The El Niño phenomenon

The shortest, or interannual, timescale relates to natural variations that are perceived as years of unusual weather—e.g., excessive heat, drought, or storminess. Such changes are so common in many regions that any given year is about as likely to be considered exceptional as typical. The best example of the influence of the oceans on interannual climate anomalies is the occurrence of El Niño and La Niña conditions in the eastern Pacific Ocean at irregular intervals of about 3–8 years. The stronger El Niño episodes of enhanced ocean temperatures (2–8 °C [3.6–14.4 °F] above normal) are typically accompanied by altered weather patterns around the globe, such as droughts in Australia, northeastern Brazil, and the highlands of southern Peru, excessive summer rainfall along the coast of Ecuador and northern Peru, severe winter storminess along the coast of central Chile, and unusual winter weather along the west coast of North America.

The effects of El Niño have been documented in Peru since the Spanish conquest in 1525. The Spanish term “la corriente de El Niño” was introduced by fishermen of the Peruvian port of Paita in the 19th century, referring to a warm, southward ocean current that temporarily displaces the normally cool, northward-flowing Humboldt, or Peru, Current. The name is a pious reference to the Christ Child, chosen because of the typical appearance of the countercurrent during the Christmas season. By the end of the 19th century, Peruvian geographers recognized that every few years this countercurrent is more intense than normal, extends farther south, and is associated with torrential rainfall over the otherwise dry northern desert. The abnormal countercurrent also was observed to bring tropical debris, as well as such flora and fauna as bananas and aquatic reptiles, from the coastal region of Ecuador farther north. Increasingly during the 20th century, El Niño came to connote an exceptional year rather than the original annual event.

As Peruvians began to exploit the guano of marine birds for fertilizer in the early 20th century, they noticed El Niño-related deteriorations in the normally high marine productivity of the coast of Peru as manifested by large reductions in the bird populations that depend on anchovies and sardines for sustenance. The preoccupation with El Niño increased after mid-century, as the Peruvian fishing industry rapidly expanded to exploit the anchovies directly. Fish meal produced from the anchovies was exported to industrialized countries as a feed supplement for livestock. By 1971 the Peruvian fishing fleet had become the largest in its history; it had extracted very nearly 13 million metric tons of anchovies in that year alone. Peru was catapulted into first place among fishing nations, and scientists expressed serious concern that fish stocks were being depleted beyond self-sustaining levels, even for the extremely productive marine ecosystem of Peru. The strong El Niño of 1972–73 captured world attention because of the drastic reduction in anchovy catches to a small fraction of prior levels. The anchovy catch did not return to previous levels, and the effects of plummeting fish meal exports reverberated throughout the world commodity markets.

El Niño was only a curiosity to the scientific community in the first half of the 20th century, thought to be geographically limited to the west coast of South America. There was little data, mainly gathered coincidentally from foreign oceanographic cruises, and it was generally believed that El Niño occurred when the normally northward coastal winds off Peru, which cause the upwelling of cool, nutrient-rich water along the coast, decreased, ceased, or reversed in direction. When systematic and extensive oceanographic measurements were made in the Pacific in 1957–58 as part of the International Geophysical Year, it was found that El Niño had occurred during the same period and was also associated with extensive warming over most of the Pacific equatorial zone. Eventually tide-gauge and other measurements made throughout the tropical Pacific showed that the coastal El Niño was but one manifestation of basinwide ocean circulation changes that occur in response to a massive weakening of the westward-blowing trade winds in the western and central equatorial Pacific and not to localized wind anomalies along the Peru coast.