El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean with on average five year intervals. It is characterized by variations in the temperature of the surface of the tropical eastern Pacific Ocean—warming or cooling known as El Niño and La Niña respectively—and air surface pressure in the tropical western Pacific—the Southern Oscillation. The two variations are coupled: the warm oceanic phase, El Niño, accompanies high air surface pressure in the western Pacific, while the cold phase, La Niña, accompanies low air surface pressure in the western Pacific. Mechanisms that cause the oscillation remain under study.
ENSO causes extreme weather such as floods, droughts and other weather disturbances in many regions of the world. Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific Ocean, are the most affected. In popular usage, the El Niño-Southern Oscillation is often called just "El Niño". El Niño is Spanish for "the boy" and refers to the Christ child, because periodic warming in the Pacific near South America is usually noticed around Christmas. The expression of ENSO is potentially subject to dramatic changes as a result of global warming, and is a target for research in this regard.
El Niño is defined by prolonged differences in Pacific Ocean surface temperatures when compared with the average value. The accepted definition is a warming or cooling of at least 0.5 °C (0.9 °F) averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of 2–7 years and lasts nine months to two years. The average period length is 5 years. When this warming or cooling occurs for only seven to nine months, it is classified as El Niño/La Niña "conditions"; when it occurs for more than that period, it is classified as El Niño/La Niña "episodes".
The first signs of an El Niño are:
- Rise in surface pressure over the Indian Ocean, Indonesia, and Australia
- Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean
- Trade winds in the south Pacific weaken or head east
- Warm air rises near Peru, causing rain in the northern Peruvian deserts
- Warm water spreads from the west Pacific and the Indian Ocean to the east Pacific. It takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific.
El Niño's warm rush of nutrient-poor tropical water, heated by its eastward passage in the Equatorial Current, replaces the cold, nutrient-rich surface water of the Humboldt Current. When El Niño conditions last for many months, extensive ocean warming and the reduction in Easterly Trade winds limits upwelling of cold nutrient-rich deep water and its economic impact to local fishing for an international market can be serious.
Early stages and characteristics of El Niño
Although its causes are still being investigated, El Niño events begin when trade winds, part of the Walker circulation, falter for many months. A series of Kelvin waves—relatively warm subsurface waves of water a few centimetres high and hundreds of kilometres wide—cross the Pacific along the equator and create a pool of warm water near South America, where ocean temperatures are normally cold due to upwelling. The weakening of the winds can also create twin cyclones, another sign of a future El Niño. The Pacific Ocean is a heat reservoir that drives global wind patterns, and the resulting change in its temperature alters weather on a global scale. Rainfall shifts from the western Pacific toward the Americas, while Indonesia and India become drier.
Jacob Bjerknes in 1969 helped toward an understanding of ENSO, by suggesting that an anomalously warm spot in the eastern Pacific can weaken the east-west temperature difference, disrupting trade winds that push warm water to the west. The result is increasingly warm water toward the east. Several mechanisms have been proposed through which warmth builds up in equatorial Pacific surface waters, and is then dispersed to lower depths by an El Niño event. The resulting cooler area then has to "recharge" warmth for several years before another event can take place.
While not a direct cause of El Niño, the Madden-Julian Oscillation, or MJO, propagates rainfall anomalies eastward around the global tropics in a cycle of 30–60 days, and may influence the speed of development and intensity of El Niño and La Niña in several ways. For example, westerly flows between MJO-induced areas of low pressure may cause cyclonic circulations north and south of the equator. When the circulations intensify, the westerly winds within the equatorial Pacific can further increase and shift eastward, playing a role in El Niño development. Madden-Julian activity can also produce eastward-propagating oceanic Kelvin waves, which may in turn be influenced by a developing El Niño, leading to a positive feedback loop.
Normal Pacific pattern. Equatorial winds gather warm water pool toward west. Cold water upwells along South American coast. (NOAA / PMEL / TAO)
The Southern Oscillation is the atmospheric component of El Niño. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin, Australia. El Niño episodes are associated with negative values of the SOI, meaning that the pressure difference between Tahiti and Darwin is relatively small.
Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water, in part because deep convection over the warm water acts to transport air. El Niño episodes are defined as sustained warming of the central and eastern tropical Pacific Ocean. This results in a decrease in the strength of the Pacific trade winds, and a reduction in rainfall over eastern and northern Australia.
Computer model simulations have given rise to three claims regarding the influence of global warming on El Niño/Southern Oscillation (ENSO) events: (1) global warming will increase the frequency of ENSO events, (2) global warming will increase the intensity of ENSO events, and (3) weather-related disasters will be exacerbated under El Niño conditions. In this section we highlight findings that suggest the virtual world of ENSO, as simulated by state-of-the-art climate models, is at variance with reality, beginning with several studies that described the status of the problem a decade ago.
In a comparison of 24 coupled ocean-atmosphere climate models, Latif et al. (2001) reported, “almost all models (even those employing flux corrections) still have problems in simulating the SST [sea surface temperature] climatology.” They also noted “only a few of the coupled models simulate the El Niño/Southern Oscillation in terms of gross equatorial SST anomalies realistically.” And they stated, “no model has been found that simulates realistically all aspects of the interannual SST variability.” Consequently, because “changes in sea surface temperature are both the cause and consequence of wind fluctuations,” according to Fedorov and Philander (2000), and because these phenomena figure prominently in the El Niño-La Niña oscillation, it is not surprising that the latter researchers concluded climate models near the turn of the century did not do a good job of determining the potential effects of global warming on ENSO.
Human ignorance likely also played a role in those models’ failure to simulate ENSO. According to Overpeck and Webb (2000), there was evidence that “ENSO may change in ways that we do not yet understand,” which “ways” had clearly not yet been modeled. White et al. (2001), for example, found that “global warming and cooling during earth’s internal mode of interannual climate variability [the ENSO cycle] arise from fluctuations in the global hydrological balance, not the global radiation balance,” and they noted that these fluctuations are the result of no known forcing of either anthropogenic or extraterrestrial origin, although Cerveny and Shaffer (2001) made a case for a lunar forcing of ENSO activity, which also was not included in any climate model of that time.
Another example of the inability of the most sophisticated of late twentieth-century climate models to properly describe El Niño events was provided by Landsea and Knaff (2000), who employed a simple statistical tool to evaluate the skill of 12 state-of-the-art climate models in real-time predictions of the development of the 1997–98 El Niño. In doing so, they found the models exhibited essentially no skill in forecasting this very strong event at lead times ranging from zero to eight months. They also determined no models were able to anticipate even one-half of the actual amplitude of the El Niño’s peak at a medium-range lead time of six to 11 months. Hence, they stated, “since no models were able to provide useful predictions at the medium and long ranges, there were no models that provided both useful and skillful forecasts for the entirety of the 1997–98 El Niño.”
Given the inadequacies listed above, it is little wonder that several scientists criticized model simulations of ENSO behavior at the turn of the century, including Walsh and Pittock (1998), who concluded, “there is insufficient confidence in the predictions of current models regarding any changes in ENSO,” and Fedorov and Philander (2000), who wrote, “at this time, it is impossible to decide which, if any, are correct.”
So what’s happened subsequently? Have things improved since then?
Huber and Caballero (2003) introduced their contribution to the subject by stating, “studies of future transient global warming with coupled ocean-atmosphere models find a shift to a more El Niño-like state,” although they also reported the “permanent El Niño state”—which has been hyped by some climate alarmists—“is by no means uniformly predicted by a majority of models.” To help resolve this battle of the models, they worked with still another model, plus real-world data pertaining to the Eocene, which past geologic epoch—having been much warmer than the recent past—provided, in their words, “a particularly exacting test of the robustness of ENSO.” More specifically, they used the Community Climate System Model of the National Center for Atmospheric Research, which they said yielded “a faithful reproduction of modern-day ENSO variability,” to “simulate the Eocene climate and determine whether the model predicts significant ENSO variability.” In addition, they compared the model results against middle Eocene lake-sediment records from two different regions: the Lake Gosiute complex in Wyoming and Eckfield Maar in Germany.
