El Niño

El Niño is a phenomenon that occurs every three to seven years in the Pacific Ocean. During a normal year sea temperatures warm up briefly during the Christmas season off the coasts of Peru and Ecuador (El Niño is Spanish for the infant Jesus). However, during an El Niño these warm temperatures persist for many months, coinciding with aberrant changes in air pressure and wind patterns throughout the southern Pacific, known as El Niño Southern Oscillations (ENSOs). Periodically, the Intertropical Convergence Zone (ITCZ) in the atmosphere, which normally sits over the Indonesian region, migrates eastwards over the central Pacific. The ITCZ is a zone of high atmospheric pressure where the rising air triggers heavy rainfall and high air temperatures. This warms the ocean surface and triggers a planetary wave that travels eastwards. This wave is slow moving and takes about a year to cross the Pacific along the equator. Its effect is to deepen the thermocline so that the upwelling that occurs along the equator no longer brings cool nutrient-rich water from below the thermocline up to the surface. Normally the cool waters that occur off the coasts of Peru and Ecuador are not only highly productive, but they also keep the coastal climate arid.

During an El Niño event the major fishery for anchovetta (Centegraulis mysticetus), normally one of the biggest in the world, collapses and there are mass mortalities of seabirds, such as pelicans and guanay cormorants, whose droppings were the main source of nitrates used to manufacture the gunpowder used in 19th-century European wars. The warm seas also trigger heavy rainfall over the normally arid coastal regions, resulting in disastrous flooding. When the planetary wave encounters the continental margin of America it is diverted both north and south, resulting in similarly anomalous warm seas and heavy rainfall along the normally arid coasts of North and South America. The repercussions are felt in other regions such as South Africa, India, and South-East Asia, where seasonal rains fail resulting in disastrous droughts and human misery.

The El Niño events of 1982 and 1997 were the most extreme on record. What determines the irregular occurrences of ENSO events is not understood, but many scientists believe that the recent increase in their frequency and severity is one expression of global climate change. If this is indeed so then these events may become the norm with severe implications for human societies.

See also environmental issues.

www.elnino.noaa.gov/

M. V. Angel