El Niño and La Niña are changes in the winds and ocean currents of the tropical Pacific Ocean that have far-reaching effects on global weather patterns. Together, El Niño and La Niña are extremes that make up a cycle called the El Niño Southern Oscillation (ENSO). An oscillation is a repeated movement or time period. El Niño and La Niña 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 Niño and La Niña Occur
El Niño 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 Niña episodes occur during the opposite conditions, when the tropical winds and currents are unusually strong.
During normal, non–El Niño 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 Niño phase of an ENSO cycle begins with a dramatic warming of the waters off of South America and a decline of marine (ocean) life.
El Niño and La Niña
El Niño and La Niña
La Niña, the opposite phase of an ENSO cycle, occurs when the southeast trades are particularly strong. La Niña events are marked by a strengthening of the South American upwelling and a good fishing season. La Niña events often, but not always, follow El Niño events.
Discovery of El Niño and La Niña
Peruvian fishermen who depended on the South American upwelling for their livelihoods recognized and named the El Niño 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 Niño, Spanish for “the boy child,” after the Christ child. The other half of the ENSO cycle was named La Niña, “the girl child,” much later.
El Niño 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 Niño 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 Niño sea warming were related.
Effects of El Niño and La Niña
The effects of El Niño 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