Background Information: El Niño Phenomenon
After the annual seasonal cycles of weather conditions which contribute to the climate of a location, the most notable aspect of climate variability is that which distinguishes one year from its neighbors. Worldwide, but particularly in the Pacific region, conditions seem to oscillate between two states, one being notably much drier than the other. The name El Niño refers to such state and takes its name, which refers to the Christ Child, from an oceanic feature that may occur perhaps on average about twice in each 10-year period with an onset near to the Christmas season.
In a normal year, a strong ocean current, the Peru or Humboldt Current, flows northwards towards the Equator along the western coast of South America. Due to the rotation of the Earth on its axis, such a situation is associated with surface water being deflected away from the continent to be replaced by cold nutrient-rich waters drawn upwards from the depths. This mechanism is called upwelling. The nutrients ensure a good harvest for the fisheries. From the point of view of climate, the presence of cold surface water communicates with the atmosphere and so affects climate.
In an El Niño year, the ocean circulation is affected and weakens the coastal current in the eastern Pacific. The surface waters are warmer than normal, again affecting climate but the lack of nutrients creates a serious situation for the fisheries. During El Niño, the easterly surface winds weaken and retreat to the eastern Pacific, allowing the central Pacific to warm and the rain area to migrate eastward.
However, the El Niño phenomenon is not restricted to the eastern south Pacific. What is more important worldwide, and for the general Pacific region in particular, takes place in the equatorial Pacific and takes the form of complex interactions of ocean and atmosphere. In an El Niño year, the warm surface water, normally resident in the western equatorial Pacific, can be found instead in the central and eastern Pacific. With sea surface temperatures 3 to 4 degrees Celsius higher than normal, convection, clouds, and rain are produced in and from the overlying atmosphere at the expense of the western equatorial Pacific. This shift serves also to block the above-mentioned upwelling feature along the South American Pacific coast.
El Niño leads to decreases in the normally prevailing east-to-west trade winds, thus weakening the upwelling process. This leads to dramatic changes in the climate. During a normal year, the surface waters of the central and eastern Pacific stay cool and rain is concentrated near the warmer waters off the coast of Papua New Guinea in the western Pacific. During an El Niño year, decreases in the easterly winds lead to a warming of the central and eastern Pacific. This causes the rain area to move eastward, which leads to heavily increased rainfall in some areas. During an El Niño, the western Pacific typically gets much less rainfall than normal. The changes in wind direction and speed may also cause heavy storms and cyclones in areas that usually experience mild weather.
The atmospheric effect of the El Niño phenomenon can be monitored by the Southern Oscillation Index (SOI), which simply indicates the atmospheric pressure gradient between Tahiti and Darwin, Australia. In an El Niño year, the gradient reverses to give pressures at Darwin considerably higher than those observed at Tahiti.
The El Niño generally means drought in Australia, Indonesia, the Philippines, and Papua New Guinea with tropical storms in the ascendancy in Tahiti and Polynesia in general. Global warming is thought to increase the occurrence of El Niño events.
