El Nino and La Nina are changes in the winds and ocean currents of the tropical Pacific Ocean that have far-reaching effects on global weather patterns. Together, El Nino and La Nina are extremes that make up a cycle called the El Nino Southern Oscillation (ENSO). An oscillation is a repeated movement or time period. El Nino and La Nina events do not occur in a regular or seasonal pattern; instead, they repeat about every two to seven years and last for a few months.
How El Nino and La Nina Occur
El Nino events occur when the trade winds and equatorial current south of the equator in the Pacific Ocean lessen in intensity. The trade winds, or trades, are strong, steady winds that blow from east to west and drive strong west-flowing ocean currents on either side of the equator. (The trade winds are named for their role in propelling sailing ships carrying cargo to trade around the world.) The equatorial current is a sustained pattern of water flowing westward near the equator. Less dramatic La Nina episodes occur during the opposite conditions, when the tropical winds and currents are unusually strong.
During normal, non-El Nino conditions, the trade winds and equatorial current in the southern Pacific push warm surface water to the west and allow cold water from the deep ocean to rise along the coast of South America. The southeasterly (northwest-blowing) trades south of the equator usually pile a mound of warm water around the islands of Indonesia, and create a zone of cool water that rises called an upwelling off the coasts of Peru and Ecuador. The cold, nutrient-rich waters of the South American upwelling nourish abundant microscopic plants (phytoplankton) and animals (zooplankton) that provide food for larger sea animals. It is a biologically rich region for fish and land animals, including humans who depend on fish for food. The pool of warm water in the western Pacific creates a warm, rainy climate, and the cold water of the upwelling causes an arid (extremely dry) climate in coastal South America.
Occasionally, for reasons not yet fully understood, the trade winds and southern equatorial current in the south Pacific lessen in strength. Warm water sloshes east toward the central coast of South America and shuts down the South American upwelling. The El Nino phase of an ENSO cycle begins with a dramatic warming of the waters off of South America and a decline of marine (ocean) life. La Nina, the opposite phase of an ENSO cycle, occurs when the southeast trades are particularly strong. La Nina events are marked by a strengthening of the South American upwelling and a good fishing season. La Nina events often, but not always, follow El Nino events.
Discovery of El Nino and La Nina
Peruvian fishermen who depended on the South American upwelling for their livelihoods recognized and named the El Nino phenomenon in the nineteenth century. The fishermen noticed that every few years, the seawater became much warmer and the pattern of ocean currents would change within about a month of Christmas day. These changes always marked the start of a very poor fishing season. Normally dry areas along the coast would receive abundant rain. As this typically happened close to Christmas, the fishermen dubbed the phenomenon El Nino, Spanish for "the boy child," after the Christ child. The other half of the ENSO cycle was named La Nina, "the girl child," much later.
Satellite photo of the Pacific Ocean taken November 10, 1997, shows sea surface height compared to normal ocean conditions. White areas show where the water is unusually warm, which is thought to cause El Nino weather patterns.
El Nino has been a well-known local occurrence in coastal South America for more than 150 years. However, scientists only began to realize that the strong El Nino events were part of a disruption that effected the entire Pacific Ocean in the late 1960s. The effects of the southern oscillation were first recognized (and named) in the western Pacific by Sir Gilbert Walker in 1923. Walker was a British scientist who studied the changes in the summer monsoons (rainy seasons) of India. Using meteorological (weather-related) data, he observed that atmospheric pressure (pressure exerted by the air) seesaws back and forth from the Indian Ocean near northern Australia, to the southwestern Pacific near the island of Tahiti. Walker also noticed that the changes in pressure patterns were related to changes in the weather that affected rainfall, fishing, and agricultural harvests in Southeast Asia and India. In the late 1960s, Jacob Bjerknes, a professor at the University of California, first proposed that the Southern Oscillation and the strong El Nino sea warming were related.
Effects of El Nino and La Nina
The effects of El Nino on the climate of the tropical Pacific are now well known. As the mound of warm water in the western Pacific collapses and spreads eastward, the area of heavy rain above it shifts to the east. Fewer rain clouds form over the Pacific Islands, Australia, and Southeast Asia. Lush, biologically diverse rain forests dry out and become fuel for forest fires. Usually arid islands in the central Pacific receive heavy rainfall. In the eastern Pacific, the ocean upwelling weakens as the warm surface water flows toward South America. The surface water off Ecuador and Peru runs low on the nutrients that support the ocean food chain. Many species of fish and birds go elsewhere to find food, and human fishermen face economic hardship. The warmer waters offshore also encourage development of clouds and thunderstorms. Normally dry areas along the west coast of South America experience torrential rains, flooding, and mud slides during the El Nino years. La Nina events are usually less dramatic, but typically cause an opposite effect on the climate (long-term temperature, rainfall, and wind conditions) of the southern Pacific.
El Nino and La Nina also seem to cause far-reaching changes in the weather and climate in other parts of the world. The altered pattern of winds and temperatures in the tropical Pacific may change the paths of the jet streams (high-level winds) that steer storms across North and South America, Africa, Asia, and Europe. El Ninos have been linked to mild, wet winters along the west coast of North America, strong storms in the Gulf of Mexico, heavy rains in the American Southwest, and droughts (lack of rain) in Central America and northern South America. In the El Nino years of 1986-87 and 1997-98, California and Chile both experienced torrential rainstorms and heavy snows that led to mudslides. El Nino may also affect the Indian monsoons and bring drought to northern Africa, thereby threatening agricultural harvests in India, Asia, and Africa. During La Nina episodes like 1998-99, the northern part of the United States may experience heavy snows, increased rainfall, and cold temperatures, while tornado activity increases in the southern states.
The far reaching climatic and economic effects of El Nino and La Nina make understanding ENSO a priority for scientists. Improved understanding and forecasting will help populations plan for the effects of El Ninos and limit economic suffering and starvation. While El Nino and La Nina do have far-reaching effects, scientists are also careful not to blame all extreme or abnormal weather on the phenomenon, or to draw too many connections between Nino and global climate variations.
WORDS TO KNOW
Equatorial current: A sustained pattern of water flowing westward near the equator.
Monsoon: A wind from the southwest that brings heavy rainfall to India and other parts of southern Asia during the summer.
Trade winds: Strong, constant easterly (west-blowing) winds on either side of the equator.
Tropics: Warm, humid region lying north and south of the equator.
Upwelling: An area where cold, often nutrient-rich water rises from the deep ocean to the surface.