In describing their findings, Huber and Caballero report the model simulations showed “little change in ... ENSO, in agreement with proxies.” They also note other studies “indicate an ENSO shutdown as recently as ~6000 years ago, a period only slightly warmer than the present.” Hence, they concluded, “this result contrasts with theories linking past and future ‘hothouse’ climates with a shift toward a permanent El Niño-like state.” This conclusion represents a significant setback to climate alarmists who have used this unsubstantiated (and now invalidated) theory to induce unwarranted fear of global warming among the general public.
Three years later, Joseph and Nigam (2006) evaluated several climate models “by examining the extent to which they simulated key features of the leading mode of interannual climate variability: El Niño -Southern Oscillation (ENSO)”—which they described as “a dominant pattern of ocean-atmosphere variability with substantial global climate impact”—based on “the Intergovernmental Panel on Climate Change’s (IPCC) Fourth Assessment Report (AR4) simulations of twentieth-century climate.” This evaluation indicated that different models were found to do well in some respects but not so well in many others. For example, they found climate models “are still unable to simulate many features of ENSO variability and its circulation and hydroclimate teleconnections.” In fact, they found the models had only “begun to make inroads in simulating key features of ENSO variability.”
According to Joseph and Nigam, “climate system models are not quite ready for making projections of regional-to-continental scale hydroclimate variability and change.” Indeed, the study raises the question of whether they are ready to make any valid projections about anything. As Joseph and Nigam conclude, “predicting regional climate variability/change remains an onerous burden on models.”
One year later, L’Ecuyer and Stephens (2007) asked how well state-of-the-art climate models reproduced the workings of real-world energy and water cycles, noting “our ability to model the climate system and its response to natural and anthropogenic forcings requires a faithful representation of the complex interactions that exist between radiation, clouds, and precipitation and their influence on the large-scale energy balance and heat transport in the atmosphere,” while further stating “it is also critical to assess [model] response to shorter-term natural variability in environmental forcings using observations.”
The two researchers used multi-sensor observations of visible, infrared, and microwave radiance obtained from the Tropical Rainfall Measuring Mission satellite for the period January 1998 through December 1999, in order to evaluate the sensitivity of atmospheric heating (and the factors that modify it) to changes in east-west SST gradients associated with the strong 1998 El Niño event in the tropical Pacific, as expressed by the simulations of nine general circulation models of the atmosphere that were utilized in the IPCC’s AR4. This protocol, in their words, “provides a natural example of a short-term climate change scenario in which clouds, precipitation, and regional energy budgets in the east and west Pacific are observed to respond to the eastward migration of warm sea surface temperatures.”
L’Ecuyer and Stephens report “a majority of the models examined do not reproduce the apparent westward transport of energy in the equatorial Pacific during the 1998 El Niño event.” They also discovered “the intermodel variability in the responses of precipitation, total heating, and vertical motion [was] often larger than the intrinsic ENSO signal itself, implying an inherent lack of predictive capability in the ensemble with regard to the response of the mean zonal atmospheric circulation in the tropical Pacific to ENSO.” In addition, they found “many models also misrepresent the radiative impacts of clouds in both regions [the east and west Pacific], implying errors in total cloudiness, cloud thickness, and the relative frequency of occurrence of high and low clouds.” In light of these much-less-than-adequate findings, they conclude, “deficiencies remain in the representation of relationships between radiation, clouds, and precipitation in current climate models,” while further stating these deficiencies “cannot be ignored when interpreting their predictions of future climate.”
Paeth et al. (2008) compared 79 coupled ocean-atmosphere climate simulations derived from 12 different state-of-the-art climate models forced by six different IPCC emission scenarios with observational data in order to evaluate how well they reproduced the spatio-temporal characteristics of ENSO over the twentieth century, after which they compared the various models’ twenty-first-century simulations of ENSO and the Indian and West African monsoons to one another. With respect to the twentieth century, this work revealed that “all considered climate models draw a reasonable picture of the key features of ENSO.” With respect to the twenty-first century, on the other hand, they say that “the differences between the models are stronger than between the emission scenarios,” while “the atmospheric component of ENSO and the West African monsoon are barely affected.” Their “overall conclusion” was that “we still cannot say much about the future behavior of tropical climate.” Indeed, they considered their study to be merely “a benchmark for further investigations with more recent models in order to document a gain in knowledge or a stagnation over the past five years.”
