What is El Nino?

El Nino is a name for the unusual warming of the ocean's surface, which occurs every 2 to 7 years along the equator in the central and eastern Pacific Ocean. During a strong El Nino, surface temperatures can rise as much as 15 deg F greater than normal over an immense area, stretching thousands of square miles from the coast of Peru to across the international date line. The addition of this heat and humidity source to the global atmospheric circulation is known to disrupt weather and climate patterns in regions around the world.

The same region of the Pacific Ocean can also turn colder than normal, though not quite to the magnitude of an El Nino event. These periods of colder than normal sea surface temperatures are known as La Nina and occur as frequently as the better-known El Nino.

Neutral is the term for when neither El Nino nor La Nina are present in the Pacific. The majority - about half - of all years are classified as neutral years. El Nino and La Nina events tend to repeat roughly every four to seven years, although one is not always followed by the other. A history of recent El Ninos helps to clarify those points.

Neutral Conditions

Under neutral conditions, trade winds blow from east to west near the Equator in the Pacific Ocean. This results in higher sea-surface heights in the Western Pacific, with about an 18 inch difference between Asia and South America. This small difference nonetheless helps to allow the water in the western Pacific to be about 14:F warmer than along the South American coast. The cold, nutrient-rich water helps to support fishing activities off of the South American coast, while the warm waters result in large amounts of precipitation near Asia.

El Nino

During an El Nino event, however, the trade winds lessen in intensity, or relax, resulting in warmer ocean temperatures in the eastern Pacific, nearly as warm as those typically found in the west Pacific. Warmer waters lead to increased rain, oftentimes bringing about flooding in the mountainous areas along the coast. Furthermore, as a result of the warmer waters, the fishing industry is negatively impacted.

On a global scale, the changes in the heat distribution over the surface of the ocean during an El Nino event result in large, far-reaching changes in global climate. These result in many of the impacts highlighted in the U.S. Climate Variations and Southeast Climate Variations sections of the site.

La Nina

During La Nina events, the equatorial trade winds instead strengthen, resulting in colder water being brought up from the ocean's floor. The change in ocean temperatures between the eastern and western Pacific approaches 20¡F; since the heat distribution across the oceans is affected, much like with El Nino, global climates are impacted.

The effects and climate variations of a La Nina event are not the same as those of El Nino and oftentimes oppose each other. Furthermore, La Nina events tend to be weaker (on average) than El Nino events as the changes in ocean temperatures are not as great, leading to lessened variations. Nonetheless, El Nino, La Nina, and neutral phase events greatly impact not only our climate, but the climate of the entire world.