El Niño and the Southern Oscillation:
A Reversal of Fortune

by Kimberly Amaral

Flooding rains and warm weather in Peru wipe out the anchovy harvest. Torrential downpours and mud slides besiege southern California while the Northeast United States has fewer hurricanes and a mild winter. Droughts strike Indonesia, Africa, and Australia--all within the period of the same few months. Could all these events possibly be related?

Absolutely. In fact, it can happen about once every four to seven years--with varying intensity. And it all can be attributed to the same event: the El Niño/Southern Oscillation (ENSO).

El Niño (meaning "The Little One" in Spanish for its tendency to arrive around Christmas) is characterized by a dwindling, or sometimes even a reversal of trade winds. Normally the winds blow east to west across the southern Pacific. These winds travel along the surface of the sea and bring warm surface water along with them to the western coasts.

As the warm surface water is pushed away from the coasts of Peru, colder, nutrient-rich water rushes up to take its place. The result is some of the coolest water found in the lower latitudes (sometimes dipping to 68 degrees F), and a plentiful plankton-filled feeding ground for the anchovy population on which Peruvians count on for much of their economic survival. In the west, the warm water that's pushed along raises sea levels. By the time winds reach Micronesia, the sea level has risen about three feet and the water has warmed about seven degrees F.

But during an El Niño, these trade winds relax, or even reverse, as was the case during the devastating 1982-1983 El Niño. Warm water sloshes back east in a vast, slow wave. Along the Peruvian coast, warm water builds up, driving the thermocline (the buffer zone between the upper layer of water and the frigid ocean below) down. The cooler, rich waters drop along with the thermocline, driving the anchovy population down with it, or killing off a large portion. (During the 1972 El Niño, the anchovy population dropped from 20 million to 2 million). This in turn reduces the number of marine birds who feed on the anchovy. The birds' excrement (guano) produces deposits on the islands off the Peru coast, which in the form of fertilizers is another important economic asset of Peru.

In these maps of four El Niño winters, the orange and red areas illustrate how much warmer (in degrees Celcius) sea surface temperatures were. Courtesy of the Center for Climate Analysis.

Since the turn of the century, scientists believed this phenomena occurred independently of any other weather patterns. But in the last few decades, they have learned that pressure changes and wind currents also play a vital role. Part of this deals with the Southern Oscillation.

The Southern Oscillation is a seesaw of air pressures on the eastern and western halves of the Pacific. Normally the atmosphere above the eastern South Pacific is dominated by a persistent high-pressure zone, while a low pressure zone dominates the west. These two systems are coupled: when the pressure rises in the east, it falls in the west and vice versa. To measure this coupling, meteorologists take the pressure at Easter Island (about 2,700 miles west of South America), and subtract it from the pressure at Darwin in northern Australia. From this they calculate the southern oscillation index.

This difference in pressures drives the trade winds from east to west along the equator under normal conditions. At the same time, high above the ocean surface, this wind circulation is completed, as it continues to blow around from west to east. This convection of air is called the Walker Circulation (named after Sir Gilbert Walker, who first identified it in the 1920s).

Illustrations of normal and El Niño conditions (above and below) from
"What is an El Niño?" Courtesy of the TAO Project at NOAA/Pacific Marine
Environmental Laboratory.
But every four to seven years, the southern oscillation index drops sharply. The east end of the pressure seesaw goes down, the west end goes up, and the Walker Circulation collapses and sometimes even reverses direction. With the collapse of the winds comes the characteristic warm flow of water to the east. All of these elements combined form the phenomena we call El Niño. A typical El Niño event lasts for 14-22 months; it decays when there is no longer enough warm water to sustain the cycle. There are, of course, exceptions to this: a wave of warm water from the 1982 El Niño--measuring only eight inches high in 1994 and traveling about five miles an hour--survived for 12 years.

