Introduction to El Nino

Keywords:

El Niño; El Niño Southern Oscillation; internal waves; trade winds; Kelvin Waves; Rossby Waves; La Nina; Peru Current

El Niño is a warm, nutrient poor, ocean current that flows southward along the coast of northern Peru [Academic American Ency., 1994] and creates sometimes catastrophic effects on local and sometimes global fishing and biogeochemical systems. This event, named El Niño (the {Christ} Child) because it occurs around Christmas time, was once thought of as happening only abnormally. It is known now, however, that El Niño is actually the result of a Pacific Ocean oriented cycle that lasts from three to five years. The event itself lasts about 12 to 18 months.

The El Niño or, more precisely, the El Niño Southern Oscillation (ENSO), for which it and its related phenomena are called, is produced primarily by the interaction between the winds in the atmosphere and the sea surface in the Pacific Ocean. This interaction is tracked by observing, through remote satellites, the changing patterns of sea surface temperatures in the Pacific, sea level changes, and pressure oscillations.

In 1904, Sir Gilbert Walker, Director General of the Observatory in India, began studying the sea level pressure swing between the East Pacific and the West Pacific. He correlated the swing with data from around the globe, including sea surface temperature, and called it the Southern Oscillation (the change in pressure is called the Southern Oscillation Index -SOI- today). Walker failed to make the connection between this oscillation and El Niño however.

Normally, when there is no El Niño (often referred to as La Niña [Willam H. Schlesinger, 1991]), the trade winds (winds that flow toward the equator) blow from east to west across the coastal waters of the eastern Pacific. In doing so, they drag the warm surface waters of the equatorial Pacific with them. This causes an upwelling of cooler, nutrient-rich waters which the fish population in the region depends on. This 'air/sea interaction' is accompanied by the circulation of large internal waves (waves that have their peaks under the ocean surface) across the Pacific ocean mainly in the equatorial region. These internal waves are referred to as Kelvin and Rossby waves. Kelvin waves travel eastward along the equator and are not subjected to the Coriolis force because of this. Rossby waves, unlike Kelvin waves, travel westward and are a result of the changes that the Kelvin waves introduced to the area. Rossby waves, since they move away, but parallel, to the equator are not relieved of the Coriolis force; thus they travel up to about three times slower.

During an El Niño, the east-to-west trade winds weaken causing the upwelling of deep water to cease. The prevailing reason for this is that cumulus clouds, produced by the warm surface waters of the Pacific, move eastward along the Pacific altering the surface winds and weakening the prevailing east-to-west trade winds. [NASA facts: El Niño, 1994] As the trade winds weaken and the ocean surface is warmed, the El Niño is strengthened and a positive feedback loop forms.

The ENSO cycle can also be explained through the movement of waves in the Pacific as mentioned above. The cycle starts with warm water traveling from the western Pacific to the eastern Pacific in the form of Kelvin waves. After roughly three to four months [Edward Laws, 1992] of traveling across the Pacific along the equator, the Kelvin waves reach the western coast of South America where they mix with the cool Peru Current system; therefore raising sea levels and sealevel temperatures in the region. Upon reaching the coast, the water forks to the north and south and causes El Niño conditions to the south.Because of the changes in sea-level and sea-temperature due to the Kelvin waves, an infinite amount of Rossby waves are formed and move back over the Pacific. The Rossby waves, as mentioned before, are much slower than the Kelvin waves and can take anywhere from nine months to four years [Edward Laws, 1992] to cross the Pacific.Waves move slower when the distance from the equator is increased. (This wave delay is key to the ENSO cycle.) When the Rossby waves arrive at the western Pacific they bounce off the coast and become Kelvin waves and again travel back across the Pacific towards South America. This time, however, the waves decrease the sea-level and sea surface temperature returning the area to normal or La Nina conditions.

This sea surface temperature (SST) oscillation is accompanied by an oscillation in pressure (called the Southern Oscillation). During an El Niño, sea level pressure in the east becomes real low while pressure in the rest rises. The situation reverses itself during La Niña years.

This 3 to 5 year cycle is then repeated and each El Niño phase is different, some being mild and some being very strong. A complete ENSO cycle requires that the Pacific be crossed by two pairs of Kelvin and Rossby waves. Because of El Niño's devastation to local and sometimes global economies and agriculture, the benefits of being able to predict an El Niño even a short while in advance are great.

Return to the Course: Phase 1.