A weakening, or in extreme cases reversal in direction, of the prevailing south-easterly trade winds over the equatorial South Pacific Ocean that is associated with a change in the distribution of air pressure over the South Pacific known as a southern oscillation. A similar phenomenon may also occur in the Atlantic. Approximately once every seven years, during the Christmas season (the name refers to the Christ child), prevailing trade winds weaken and the Equatorial countercurrent strengthens. Warm surface waters, normally driven westward by the wind to form a deep warm pool off Indonesia, flow eastwards to overlie the cold waters of the Peru current. In exceptional years, e.g. 1891, 1925, 1953, 1972–3, 1982–3, 1986–7, 1994–5, and 1997–8, the extent to which the upwelling of the nutrient-rich cold waters is suppressed causes the death of a large proportion of the plankton population and a decline in the numbers of surface fish. El Niño conditions bring heavy rain to the ordinarily arid western coast of S. America, drought to Indonesia, and produce climatic effects throughout the Pacific region and beyond. See teleconnections.
El Niño
A disruption of the equatorial Pacific, east–west Walker circulatory cell. The gradient between the low pressure of the warm, western tropical Pacific and the high pressure of the cold, eastern tropical Pacific decreases, weakening the easterly trade winds, and allowing warm surface water to move eastwards. The rising branch of the circulation cell, and therefore the precipitation associated with it, moves with the water. An El Niño is only one element of a dual-phase oscillating ocean–atmosphere system; when the system reverts to its ‘normal’ phase, Pacific waters cool off the coast of Ecuador and Peru, and become warm again in the western Pacific. Sometimes the eastern Pacific becomes unusually cool; this is a La Niña event. This entire ocean–atmosphere system is the El Niño, and the ‘normal’ phase (sometimes developing into La Niña) is the El Niño–Southern Oscillation. Dawson and O’Hare (2000) Geog. 85, 3 give an exceptionally clear explanation. See Moy et al. (2002) Nature 420 on the variability of the El Niño–Southern Oscillation.
El Niño events have been teleconnected to abnormally heavy rain in the southern USA (Science Daily, 2008/01/080128113104) and western South America, a decrease in tropical cyclones in the Atlantic, and to droughts in Indonesia and Australia; see M. E. Mann, R. S. Bradley, and M. K. Hughes (2000). See Nicholls (1988) Bull. Am. Met. Soc. 69, 2 on El Niño–Southern Oscillation Impact Prediction.
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