Many have wondered what causes this abnormal string of events to occur in the first place. Some attribute it to activity that occurs on the ocean floor. One scientist, Daniel Walker, has proposed a connection between underwater earthquakes and the incidence of El Niños. Walker, a seismologist at the University of Hawaii at Manoa, found that the timing of five El Niños since 1964 coincides closely with the occurrence of earthquakes on the East Pacific Rise, a mountain chain on the ocean floor that passes just west of Easter Island. Walker says if the volcanic heat were to reach the ocean surface, that would warm the air surrounding it and could trigger an ENSO event. Most other scientists consider this an unlikely possibility: heat could not reach the surface quickly enough, and a rise of heat from the Pacific floor would have been noticed before.

"It's a coupled ocean and atmosphere problem. The ocean is sensitive to the winds and the winds vary due to ocean temperatures," says John Toole, an associate scientist in physical oceanography at Woods Hole Oceanographic Institution in Massachusetts. Toole worked for several years on the Tropical Ocean and Global Atmosphere Programme (TOGA), a ten-year, international effort to understand tropical ocean and global atmosphere variability.

"I think the root cause of (El Niño) is buried in the ocean," says Toole. "The long time scale for El Niño is fundamentally linked to the width of the Pacific and the time it takes for that information to propagate across the Pacific...The Pacific tries to respond on an annual time scale (with the sun), but it can't because it's too wide." This, he says, causes anomalies like El Niño.

El Niño is not the only variation from the norm to occur in the Pacific. Sometimes, an anti-El Niño event will occur, where there is a cold phase along the eastern Pacific--also known as La Niña.

Like the rippling effect of a stone dropped in a pond, these events have also proven to have far-reaching global effects. El Niño events tend to dry out Australia and India while bringing heavy rains to the west coast of South and Central America. They also nudge the jet stream off course, supplying California with a more-than-healthy supply of rain, and suppressing Atlantic hurricanes. Scientists have also found a distinct correlation between the rise and fall of sea surface temperatures in the Pacific, and corn harvests in Zimbabwe. The link between the two is so tight, they could accurately predict Zibabwean harvests for the last 20 years using El Niño data for the previous year.

The effects of El Niño have even been felt as far south as Antarctica. More than 6,000 kilometers away, Weddell seals feel the brunt of ENSO events. James W. Testa of the University of Alaska in Fairbanks noticed that the number of births declines every four to six years--coinciding with ENSO events. He suggests that the seal declines may result from changes in the fish populations, caused possibly by shifts in ocean currents.

Much can be gained then, if El Niño could be predicted accurately. Peru, for instance, could anticipate a rainy season, and plant crops--like rice--adapted for the weather. So far, several scientists have been successful developing models to predict another El Niño onslaught.

As part of the TOGA study, scientists have developed computer models for predicting when the tropical Pacific will swing toward warm or cool temperatures. Ants Leetmaa of the Climate Analysis Center is developing a long-range forecasting strategy for the U.S. weather service. Using a tool called a coupled global circulation model, he similates how streams in the oceans and atmosphere shuttle heat and moisture around the planet. After plugging in the necessary data, the model spins through 270 simulated days. This model can predict sea surface temperatures over the next six months.

Mark Cane and Stephen Zebiak of Columbia University's Lamont-Doherty Geological Survey have developed a model they think can predict an El Niño event at least six months--and perhaps even a year--ahead of time. Cane and Zebiak first map out what goes into the making of an El Niño. They then plug in the essential elements: sea-surface temperatures and wind data, and estimate thermocline depth using information about surface winds. Their model correctly predicted El Niño events for three-fourths of the time over 15 years. And as for the remainder of their findings? They weren't exactly wrong--just cloudy.

Tim Barnett of the Scripps Institution of Oceanography in La Jolla, California has also developed an effective prediction model. His statistical approach looks for unusually high pressure moving eastward along the equator. After assembling 32 years of data on the western and eastern Pacific, his model correctly predicted the 1982 El Niño.

Currently the National Oceanic and Atmospheric Administration is operating a network of buoys to measure temperature, currents and winds near the equator. This data is can be used in predicting short term climate variations. Research is ongoing to understand exactly what brings on these variations in ocean and atmosphere, and how we can accurately predict it. But until then, the people of Peru will keep a watchful eye out each Christmas.

Table listing years that El Niño and La Niña occurred. (If your browser does not support tables, click here.)

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