El Niño is a warming of the tropical Pacific Ocean that occurs roughly every 3 to 7 years. It develops in association with swings in atmospheric pressure known as the Southern Oscillation. During El Niño, the tradewinds weaken along the Equator as atmospheric pressure rises in the western Pacific and falls in the eastern Pacific. This condition allows warm water, normally confined to the far western Pacific, to migrate eastward. Upwelling, a process which brings nutrient-rich cold water to the surface along the coast of South America and along the Equator, is shut down, and sea surface temperatures warm in the central and eastern Pacific. Deep cumulus clouds and heavy rains, normally occurring in the western Pacific over the warmest water, migrate eastward in response to these surface temperature changes. These changes leave the western Pacific dry but bring torrential rains to the islands of the central Pacific and the west coast of South America.
Changing air currents
Tropical rainfall also releases heat into the upper troposphere, providing a source of energy to drive global wind fields. Shifts in these precipitation patterns cause changes in the atmospheric circulation that carry the influence of El Niño to parts of the globe remote from the tropical Pacific. The jet streams in both hemispheres of the Pacific intensify and shift equatorward during El Niño, steering wintertime storms into southern California and northern Chile. Northward deflection of air currents at higher latitudes over the North Pacific during El Niño years also brings warmer winter temperatures to parts of Alaska, Canada, and the northern tier of the United States.
La Niña is characterized by stronger than normal tradewinds, colder tropical Pacific sea surface temperatures, and a shift in heavy rainfall to the far western tropical Pacific. It often produces effects on global weather patterns opposite to those of El Niño. As a result, El Niño, La Niña, and Southern Oscillation are often referred to collectively as ENSO, a cycle which oscillates between warm, cold, and neutral states in the tropical Pacific.
Drastic weather changes
In 1997–1998 El Nino brought torrential rainfalls and flooding to parts of California, the southeastern United States, equatorial east Africa, and Chile. It was also responsible for severe droughts in Mexico, Indonesia, and northeast Brazil. It virtually shut down the Atlantic hurricane season in 1997, yet spawned deadly swarms of tornadoes in nine southeastern states in the spring of 1998. Parts of the Midwest and the Great Lakes region experienced their mildest winter in over 100 years, as temperatures soared to record highs between November 1997 and February 1998.
Marine ecological shifts
Effects of the 1997–1998 El Niño on Pacific marine ecosystems were dramatic. The anchovy fishery collapsed off the coast of Peru, and thousands of marine mammals and seabirds perished for lack of food off the coast of California. Sportfishing along the west coast of the United States enjoyed a banner year as exotic tropical fish species migrated northward with El Niño warmed waters.
Economic and environmental losses
One preliminary estimate has put the total cost of the 1997–1998 El Niño at $33 billion due to crop failures, damaged infrastructure (such as roads, bridges, and buildings), reduced energy production and industrial output, and other economic losses. It has also been estimated that over 23,000 lives were lost worldwide as a result of weather-related disasters, and millions more were affected by the damage left in the wake of El Niño. El Niño was an environmental disaster in places such as Indonesia, northeast Brazil, and Mexico, where forest fires raged out of control for months. Among its side effects was the spread of infectious diseases such as malaria, dengue fever, and cholera in Southeast Asia, South America, and Africa.
Observing and predicting trends
The 1997–1998 El Niño was, by some measures, the strongest on record, surpassing the record 1982–1983 occurrence (Fig. 1). These two climate events delimit a remarkable chapter in climate research. The 1982–1983 El Niño was neither predicted nor even detected until nearly at its peak. This failure shocked the scientific community which had been planning a major decade-long international program to study the ENSO cycle. The 1982–1983 El Niño made it starkly clear that both observational and forecasting capabilities were woefully inadequate, so that developing such capabilities became a central theme of the Tropical Ocean-Global Atmosphere (TOGA) research program, which took place from 1985 to 1994.
ENSO observing system
Within the context of TOGA, a new ENSO observing system was developed. It consists of arrays of moored and drifting buoys, shipboard measurements, and a network of island and coastal sea-level measurement stations (Fig. 2). It required financial support from many nations to complete, and was not finished until the final month of the TOGA program (December 1994). A key feature of the observing system is the fast delivery of data via satellite relay, often within hours of collection. These data are used for monitoring evolving climatic conditions, scientific analyses, and ENSO forecasting. Complementing this suite of ocean based measurements is a constellation of space satellites measuring key environmental parameters.