Jin et al. (2008) investigated the overall skill of ENSO prediction in retrospective forecasts made with ten different state-of-the-art ocean-atmosphere coupled general circulation models with respect to their ability to hindcast real-world observations for the 22 years from 1980 to 2001. They found almost all models have problems simulating the mean equatorial SST and its annual cycle. They write, “none of the models we examined attain good performance in simulating the mean annual cycle of SST, even with the advantage of starting from realistic initial conditions.” They also note that “with increasing lead time, this discrepancy gets worse,” and that “the phase and peak amplitude of westward propagation of the annual cycle in the eastern and central equatorial Pacific are different from those observed.” What is more, they found, “ENSO-neutral years are far worse predicted than growing warm and cold events,” and “the skill of forecasts that start in February or May drops faster than that of forecasts that start in August or November.” They and others call this behavior “the spring predictability barrier,” which gives an indication of the difficulty of what they were attempting to do. Jin et al. conclude that “accurately predicting the strength and timing of ENSO events continues to be a critical challenge for dynamical models of all levels of complexity.”
McLean et al. (2009) quantified “the effect of possible ENSO forcing on mean global temperature, both short-term and long-term,” using Southern Oscillation Index (SOI) data provided by the Australian government’s Bureau of Meteorology. This parameter is defined as “the standardized anomaly of the seasonal mean sea level pressure difference between Tahiti and Darwin, divided by the standard deviation of the difference and multiplied by 10.” The temperature data employed in this endeavor were “the University of Alabama in Huntsville lower-tropospheric (LT) temperature data based on measurements from selected view angles of Microwave Sounding Unit (MSU) channel LT 2” for the period December 1979 to June 2008, supplemented by “balloon-based instrumentation (radiosondes).” More specifically, in the case of the latter data going back in time to 1958, they employed the Radiosonde Atmospheric Temperature Products for Assessing Climate (RATPAC) product (A) of the U.S. National Climatic Data Center, which represents the atmospheric layer between approximately 1500 and 9000 meters altitude.
When their work was completed, McLean et al. found “change in SOI accounts for 72% of the variance in GTTA [Global Tropospheric Temperature Anomalies] for the 29-year-long MSU record and 68% of the variance in GTTA for the longer 50-year RATPAC record,” as well as “81% of the variance in tropospheric temperature anomalies in the tropics,” where they say ENSO “is known to exercise a particularly strong influence.” In addition, they determined that “shifts in temperature are consistent with shifts in the SOI that occur about 7 months earlier.” Consequently, the three researchers state as their final conclusion, “natural climate forcing associated with ENSO is a major contributor to variability and perhaps recent trends in global temperature, a relationship that is not included in current global climate models.”
Noting that “coral records closely track tropical Indo-Pacific variability on interannual to decadal timescales,” Ault et al. (2009) employed 23 coral δ18O records from the Indian and Pacific Oceans to extend the observational record of decadal climate variability back in time to cover the period of AD 1850–1990. In so doing they identified “a strong decadal component of climate variability” that “closely matches instrumental results from the twentieth century.” In addition, they report the decadal variance they uncovered was much greater between 1850 and 1920 than it was between 1920 and 1990. As for what this observation means, the researchers say they “infer that this decadal signal represents a fundamental timescale of ENSO variability,” which has an enhanced variance in the early half of the record that “remains to be explained.”
In conclusion, there remain multiple unknowns with respect to ENSO and long-term climate change, and many of these unknowns raise serious questions about the ability of current climate models to adequately anticipate the multiplicity of climatic effects that the ongoing rise in the air’s CO2 content may or may not impose on Earth’s atmospheric and oceanic environments.
During non-El Niño conditions, the Walker circulation is seen at the surface as easterly trade winds which move water and air warmed by the sun towards the west. This also creates ocean upwelling off the coasts of Peru and Ecuador and brings nutrient-rich cold water to the surface, increasing fishing stocks. The western side of the equatorial Pacific is characterized by warm, wet low pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 centimetres (24 in) higher in the western Pacific as the result of this motion.
