El Niño was first recognized by fishermen from Ecuador and Peru, who noticed that the waters of the eastern Pacific sometimes got warmer in December or January. Because the warming usually occurred near the Christmas season, the event was named El Niño—“little boy,” or “Christ child,” in Spanish. In the 1920s, scientists recognized that this seasonal event of warm equatorial currents related to an east-west atmospheric pressure seesaw and called the phenomena the Southern Oscillation. In the 1960s, scientists discovered that El Niño is countered by the upwelling of cold waters off the west coast of South America that push strong, warm equatorial currents toward the western Pacific, an event that in the 1980s came to be called La Niña, Spanish for “little girl.” Both El Niño and La Niña are part of the global atmospheric circulation pattern that affects the weather at great distances from the equatorial Pacific. This recurring climate pattern involving interactions between the atmosphere and ocean in this area is now called the El Niño–Southern Oscillation, or ENSO.

Thus, ENSO is recognized by El Niño with its warmer-than-average temperatures (▼A) and La Niña with its cooler-than-average temperatures (▼B). Between these two extremes, a neutral phase occurs where atmospheric and oceanic conditions are close to average.

The seesaw pattern of ENSO involves atmospheric pressure changes between the eastern and western Pacific that occurs over irregular intervals of 2–7 years. Low pressure in the western Pacific strengthens the trade winds, which move warm, tropical water toward Australia and Indonesia. By contrast, a drop in pressure in the eastern Pacific weakens the trade winds and strengthens the equatorial countercurrents, which amass large quantities of warm water along the coasts of Peru and Chile. The changes in atmospheric pressure create the wind patterns that produce the weather associated with El Niño and La Niña events. During these periods, an oscillating warming and cooling pattern can change surface ocean temperatures by as much as 3°C compared to normal.

Global Impact of El Niño

El Niño is noted for its potentially catastrophic impact on the weather and economies of Peru, Chile, and Australia, among other countries. As shown in ▲A, during an El Niño, strong equatorial countercurrents amass large quantities of warmer-than-normal water along the west coast of South America. The unusually warm water and associated low pressure cause arid areas of Peru and Chile to receive unusually heavy rains that can lead to major flooding. In addition, the warm surface water blocks the upwelling of colder, nutrient-filled water—the primary food source for millions of small feeder fish (mainly anchovies). El Niño events are associated with drastic fish population declines and drought or flooding impacts to agricultural areas, resulting in significant lost revenues for local and national businesses.

During an El Niño event, winter temperatures are warmer than normal in the north-central United States and parts of Canada (▼A). In addition, significantly wetter winters are experienced in the southwestern United States and northwestern Mexico, while the southeastern United States experiences wetter and cooler conditions. One major benefit of El Niño is a lower-than-average number of Atlantic hurricanes.

Global Impact of La Niña

The atmospheric circulation in the equatorial Pacific during La Niña is dominated by strong trade winds. These wind systems, in turn, generate a strong equatorial current that flows westward from South America toward Australia and Indonesia. This circulation pattern is often associated with flooding in northeastern Australia and Indonesia, whereas especially dry conditions prevail along the west coast of South America.

During La Niña, the cold Peru Current that flows equatorward along the coast of Chile and Peru intensifies, which encourages upwelling of deep nutrient-rich water. As a result, La Niña has come to be known as the “gift giver” in that region because the added nutrients are a boon to marine life, making fishing particularly good.

Typical La Niña winter weather includes cooler and wetter conditions over the northwestern United States and especially cold winter temperatures in the Northern Plains states, while unusually warm conditions occur in the Southwest and Southeast (▲C). In the western Pacific, La Niña events are associated with wetter-than-normal conditions. Another La Niña impact is more frequent hurricane activity in the Atlantic. A recent study concluded that the cost of hurricane damages in the United States is 20 times greater in La Niña years than in El Niño years.

Although what triggers El Niño and La Niña is not fully understood, we do know that they are intimately related to changes in global pressure patterns. Each time an El Niño occurs, the barometric pressure drops over large portions of the eastern Pacific and rises in the tropical portions of the western Pacific. Then, as a major El Niño event comes to an end, the pressure difference between these two regions swings back in the opposite direction, resulting in a La Niña event.

Clearly, weather in one place impacts weather in another place. The link between weather in widely separated regions of the globe is called a teleconnection. For example, the warming of the eastern Pacific associated with an El Niño event is linked to winter flooding in southern California. Further, strong El Niño events are known to significantly weaken monsoon precipitation over India. Because sea-surface temperatures are often predictable months in advance, understanding teleconnection patterns allows meteorologists to make climate predictions for distant locations. For instance, predicting the start of an El Niño enables prediction of North American precipitation and temperature patterns weeks or even months in advance.

• El Niño and La Niña events are part of the global circulation and are related to a seesaw pattern of atmospheric pressure between the eastern and western Pacific called the El Niño-Southern Oscillation (ENSO). El Niño and La Niña events influence weather on both sides of the tropical Pacific Ocean as well as in the United States.

• El Niño refers to episodes of ocean warming in the eastern Pacific along the coasts of Ecuador and Peru. It is associated with weak trade winds, a strong eastward-moving equatorial countercurrent, a weakened Peru Current, and diminished upwelling along the western margin of South America.

• A La Niña event is associated with colder-than-average surface temperatures in the eastern Pacific. La Niña is linked to strong trade winds, a strong westward-moving equatorial current, and a strong Peru Current with significant coastal upwelling.

• El Niño: The name given to the periodic warming of the ocean that occurs in the central and eastern Pacific. A major El Niño episode can cause extreme weather in many parts of the world. The opposite of La Niña.

• El Niño-Southern Oscillation (ENSO): A recurring climate pattern driven by interactions between the atmosphere and ocean and characterized by changes in water temperature and in the central and eastern Pacific Ocean. Also called ENSO.

• La Niña: The name given to the periodic warming of the ocean that occurs in the central and eastern Pacific. A major El Niño episode can cause extreme weather in many parts of the world. The opposite of La Niña.

• Southern Oscillation: The seesaw pattern of atmospheric pressure change that occurs between the eastern and western Pacific. The interaction of this effect and that of El Niño can cause extreme weather events in many parts of the world.