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image showing sea surface temperatures in normal and El Nino conditions.


What is El Niño?


El Niño is defined as warmer than normal sea-surface temperatures (SSTs) in the tropical Pacific ocean that impact global weather patterns. This deviation from normal SSTs can have large scale impacts, not only on ocean processes, but also on the atmosphere. The graphic shown at the upper-left shows a normal SST pattern from December 1993, indicating no large scale deviations in the SST pattern.

The bottom graphic shows the SST pattern from December 1997 which happened to be during one of the largest El Niño events recorded during the 20th century. It is easy to see a large broad area of red, centered on the equator over the eastern Pacific Ocean. This area of red denotes SSTs which are 2-6 degrees warmer than normal values at that time of year. This is the signal that scientists look for when diagnosing El Niño events. As can be seen, the size of this warm pool of ocean water can be larger than the entire United States!


Image showing circulations in normal and El Nino years.

So how does this large pool of warm water impact the atmosphere?

In normal, non-El Niño conditions (top panel of diagram to the left, land masses represented in brown), the trade winds blow toward the west across the tropical Pacific Ocean. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.

The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper layers. This cold water is nutrient-rich, supporting a diverse marine ecosystem and major fisheries. Rainfall is found in rising air over the warmest water in the western Pacific, and the east Pacific is relatively dry. The diagram shows the typical convective loop with rising air over the western Pacific and sinking air over the eastern Pacific.

During El Niño (bottom panel of diagram to the left), the trade winds relax in the central and western Pacific leading to a movement to the east of the warm pool of water. This warm pool of water moves eastward until it spreads across the entire Pacific Ocean. The normally cool water located off the South American coast is replaced by much warmer waters, and without the trade winds, the upwelling which supplies the cooler sub-surface ocean water is weakened. This leads to the warm SSTs that were shown at the figure at the top of this webpage, with the strongest deviations from normal just off the South American coastline. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in the normally wet regions of Indonesia and Australia. This eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation pattern as can be seen in the diagram. This large change in the circulation pattern in turn forces changes in weather in regions far removed from the tropical Pacific, such as Arizona.


How did it get the name El Niño?

El Niño means The Little Boy, or Christ Child in Spanish. This name was chosen based on the time of year in which the warm waters developed off the South American coast. El Niño was originally recognized by fishermen off the coast of South America in the 1600s, with the appearance of unusually warm water in the Pacific Ocean. This phenomenon typically occurred near the beginning of the year and meant that fishing would be more difficult due to the lack of nutrients brought up to the ocean surface by upwelling. Since fishing villages depended on the fish that were caught by the fishermen, these episodes were documented as harsh times for the villages due to lack of food and the prevalence of flooding rains.

Effects of El Nino on winter weather conditions.

What are the global impacts of El Niño?

As mentioned, El Niño has impacts that are felt not only near the equator, but far removed from the equator as can be seen in the figure to the left. A large area of dry and warm conditions are found in normally wet regions of the world such as Indonesia and Australia. Places that are normally dry, like western South America and the southwest United States tend to be much wetter than normal. Western Canada and Alaska along with portions of the Midwestern United States tend to enjoy a much warmer than normal winter as well. The southeast United States tends to experience wet and cool conditions throughout much of an El Niño winter.


What about the impact of El Niño on northern Arizona?

The general effects of El Niño on northern Arizona tend to be felt most during the winter months. This is due to the stronger jet stream that typically occurs during the winter, when the effects of the warmer pool of water on the atmosphere are strongest. Across northern Arizona, El Niño winters tend to have more prolonged story periods with fewer and shorter dry breaks than normal. The storm track tends to have fewer storm systems originating from colder regions, and more originating from warmer regions with more moisture associated with them. Because of this, El Niño winters tend to be warmer and wetter than a normal winter across northern Arizona. For more information, please check out the Science Corner webpage called How can El Niño and La Niña affect Arizona?

How often do El Niño events occur?

El Niño events tend to occur on average about every 3 to 5 years and typically last for a year. However, El Niño events can last for two years on occasion. For a list of El Niño (warm) events, please go to the Climate Prediction Center List of Cold and Warm Episodes by Season.

How can I find out more about El Niño ?

Numerous webpages can be found on the internet with information regarding El Niño. Those links provided below are only a few of the locations on the web where you can find more information regarding this ocean-atmospheric phenomenon.


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