Effects of ENSO's warm phase (El Niño)
Because El Niño's warm pool feeds thunderstorms above, it creates increased rainfall across the east-central and eastern Pacific Ocean including several portions of the South American west coast. The effects of El Niño in South America are direct and stronger than in North America. An El Niño is associated with warm and very wet weather months December–April along 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. Along the west coast of South America, El Niño reduces the upwelling of cold, nutrient-rich water that sustains large fish populations, which in turn sustain abundant sea birds, whose droppings support the fertilizer industry. This leads to fish kills offshore Peru.
The local fishing industry along the affected coastline can suffer during long-lasting El Niño events. The world's largest fishery collapsed due to overfishing during the 1972 El Niño Peruvian anchoveta reduction. During the 1982–83 event, jack mackerel and anchoveta populations were reduced, scallops increased in warmer water, but hake followed cooler water down the continental slope, while shrimp and sardines moved southward so some catches decreased while others increased. Horse mackerel have increased in the region during warm events. Shifting locations and types of fish due to changing conditions provide challenges for fishing industries. Peruvian sardines have moved during El Niño events to Chilean areas. Other conditions provide further complications, such as the government of Chile in 1991 creating restrictions on the fishing areas for self-employed fishermen and industrial fleets.
The ENSO variability may contribute to the great success of small fast-growing species along the Peruvian coast, as periods of low population removes predators in the area. Similar effects benefit migratory birds that travel each spring from predator-rich tropical areas to distant winter-stressed nesting areas.
Southern Brazil and northern Argentina also experience wetter than normal conditions but mainly during the spring and early summer. Central Chile receives a mild winter with large rainfall, and the Peruvian-Bolivian Altiplano is sometimes exposed to unusual winter snowfall events. Drier and hotter weather occurs in parts of the Amazon River Basin, Colombia and Central America.
Winters, during the El Niño effect, are warmer and drier than average in the Northwest, Northmidwest, and Northmideast United States, and therefore those regions experience reduced snowfalls. Meanwhile, significantly wetter winters are present in northwest Mexico and the southwest United States including central and southern California, while both cooler and wetter than average winters in northeast Mexico and the southeast United States (including the Tidewater region of Virginia) occur during the El Niño phase of the oscillation.
In Canada, both warmer and drier winters (due to forcing of the Polar Jet further north) over much of the country occur, although less variation from normal is seen in the Maritime Provinces. The following summer is warmer and sometimes drier creating a more active than average forest fire season over Central/Eastern Canada. Some believed that the ice-storm in January 1998, which devastated parts of Southern Ontario and Southern Quebec, was caused or accentuated by El Niño's warming effects. El Niño warmed Vancouver for the 2010 Winter Olympics, such that the area experienced a subtropical-like winter during the games.
Summers, during the El Niño effect, are wetter than average in the Northwest, Northmidwest, Northmideast, and mountain regions of the United States.
El Niño is credited with suppressing hurricanes and made the 2009 hurricane season the least active in twelve years. El Niño is also associated with increased wave-caused coastal erosion along the United States Pacific Coast.
There is some evidence that El Niño activity is correlated with incidence of red tides off the Pacific coast of California.
Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies. When the subtropical ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E, which would favor the Japanese archipelago. During El Niño years, Guam's chance of a tropical cyclone impact is one-third of the long term average. The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.
In Africa, East Africa, including Kenya, Tanzania and the White Nile basin experiences, in the long rains from March to May, wetter than normal conditions. There are also drier than normal conditions from December to February in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique and Botswana. Direct effects of El Niño resulting in drier conditions occur in parts of Southeast Asia and Northern Australia, increasing bush fires and worsening haze and decreasing air quality dramatically. Drier than normal conditions are also generally observed in Queensland, inland Victoria, inland New South Wales and eastern Tasmania from June to August. West of the Antarctic Peninsula, the Ross, Bellingshausen, and Amundsen Sea sectors have more sea ice during El Niño. The latter two and the Weddell Sea also become warmer and have higher atmospheric pressure. El Niño's effects on Europe are not entirely clear, but certainly it is not nearly as affected as at least large parts of other continents. There is some evidence that an El Niño may cause a wetter, cloudier winter in Northern Europe and a milder, drier winter in the Mediterranean Sea region. The El Niño winter of 2006/2007 was unusually mild in Europe, and the Alps recorded very little snow coverage that season.
