El NiñoEl Niño and
La Niña (often written in English as
El Nino and
La Nina) and are major temperature fluctuations in the tropical Pacific Ocean. They are Pacific signatures of the global ENSO phenomenon (El Niño-Southern Oscillation). Their effect on climate in the southern hemisphere is profound. Their role in global warming or cooling is an area of active research, with no clear consensus yet.
Chart of ocean surface temperature anomaly [°C] during the last strong El Niño in December 1997.El NiñoEl Niño was originally recognized by fishermen off the coast of South America (Peru and Ecuador) as the appearance of unusually warm water in the Pacific Ocean. El Niño is the warming of the surface waters of the eastern equatorial Pacific Ocean that occurs at irregular intervals of 2-7 years, usually lasting 1-2 years. Along the west coast of South America near the end of each calendar year, a warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water of the Humboldt Current which fish prefer. Because this condition often occurs around Christmas, local fishermen named it El Niño (Spanish for boy child, referring to the Child Jesus). In most years the warming lasts only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Niño conditions last for many months, more extensive ocean warming occurs and economic results can be disastrous.
Recent El Niños have occurred in 1986-1987, 1991-1992, 1993, 1994, 1997-1998, 2002-2003, and a new El Niño has been in effect since September 2004. The El Niño of 1997-1998 was particularly strong, while the period from 1990-1994 was unusual in that El Niños rarely occur in such rapid succession. They were generally weak, however.
ENSO/El Niño state.
- Sea surface is warm in central and eastern Pacific.
- Less cold water is pulled up along west coast of South America.
- Hot air rises in central Pacific, travels east and west before cooling and descending. ENSO normal state.
- Normal equatorial winds warm as they flow westward across the Pacific.
- Cold water is pulled up along west coast of South America.
- Warming water is pushed toward west side of Pacific.
- Sea surface is warm in the west.
- Hot air rises in western Pacific, travels eastward and cool air descends on South America.ENSO/La Niña state.
- Similar to ENSO normal state.
- Warm water accumulates in far western Pacific.
- Equatorial water is cooler than in the normal state.El Niño's effectsEl Niño's weather effects depend on the location, time of year, and the particular episode. In North America, typically, winters are warmer than normal in the upper midwest states and Canada, while central and southern California, northwest Mexico and the southeastern U.S., are wetter than normal. The Pacific Northwest states, on the other hand, tend to be drier during an El Niño. During a La Niña, by contrast, the midwestern U.S. tends to be drier than normal.
In South America, the effects of El Niño are rather direct and stronger than in North America. An El Niño is associated with very wet summers in the coasts of Northern Peru and Ecuador causing major flooding whenever the event is strong or extreme. The effects during the months of February, March and April may become critical. Southern Brazil and Northern Argentina also experience wetter than normal conditions but mainly during the spring and early summer. Central Chile receives large rainfall during winter, and the Peruvian-Bolivian Altiplano is sometimes exposed to unusual winter snowfall events. The entire peruvian coast and parts of northern Chile experience above-normal temperatures and moist conditions for several consecutive months as well. Drier and hotter weather occurs in parts the Amazon River Basin, Colombia and Central America.
Australia and Indonesia experience as well direct effects of El Niño resulting in drier conditions, especially in Indonesia and along the eastern half of Australia.
Finally, Africa experiences droughts both in the Sahel region (northwest Africa) and in south-central Africa (Zambia, Zimbabwe, Mozambique and Botswana mainly).
Non-climate effectsAlong the west coast of South America, El Niño reduces the upwelling of cold, nutrient-rich water that sustains large fish populations, that sustain abundant sea birds, whose droppings support the fertilizer industry. The local fishing industry, who named the annual appearance near Christmas of the El Niño effect, along the affected coastline suffers greatly during long-lasting El Niño events.
La NiñaIn the Pacific, La Niña is characterized by unusually cold ocean temperatures in the Equatorial Pacific, compared to El Niño, which is characterized by unusually warm ocean temperatures in the same area. The La Niña condition often follows the El Niño, especially when the latter is strong. Strong La Niñas occurred in 1988-1989 and 1998-2001, and weakly in 1995-1996.
SOIThe Southern Oscillation (SO) is a global-scale seesaw in atmospheric pressure between Indonesia/North Australia, and the southeast Pacific. Its measure is through the Southern Oscillation Index (SOI). The SOI is the normalised difference in surface pressure between Tahiti and Darwin.
SOI index, 1876-2004. By William M. Connolley.
Red: monthly averages; Blue: 12 month running meanENSOENSO (El Niño, Southern Oscillation) is a set of interacting parts of a single global system of climate fluctuations that come about as a consequence of atmospheric circulation. ENSO is the most prominent known source of interannual variability in weather and climate around the world (~3 to 8 years), though not all areas are affected. Global ENSO has signatures in the Pacific, Atlantic and Indian Oceans. In the Pacific, during major warm events El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SOI intensity. While ENSO events are basically in phase between the Pacific and Indian Oceans, ENSO events in the Atlantic Ocean lag those in the Pacific by 12-to-18 months. Many of the countries most affected by ENSO events are developing countries within main continents (South America, Africa...), with economies that are largely dependent upon their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events in the three oceans can have global socio-economical impacts. While ENSO is a global and natural part of the Earth's climate, whether its intensity or frequency may change as a result of global warming is an important concern. Low-frequency variability has been evidenced. Interdecadal modulation of ENSO might exist.
Western Hemisphere Warm PoolStudy of climate records has found that about half of the summers after an El Niño have unusual warming in the Western Hemisphere Warm Pool (WHWP). This affects weather in the area and seems to be related to the North Atlantic Oscillation.
Atlantic effectAn effect similar to El Niño sometimes takes place in the Atlantic Ocean, where water along equatorial Africa's Gulf of Guinea becomes warmer and eastern Brazil becomes cooler and drier. This may be related to El Niño Walker circulation changes over South America.
Issue #4