermocline tilt diminishes. But the trade winds are usually replenished by the Asian summer monsoon, and the delicate balance of the thermocline tilt is again maintained.

Sometimes, and for reasons not fully understood, the trade winds do not replenish, or even reverse direction to blow from west to east. When this happens, the ocean responds in a several ways. Warm surface waters from the large, warm pool east of Indonesia begin to move eastward. Moreover, the natural spring warming in the central Pacific is allowed to continue and also spread eastward through the summer and fall. Beneath the surface, the thermocline along the equator flattens as the warm waters at the surface effectively act as a 300-foot-deep cap preventing the colder, deeper waters from upwelling. As a result, the large central and eastern Pacific regions warm up (over a period of about 6 months) into an El Niño. On average, these waters warm by 3° to 5°F, but in some places the waters can peak at more than 10°F higher than normal (up from temperatures in the low 70s Fahrenheit, to the high 80s).

 

This animation (left) shows the relationship between the direction and intensity of the Pacific trade winds, and the formation of El Niño. The arrows show surface wind dynamics, while the colors represent sea surface temperature. Notice how the warmer water expands, while cooler water contracts. This animation was made using ADEOS scatterometry data to show winds and Topex-Poseidon data to show water temperature and topography. (Courtesy of Greg Shirah, Goddard SVS) (3.4MB)

Relevant Satellite Missions:
SeaWiFS
Terra

Related Sites:
El Niño/La Niña Updates

   
Thermocline tilt January 1997Thermocline tilt April 1997Thermocline tilt July 1997

In the east, as temperatures increase, the water expands, causing sea levels to rise anywhere from inches to as much as a foot. But in the western Pacific, sea level drops as much of the warm surface water flows eastward. During the 1982-83 El Niño, this drop in sea level exposed and destroyed upper layers of coral reefs surrounding many western Pacific islands.

next:El Niño's impacts on the biosphere

 

This series of images shows how the thermocline tilt changes during the onset of an El Niño. Notice how in January 1997 (left) there is a steep temperature gradient between the western and eastern Pacific Ocean. In April 1997 (center), much of the deep pool of warm water in the west has migrated eastward, lessening the temperature gradient. By July 1997 (right), there is a full-blown El Niño and the thermocline is flat. (Courtesy of Greg Shirah, Goddard SVS)

 What
is El Nino?

Phytoplankton bloomDuring normal years, when there is a steep thermocline tilt, the cold, deep currents flowing from Antarctica up the west coast of South America are allowed to upwell, bringing essential nutrients that would otherwise lie at the bottom. Phytoplankton living near the surface depend upon these nutrients for survival. In turn, fish and mammals depend upon phytoplankton as the very foundation of the marine food chain. As previously explained, the warm surface waters of an El Niño prevent this upwelling, effectively starving the phytoplankton population there and those animals higher up the food chain that depend upon it. Fishermen in Peru and Ecuador generally suffer heavy losses in their anchovy and sardine industries.

At Christmas Island, as a result of the sea level rise during the 1982-83 El Niño, sea birds abandoned their young and flew out over a wide expanse of ocean in a desperate search for food. Along the coast of Peru during that same time period, 25 percent of the adult fur seal and sea lion populations starved to death, and all of the pups in both populations died. Similar losses were experienced in many fish populations.

Meanwhile, over a six-month period about 100 inches of rainfall fell in Ecuador and northern Peru--ordinarily a desert region. Vegetation thrived and the region grew lush with grasslands and lakes, attracting swarms of grasshoppers and, subsequently, birds and frogs that fed on the grasshoppers. Many fish that had migrated upstream during the coastal flooding became trapped in the drying lakes and were harvested by local residents. Shrimp harvests were also very high in some of the coastal flood regions, but so too was the incidence of malaria cases due to thriving mosquito populations.

El Niño's impacts on weather patterns
El Niño's effects are not limited to the tropical regions off the western coasts of Peru and Ecuador. Its effects are felt all over the world, where the disruption of normal local weather patterns can have tragic and/or profound economic consequences. As warm water migrates eastward, increased heat and moisture rises into the atmosphere, altering the weather patterns in neighboring regions, which in turn can ripple out to affect still other region weather patterns around the globe. For instance, a severe El Niño will enhance the jet stream over the western Pacific and shift it eastward, leading to stronger winter storms over California and the southern United States, with accompanying floods and landslides. In contrast, El Niño can also cause severe droughts over Australia, Indonesia, and parts of southern Asia.

While El Niño is known to lower the probability of hurricanes in the Atlantic, it increases the chances of cyclones and typhoons in the Pacific. The 1982-83 El Niño is estimated by NOAA to have caused some $8 billion in damages due to floods, severe storms, droughts and fires around the world.

return to:What is El Niño?

 

This animation shows sea surface temperature across the equatorial Pacific Ocean (top) during the 1997-98 El Niño. The lift-out shows a higher-resolution image of ocean color in the region surrounding the Galapagos Islands. Play the animation several times and note the transitions that occur between May 10 and May 25, 1998. As the El Niño recedes, surface temperatures cool allowing colder, nutrient-rich currents to upwell. There is a large, almost immediate bloom of phytoplankton in response to the replenished food source. (Courtesy of Greg Shirah, Goddard SVS) (853k)

pullquote

What is El Niño?
El Niño: a temperature anomaly
El Niño's impacts on the biosphere

What is La Niña?

Relevant Satellite Missions:
SeaWiFS
Terra

Related Sites:
El Niño/La Niña Updates