Most recently, Singapore experienced the driest February in 2010 since records begins in 1869. With only 6.3 millimetres of rain fell in the month and temperatures hitting as high as 35 degrees Celsius on 26 February. 1968 and 2005 had the next driest Februaries when 8.4 mm of rain fell.
Effects of ENSO's cool phase (La Niña)
Sea surface skin temperature anomalies in November 2007 showing La Niña conditions
La Niña is the name for the cold phase of ENSO, during which the cold pool in the eastern Pacific intensifies and the trade winds strengthen. The name La Niña originates from Spanish, meaning "the girl", analogous to El Niño meaning "the boy". It has also in the past been called anti-El Niño, and El Viejo (meaning "the old man").
La Niña results in wetter than normal conditions in Southern Africa from December to February, and drier than normal conditions over equatorial East Africa over the same period.
During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific ocean, which increases the landfall threat to China. In March 2008, La Niña caused a drop in sea surface temperatures over Southeast Asia by an amount of 2 °C. It also caused heavy rains over Malaysia, Philippines and Indonesia.
During a time of La Niña, drought plagues the coastal regions of Peru and Chile. From December to February, northern Brazil is wetter than normal.
La Niña causes mostly the opposite effects of El Niño. La Niña causes above average precipitation across the North Midwest, the Northern Rockies, Northern California, and in the Pacific Northwest's southern and eastern regions. Meanwhile there is below average precipitation in the southwestern and southeastern states.
La Niñas occurred in 1904, 1908, 1910, 1916, 1924, 1928, 1938, 1950, 1955, 1964, 1970, 1973, 1975, 1988, 1995.
In Canada, La Niña will generally cause a cooler, snowier winter, such as the near record-breaking amounts of snow recorded in the La Niña winter of 2007/2008 in Eastern Canada.
There was a strong La Niña episode during 1988–1989. La Niña also formed in 1995, from 1998–2000, and a minor one from 2000–2001. Recently, an occurrence of El Niño started in September 2006 and lasted until early 2007. From June 2007 on, data indicated a moderate La Niña event, which strengthened in early 2008 and weakened by early 2009; the 2007–2008 La Niña event was the strongest since the 1988–1989 event. The strength of the La Niña made the 2008 hurricane season one of the most active since 1944; there were 16 named storms of at least 39 mph (63 km/h), eight of which became 74 mph (119 km/h) or greater hurricanes.
According to NOAA, El Niño conditions were in place in the equatorial Pacific Ocean starting June 2009, peaking in January–February. Positive SST anomalies (El Niño) lasted until May 2010. Since then, SST anomalies have been negative (La Niña) and expected to stay negative for the next northern winter.
Remote influence on tropical Atlantic Ocean
A study of climate records has shown that El Niño events in the equatorial Pacific are generally associated with a warm tropical North Atlantic in the following spring and summer. About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool (WHWP) to become unusually large in summer. Occasionally, El Niño's effect on the Atlantic Walker circulation over South America strengthens the easterly trade winds in the western equatorial Atlantic region. As a result, an unusual cooling may occur in the eastern equatorial Atlantic in spring and summer following El Niño peaks in winter. Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.
ENSO and global warming
During the last several decades the number of El Niño events increased, and the number of La Niña events decreased. The question is whether this is a random fluctuation or a normal instance of variation for that phenomenon, or the result of global climate changes towards global warming.
The studies of historical data show that the recent El Niño variation is most likely linked to global warming. For example, one of the most recent results is that even after subtracting the positive influence of decadal variation, shown to be possibly present in the ENSO trend, the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years.