Mapping 1997–1998 El Niño
This El Niño was the first for which the ENSO observing system was in place from start to finish, so that this event was not only the strongest on record but also the best documented. Though its development was similar in many respects to that of previous El Niño events, enhanced definition and fast delivery of the data from the observing system provided crucial information on its rapid evolution. This El Niño developed so explosively that from June to December 1997 each month set a new record high for sea surface temperatures in the eastern equatorial Pacific. By December 1997, most of the equatorial Pacific was covered with water at 28–29°C (82–84°F), which is near the maximum sustainable temperatures possible in the open ocean (Fig. 3). The global impacts of this El Niño were equally spectacular in keeping with the extreme conditions observed in the equatorial Pacific. Then, even more suddenly than it developed, El Niño ended with an unprecedented drop in sea surface temperatures in the eastern and central Pacific, falling at some locations nearly 8°C (14°F) in 30 days. The climate system shifted from the strongest El Niño on record to cold La Niña conditions in the span of a month.
Great strides have been made in ENSO forecasting since the 1982–1983 El Niño. A wide variety of forecasting approaches have been developed, ranging from statistical models based on the average behavior of previous ENSO events, to complex dynamical models that try to represent the physical processes at work in the coupled ocean-atmosphere system. Many of these models had success in predicting, at least one to three seasons in advance, that 1997 would be unusually warm in the tropical Pacific. Many of them also predicted that El Niño would give way to La Niñas in 1998. Long-range weather forecasting schemes that included information about tropical Pacific sea temperature conditions were successful in predicting temperature and precipitation patterns in widely disparate parts of the globe many months in advance. For example, the forecast for wintertime precipitation issued by the National Centers for Environmental Prediction in the fall of 1997 was the most accurate ever for the continental United States.
Successful forecasts, unprecedented high-definition ocean measurements, and record warmth in the tropical Pacific all combined to capture the public attention in 1997–1998. Media coverage was so intense that El Niño became a household word all over the world. As a result, many individuals, municipalities, businesses, and in some cases national governments mobilized resources in an effort to prepare for El Niño's onslaught. It is likely that without the advance warning the toll from El Niño would have been much higher.
However, there were some forecasting failures related to the 1997–1998 El Niño. None of the forecast models predicted the rapid development or intensity of the El Niño before its onset, and none predicted the suddenness of its demise. Expectations of severe droughts in Australia and Zimbabwe and reduced summer monsoon rainfall over India failed to materialize. The reasons for these failures have yet to be fully determined. Factors that can influence the ENSO cycle and its global consequences include chaotic or random processes in the climate system that might enhance or obscure ENSO-related variations. Ocean-atmosphere interactions originating in regions outside the tropical Pacific may also be important, such as in the Indian Ocean where there has been a warming trend in tropical sea temperatures for the past 20 years. In addition, temperatures have been elevated in the tropical Pacific since the mid-1970s in association with a naturally occurring basin-scale phenomenon with a period of several decades. This Pacific Decadal Oscillation affects the background conditions on which ENSO events develop, potentially altering ENSO's character. Finally, 1998 and 1997 were, in that order, the warmest years on record. Occurrence of the 1997–1998 El Niño contributed in part to these extremes, since it is known that global temperatures rise a few tenths of a degrees Celsius following the peak El Niño warming in the tropical Pacific. However, aside from record warmth in 1997–1998, there has been a century-long trend of rising global temperatures, which may be due to anthropogenic greenhouse gas warming. See also: Global warming
Exactly how global warming, influences from outside the tropical Pacific, decadal time-scale variations, and random and chaotic elements of the climate system interact with one another and with ENSO is not entirely clear. It is clear, though, that there have been more El Niños than La Niñas since the mid-1970s, the early 1990s was a period of extended warmth in the tropical Pacific, and the extremely strong 1997–1998 El Niño followed by only 15 years the record-setting El Niño of 1982–1983. Further research is required to better understand the interactions between El Niño and other climate phenomena and to translate that understanding into improved forecasting capabilities.