El Niño

El Niño During the nineteenth century Peruvian fishermen were aware of a warm water current that flowed southwards along the coast. Because it usually started after Christmas, Los Dias del Niño, the days of the (Christ) Child, they termed it Corriente del Niño or the El Niño current. The tropical marine conditions that El Niño brings are in great contrast to the habitual coolness caused by upwelling of cold water from Antarctica along the western South American coast. The annual El Niño current heralds summer rains over the coastal desert and Andean Mountains, bringing much-needed run-off for the irrigation of crops. The fishing industry also has to adapt from pursuing schooling fishes of the cool, dense, nutrient-rich upwelling water to foraging predator species of the less productive and less saline warmer waters.

At irregular intervals the El Niño current is markedly stronger than normal and carries the warmer, less saline water much further south. Scientists now use the term El Niño to refer to these irregular, stronger events, whose effects can last for more than a year and have important repercussions for weather and climate, and for social and economic well-being, throughout the Pacific basin and even further afield. The first written evidence for an El Niño event (though not named as such) is found in the campaign diaries of Pizarro in 1525–6. El Niño events seem to have occurred at intervals of three to four years, with the strongest type occurring at intervals of more than 20 years. During the twentieth century there were nine strong and 16 moderate El Niño events.

The atmospheric equivalent of El Niño is termed the Southern Oscillation (SO). The Southern Oscillation is a large-scale atmospheric pressure change over the south-eastern Pacific and Indian Ocean. It was first documented in 1932 by Sir Gilbert Walker as Director-General of Observatories in India was studying the causes of variation in the Indian Monsoon. During the ‘high phase’ of the Southern Oscillation, strong tropical convection causes air to ascend over the western Pacific and Indonesia, producing low pressure which draws in strong trade winds over the ocean surface from the south-east. The air rises and returns at higher altitudes before descending and causing high pressure in the south-eastern Pacific. During a ‘low phase’ the pressure pattern is reversed as the zone of low pressure associated with tropical convection moves to the middle and eastern Pacific, resulting in weaker than normal trade winds.

The Southern Oscillation Index (SOI) is a measure of the pressure difference between Darwin in Australia and Tahiti (Fig. 2). Troughs in the index correspond to El Niño events when pressure is low in Tahiti and high in the western Pacific, whereas peaks in the Southern Oscillation Index correspond to an accentuated ‘normal’ phase (now termed La Niña) characterized by strong trade winds. Scientists were unaware of the link between the Southern Oscillation and El Niño until 1957, when a strong correlation was noted between the SOI and sea surface temperatures off Peru.

The Norwegian scientist Jacob Bjerknes was the first to suggest a mechanism for linking these atmospheric and oceanic phenomena. A simplified structure of the tropical ocean can be thought of as comprising a layer of less dense, fresher, warmer water overlying a layer of denser, colder, more saline water, the two separated by a zone of rapidly changing density and temperature known as the thermocline. During ‘normal’ conditions, the west-blowing trade winds move the upper warmer layer towards the western Pacific, where it increases in thickness such that the thermocline can be at depths of 150 to 200 m, leaving only a shallow upper layer and thermocline (30–50 m) in the east. At the same time the west-blowing trade winds and the Earth's rotation combine to produce a surface movement of water away from the Equator, both to north and south. This in turn induces upwelling of cold water from below, and an elongated tongue of cool surface water is produced. The cool tongue does not extend to the western Pacific, because here the trade winds are weaker and the thermocline is deeper. When the trade winds relax at the start of an El Niño event, the upwelling ceases and the upper warm water moves eastwards, bringing with it the zone of low pressure and strong atmospheric convection.

The relaxation of the trade winds initiates a large-scale oceanic wave termed a Kelvin wave, which travels from west to east in the equatorial region at maximum speeds of 2.8 m s⁻¹, crossing the basin in one to two months, lowering the thermocline and raising sea level as it passes. Once a Kelvin wave reaches the eastern side of the Pacific ocean, some of the wave energy may also be translated into coastal Kelvin waves which travel north and south parallel to the coast of the Americas, taking up to three to four months to reach high latitudes. In regions away from the Equator the principal large-scale wave form is a westward-travelling Rossby wave, which can be produced by reflection of Kelvin or coastal Kelvin waves. The speed of a Rossby wave decreases with distance from the Equator, and so El Niño effects that are carried into middle and high latitudes may persist for months and even years. At the latitude of Japan, for instance, a Rossby wave reflected westwards across the Pacific would take several years to cross the basin. Research has indicated that after particularly strong El Niño events, such as in 1982–3, the effects on sea surface temperature and, by implication, weather conditions have been found at higher latitudes up to 11 years later.

A typical El Niño event seems to be presaged by a period of stronger than normal trade winds. In the onset phase, which usually begins in the early part of the year, there is a westerly (east-blowing) component to the wind in the western and central equatorial Pacific. In April, peak sea-surface temperatures are noted off Peru and Ecuador corresponding to the arrival of the oceanic Kelvin wave. Sea level also rises because of the lower atmospheric pressure and the arrival of warmer surface water from the west. High sea-surface temperatures can last until the following boreal spring of year 2, perhaps with another small peak during the winter before collapsing at the end of the sequence.

The effects of an El Niño event can be global. The eastward movement of the low-pressure convection area results in drought conditions in Australia, Indonesia, Africa, and India. The reverse occurs in the central Pacific, where there is intense rainfall and hurricane initiation. Western South America experiences heavy rainfall, resulting in flooding and erosion, and the effects can be felt in North America, where the change induced in the pressure patterns results in frontal systems bringing high rainfall further south to the western and southern United States.

The 1982–3 El Niño was exceptional in its severity and was undoubtedly the strongest of the twentieth century; it was also one of the best recorded so far. There were record-breaking amounts of rainfall in Peru and Ecuador in early 1983, resulting in record amounts of run-off, flooding, avalanches, and erosion. Translation of the effects took place to higher latitudes with concomitant flooding and costal erosion. Further afield there was a record drought in Australia. New types of disturbance were noted to the marine biota on a global scale. For instance, the deepening of the thermocline and the associated fall in nutrients within the surface waters of the eastern Pacific resulted in the devastation of many coral reefs which had experienced almost uninterrupted growth for several centuries. The loss of the nutrients led to a decrease in the zooplankton, which resulted in fish and squid mortality, reproductive failure in marine birds, and food shortages for penguins, cormorants, seals, and sea lions. In addition, rough seas and high sea levels made feeding very difficult for near-shore feeders such as marine iguanas, and kelp colonies were devastated.

Historical studies over longer timescales using a wide range of approaches suggests that El Niño events can be detected using such proxy evidence as changes in tree ring widths in the south-west of the United States and Mexico; Nile River flood data; tropical and subtropical ice cores; coral growth records; fishery catch records; and from a wide range of evidence derived from marine and lake sediments.

It is now evident that El Niño teleconnections (atmospheric and oceanic responses away from the equatorial Pacific) indicate that the El Niño phenomenon is the largest source of interannual climatic variability on the global scale. Attempts at predicting the onset and severity of El Niño events have not so far been satisfactory. Much more work will have to be done by ocean modellers and palaeoclimatologists before the factors behind the variability are understood.

B. A. Haggart

Bibliography

Bigg, G. R. (1990) El Niño and the Southern Oscillation. Weather 45, 2–8.
Enfield, D. B. (1989) El Niño, past and present. Reviews of Geophysics 27, 159–87.