El Niño is a warm event associated with an oscillation of the ocean-atmosphere system in the tropical Pacific. This oscillation has important consequences for weather around the globe, though El Niño is typically associated with South America and southern California. Originally, the term “El Niño” (literally “the little boy” in reference to the Christ child) denoted a warm southward-flowing ocean current that occurred around Christmas time off the west coast of Peru and Ecuador. The term was later restricted to unusually strong warming that disrupted local fish and bird populations every few years. However, as a result of the frequent association of South American coastal temperature anomalies with globally warm events, El Niño has now become synonymous with large-scale, climatically significant, warm events (National Oceanic and Atmospheric Administration 2009). Every three to seven years El Niño can be expected to move up the western coast of the Americas, flooding coastal areas, while providing mild winters for more northerly locales.
Under “normal” (non–El Niño) conditions, the tropical trade winds blow from east to west, piling up warm water in the western Pacific. In the eastern Pacific, the trade winds pull up cold, deep, nutrient-rich water along the equator, from the coast of Ecuador to the central Pacific. The warmed western Pacific Ocean drives a particularly vigorous hydrologic cycle with towering cumulus clouds and tropical storms. Heat and moisture lofted into the upper atmosphere by the clouds and storms are distributed by high-altitude winds across vast regions of the globe (Reynolds et al. 2003b).
During an El Niño, this typical situation is disrupted and the trade winds weaken, reducing the upwelling of cool waters in the eastern Pacific and allowing the pool of warm water in the west to drift eastward toward South America. As the central and eastern Pacific Ocean warms, atmospheric pressure gradients along the equator weaken, and the trade winds diminish even more (Reynolds et al. 2003b).
Effects of El Niño on the World
In the United States, particularly southern California, El Niño results in a lot of rain and devastating flooding; this is also the result in South America. However, other parts of the United States and the world are affected in vastly different ways:
Farther up the U.S. Pacific coast, Oregon and Washington have mild winters as a result of warm ocean currents.
El Niño creates a high air current that travels across the United States. In the Rocky Mountains, this will typically bring more snow, creating a boon for skiers and winter recreation enthusiasts.
In the U.S. Northeast, El Niño lessens the effect of hurricanes and creates mild winters.
In the U. S. Southwest, El Niño causes enhanced streamflow, which can lead to flooding, rising water levels in terminal (closed-basin) lakes such as the Great Salt Lake, and landslides and debris flows. Over the past few decades some of the most destructive landslides and debris flows in the Southwest have occurred during El Niño events (Reynolds et al. 2003a).
Across the entire southern United States and as far north as South Dakota, El Niño significantly increases winter precipitation, with the strongest effects occurring in southern Arizona, central Nebraska, and along the Gulf of Mexico (Kurtzman and Scanlon 2007).
In the Western Pacific, near Australia and Indonesia, large droughts and cooler ocean temperatures occur. When the trade winds blow the warm water back to the west, the warm water causes more storms, relieving the droughts.
In contrast to the warm El Niño event, La Niña is a cold event. Likewise, the name—La Niña—is the opposite of El Niño and means “the little girl.” When the trade winds along the Pacific coast move the warm waters associated with El Niño westward toward eastern Asia and Australia, La Niña starts to affect the world. Typically, the effects are the opposite of El Niño: where the ocean was warm, now it is significantly cooler. The coasts of South America and southern California are much less rainy. The northern coast of the United States becomes significantly cooler. The Rocky Mountains experience much less snow, and the Northeast has many more storms associated with its winters. Eventually the trade winds oscillate and El Niño becomes the prominent weather feature in the Pacific, which in turn cycles back to La Niña in just a few years.