The Year of the El Niño

This past year will undoubtedly go down in climate history as the year of the El Niño—one of the most profound events in recorded climate history. It first came to the attention of Americans in the form of heavy rains and storms, unsuitably timed for a visit by the Queen of England. However, as reports from other parts of the world started coming in, it became apparent that more than royal discomfort was involved.

Crippling drought in southern Africa, Sri Lanka, southern India, Indonesia, the Philippines, and Australia; floods and heavy rains in the U.S. Gulf states and the Northeast, in Ecuador, northern Peru, Bolivia (where 26,000 people were left homeless), and in western Europe all followed a rise in the surface temperature of the eastern Pacific ocean and an invasion of warm water known as El Niño. El Niño—Spanish for the Christ Child—is so named in South America because it usually occurs during the Christmas season.

The cost of these events can be only estimated. Hundreds of people died in Peru and Ecuador from floods and mudslides. Total damage to crops and property world-wide is estimated at $13 billion by the National Oceanic and Atmospheric Administration (NOAA). And this does not measure the misery of the homeless or those who died or will die from the famine and pestilence that accompany drought.

El Niño is not a new phenomenon, but a cyclical warming of the ocean that has long been known to Peruvian fishermen (whose anchovy fishery usually is destroyed by it) and by scientists at least since 1877.

However, the most recent El Niño was different in several ways. For one thing, it started in late spring instead of in December. The magnitude of the rise in sea temperature was remarkable—in some places 15° to 18°F above normal temperatures for that time of year. A typical El Niño has a rise of 5° to 6°F. The 1983 El Niño began in the mid-Pacific, not off the coast of South America as it usually does, and involved changes in sea level that covered millions of square miles of water and massive transfers of heat and water.

The Southern Oscillation

Scientists who have been exploring the El Niño phenomenon are reluctant to use terms like "causes" or "produces." Instead, they prefer the more cautious "associated with." One of the events that appears to be associated with an El Niño is a large-scale seesaw of air between the Pacific and Indian oceans in the tropics and subtropics that occurs every two to seven years. This seesaw, called the Southern Oscillation, is suspected of being linked to a whole family of ocean and atmospheric fluctuations around the globe. For example, precipitation and pressure during the monsoon in India vary with changes in the Southern Oscillation. Significantly, there are also huge changes in sea surface temperature in the equatorial Pacific that are associated with the Southern Oscillation.

In fact, the oceans and the atmosphere are in constant communication—the ocean storing, rearranging, and releasing heat to drive the atmospheric circulation system while the heated air masses above help move the ocean current system and affect the release of heat from the ocean surface. This interaction operates most effectively in the tropics. If the physics and dynamics of this interaction were understood, they might provide a key to understanding and anticipating global fluctuations in climate. For instance, there were early signals of the recent El Niño in the form of sharp changes in the usual pattern of air pressure and flow between May and June and large-scale changes in rainfall in the Pacific, but they were not recognized as precursors of an El Niño.

An improved understanding of how the oceans and atmosphere interact not only would allow meteorologists to anticipate an El Niño, but also could lead to improved medium- and long-term weather prediction for the North Pacific and North America. For example, it might be possible to predict extremely cold winters several seasons in advance with reasonable accuracy in some parts of the United States since there seems to be a link between temperature patterns in the North Pacific and cold winters in the eastern United States.

The TOGA study

A decade-long international program to study large-scale ocean-atmosphere interactions has been in planning for some time and will get under way this year. Dubbed TOGA, for Tropical Ocean Global Atmosphere, it will attempt to answer some key questions about climate changes. Among them are:

• What triggers the sequence of events that are connected with warming of the sea surface in the first place?
• What determines the frequency of the Southern Oscillation? An El Niño occurs every four to five years with a corresponding major swing in the Southern Oscillation, but individual events can range from two to ten years.
• Why do the sea surface changes leading to an El Niño begin early in the calendar year, with maximum warming taking place a few months after the normal maximum sea surface temperature in February? Does this offer any clues to ocean-atmosphere coupling?
• There is also the question of El Niños that occur at unexpected times or that last longer than the typical two years.
• How is the El Niño-Southern Oscillation signal transmitted from the western Pacific, where the weakening of the trade winds occurs, to the coast of South America, the site of the greatest changes in ocean temperature?
• Allied with this is the question of how these events interact with conditions in the midlatitudes. Anomalies in midlatitude sea surface temperature occur too soon after the equatorial changes for them to be the consequence entirely of ocean processes. It is possible that changes in sea surface temperature in the tropics affect air circulation in the midlatitudes, but this is not at all certain.

Research will be carried out by NOAA, the Office of Naval Research, the National Science Foundation, and the National Aeronautics and Space Administration.

The TOGA research program will seek answers to these questions by monitoring wind fields, atmosphere and ocean thermal structure (this is a description of how temperature changes over vertical and horizontal space), and sea surface topography. Some of these observations are already being done as part of research on the equatorial Pacific. Additional studies are planned for the eastern, central, and western Pacific. Monitoring equipment includes fixed and drifting buoys, expendable bathythermographs, and meters to measure current. A variety of observation platforms will also be needed, including aircraft, ships, satellites, and island-based stations.

The weather satellites are critical for this project since they are the only way atmospheric measurements can be obtained on a global scale. Currently the geostationary and polar-orbiting satellites daily produce information on cloud cover, snow or ice, and thermal radiation, as well as data that can be used to determine the vertical distribution of atmospheric temperature and moisture, and wind field. Continued and reliable coverage by both sets of satellites is essential to research on the El Niño—Southern Oscillation connection.

Despite its human costs, last year's El Niño was a scientific bonanza. Not only has the rapid exchange of information among meteorologists, oceanographers, and fishermen provided information far superior to anything previously existing, but for the first time it was apparent that El Niño is not the result of random, unconnected events, but of events that may be driven by physical laws. If these laws can be discovered, it may be possible to correctly interpret the signals that precede an El Niño and plan for the weather changes that accompany it.

While the old saw that you can't do anything about the weather is largely true, if an El Niño can be anticipated early enough, it may be possible to mitigate its effects and its costs.