El Niño and La Niña

El Niño and La Niña are disruptions of the oceanic and atmospheric systems of the equatorial Pacific Ocean that have far reaching effects on Earth's weather patterns. El Niño and La Niña do not change with the regularity of the seasons; instead, they repeat about every two to seven years. They are the extremes in an aperiodic, or irregular, cycle called the El Niño Southern Oscillation (ENSO), during which warm surface waters from the western Pacific Ocean spread toward the South American coastline.

During normal, non-ENSO periods, southeasterly (east to west) Trade Winds push equatorial surface waters into western half of the Pacific, driving the northwest-flowing Southern Equatorial Current, and creating a mound of warm water around Indonesia. Sea surface temperatures near Indonesia are typically about 46°F (8°C) higher than those near Ecuador, and the sea surface is about 1.6 ft (0.5 m) higher. The pool of warm water in the western Pacific warms the air above it, creating an upward current of moist air that rises to form rain clouds. The coastal areas and islands of the western Pacific typically enjoy abundant rainfall and support lush, biologically diverse rainforests, including those of Borneo and New Guinea. Meanwhile, along the coast of South America, cold, nutrient-rich waters from the deep ocean rise to the sea surface, since the warmer surface waters have been blown westward. The result is called an upwelling, which nourishes abundant phytoplankton and zooplankton, the tiny sea plants and animals that provide food for many other types of sea life. The South American upwelling is a very productive region for fish and the animals, including humans, who depend on fish for food. The cold water of an upwelling cools the air above it, creating high-pressure zones of sinking dry air. Regions near upwellings, like coastal Peru and Ecuador, tend to be arid (desert-like).

An ENSO event begins with a lessening of the trade winds in the equatorial Pacific, and a corresponding collapse of the sea surface slope between Indonesia and South America. The pile of warm water in the western Pacific sloshes toward the coast of South America, and shuts down the South American upwelling. A dramatic warming of the waters off of South America and a corresponding decline of marine productivity indicates the El Niño phase of the southern oscillation. La Niña, the opposite phase of an ENSO cycle, occurs when the southeast trade winds are particularly strong. La Niña events are accompanied by colder-than-normal temperatures off South America, and an intensification of the South American upwelling. La Niña events often, but not always, follow El Niño events.

Although El Niño and La Niña take place in a small portion of the southern tropical Pacific, the changes caused by ENSO events affect the weather in large parts of Asia, Africa, Polynesia, and North and South America. El Niño events occurred during 1982–1983, 1986–1987, 1991–1992, 1993, 1994, 1997–1998, and 2002–2003. (It is unusual to have two El Niños in row, as happed in 1993 and 1994.) Disruption of the vertical atmospheric circulation over the southern equatorial Pacific accompanies slackening of the Southern Equatorial Current during El Niño years. As a result, the intensity of the low-pressure system over the southwest Pacific lessens, as does the high-pressure system over the Andes, bringing abnormally dry conditions to Indonesia, and unusually wet weather to the west coast of South America. The more distant climatic effects of El Niños include dryer-than-normal conditions in eastern Africa and western South America; wetter-than-normal weather in equatorial Africa, southern South America, and the southern United States; and abnormally warm winters in Japan and northern North America. In the United States, the 1982–83 El Niño, was associated with record snowfall in parts of the Rocky Mountains, flooding in the southern United States, and heavy rain storms in southern California, which brought about floods and mud slides. The opposite effects, drought in the southern United States, and unusually cold winters in Japan, for example, often accompany La Niña episodes, which occurred recently in 1995–1996 and 1998–1999.