It is not certain what exact changes will happen to ENSO in the future: different models make different predictions It may be that the observed phenomenon of more frequent and stronger El Niño events occurs only in the initial phase of the global warming, and then (e.g., after the lower layers of the ocean get warmer as well), El Niño will become weaker than it was. It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other. More research is needed to provide a better answer to that question, but the current results do not completely exclude the possibility of dramatic changes. The ENSO is considered to be a potential tipping element in Earth's climate.
El Niño "Modoki" and Central-Pacific El Niño
The traditional Niño, also called Eastern Pacific (EP) El Niño, involves temperature anomalies in the Eastern Pacific. However, in the last two decades non-traditional El Niños were observed, in which the usual place of the temperature anomaly (Nino 1 and 2) is not affected, but an anomaly arises in the central Pacific (Nino 3.4). The phenomenon is called Central Pacific (CP) El Niño, "dateline" El Niño (because the anomaly arises near the dateline), or El Niño "Modoki" (Modoki is Japanese for "similar, but different").
The effects of the CP El Niño are different from those of the traditional EP El Niño—e.g., the new El Niño leads to more hurricanes more frequently making landfall in the Atlantic.
The recent discovery of El Niño Modoki has some scientists believing it to be linked to global warming. However, Satellite data only goes back to 1979. More research must be done to find the correlation and study past El Niño episodes.
The first recorded El Niño that originated in the central Pacific and moved towards the east was in 1986.
A joint study by the National Aeronautics and Space Administration and the National Oceanic and Atmospheric Administration concluded that climate change may contribute to stronger El Niños. El Niño "Modoki" events occurred in 1991-92, 1994–95, 2002–03, 2004–05 and 2009-10. The strongest such Central Pacific El Niño event known occurred in 2009-2010.
Health Impact of El Niño
Extreme weather conditions related with the El Niño cycle are associated with changes in the incidence of epidemic diseases. For example, the El Niño cycle is associated with increased risks of some of the diseases transmitted by mosquitoes, such as malaria, dengue and Rift Valley fever. Cycles of malaria in India, Venezuela and Colombia have now been linked to El Niño. Outbreaks of another mosquito-transmitted disease, Australian Encephalitis (Murray Valley Encephalitis - MVE), occur in temperate south-east Australia after heavy rainfall and flooding, which are associated with La Nina events. A severe outbreak of Rift Valley fever occurred after extreme rainfall in north-eastern Kenya and southern Somalia during the 1997-98 El Niño.
Cultural history and pre-historic information
ENSO conditions have occurred at two- to seven year intervals for at least the past 300 years, but most of them have been weak. There is also evidence for strong El Niño events during the early Holocene epoch 10,000 years ago.
El Niño affected pre-Columbian Incas and may have led to the demise of the Moche and other pre-Columbian Peruvian cultures. A recent study suggests that a strong El-Niño effect between 1789–93 caused poor crop yields in Europe, which in turn helped touch off the French Revolution. The extreme weather produced by El Niño in 1876–77 gave rise to the most deadly famines of the 19th century.
An early recorded mention of the term "El Niño" to refer to climate occurs in 1892, when Captain Camilo Carrillo told the Geographical society congress in Lima that Peruvian sailors named the warm northerly current "El Niño" because it was most noticeable around Christmas. The phenomenon had long been of interest because of its effects on the guano industry and other enterprises that depend on biological productivity of the sea.
Charles Todd, in 1893, suggested that droughts in India and Australia tended to occur at the same time; Norman Lockyer noted the same in 1904. An El Niño connection with flooding was reported in 1895 by Pezet and Eguiguren. In 1924 Gilbert Walker (for whom the Walker circulation is named) coined the term "Southern Oscillation".
The major 1982–83 El Niño led to an upsurge of interest from the scientific community. The period from 1990–1994 was unusual in that El Niños have rarely occurred in such rapid succession. An especially intense El Niño event in 1998 caused an estimated 16% of the world's reef systems to die. The event temporarily warmed air temperature by 1.5 °C, compared to the usual increase of 0.25 °C associated with El Niño events. Since then, mass coral bleaching has become common worldwide, with all regions having suffered "severe bleaching".
Major ENSO events were recorded in the years 1790–93, 1828, 1876–78, 1891, 1925–26, 1972–73, 1982–83, and 1997–98.
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