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89-589: The SPCZ occurs where the southeast trades from transitory anticyclones to the south meet with the semipermanent easterly flow from the eastern South Pacific anticyclone. The SPCZ exists in summer and winter but can change its orientation and location. It is often distinct from the ITCZ over Australia, but at times they become one continuous zone of convergence . The location of the SPCZ is affected by ENSO and Interdecadal Pacific oscillation conditions. It generally stretches from

178-471: A positive feedback loop develops between the convective tropical cyclone and the upper level high, the two systems are strengthened. This loop stops once ocean temperatures cool to below 26.5 °C (79.7 °F), reducing the thunderstorm activity, which then weakens the upper-level high-pressure system. When the subtropical ridge in the Northwest Pacific is stronger than in other areas, it leads to

267-462: A subtropical ridge . The evolution of an anticyclone depends upon variables such as its size, intensity, and extent of moist convection , as well as the Coriolis force . Sir Francis Galton first discovered anticyclones in the 1860s. High-pressure systems are alternatively referred to as anticyclones. Their circulation is sometimes referred to as cum sole . Subtropical high-pressure zones form under

356-621: A buildup of particulates in urban areas under the high pressure, leading to widespread haze . If the surface level relative humidity rises towards 100 percent overnight, fog can form. The movement of continental arctic air masses to lower latitudes produces strong but vertically shallow high-pressure systems. These systems affect their pressure. The surface level, sharp temperature inversion can lead to areas of persistent stratocumulus or stratus cloud , colloquially known as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of

445-556: A cooler West Pacific and a warmer East Pacific, leading to a shift of cloudiness and rainfall towards the East Pacific. This situation is called El Niño. The opposite occurs if trade winds are stronger than average, leading to a warmer West Pacific and a cooler East Pacific. This situation is called La Niña and is associated with increased cloudiness and rainfall over the West Pacific. The close relationship between ocean temperatures and

534-581: A decrease in the strength of the Pacific trade winds , and a reduction in rainfall over eastern and northern Australia. La Niña episodes are defined as sustained cooling of the central and eastern tropical Pacific Ocean, thus resulting in an increase in the strength of the Pacific trade winds , and the opposite effects in Australia when compared to El Niño. Although the Southern Oscillation Index has

623-485: A larger EP ENSO occurrence, or even displaying opposite conditions from the observed ones in the other Niño regions when accompanied by Modoki variations. ENSO Costero events usually present more localized effects, with warm phases leading to increased rainfall over the coast of Ecuador, northern Peru and the Amazon rainforest , and increased temperatures over the northern Chilean coast, and cold phases leading to droughts on

712-533: A long station record going back to the 1800s, its reliability is limited due to the latitudes of both Darwin and Tahiti being well south of the Equator, so that the surface air pressure at both locations is less directly related to ENSO. To overcome this effect, a new index was created, named the Equatorial Southern Oscillation Index (EQSOI). To generate this index, two new regions, centered on

801-411: A negative SSH anomaly (lowered sea level) via contraction. The El Niño–Southern Oscillation is a single climate phenomenon that quasi-periodically fluctuates between three phases: Neutral, La Niña or El Niño. La Niña and El Niño are opposite phases which require certain changes to take place in both the ocean and the atmosphere before an event is declared. The cool phase of ENSO is La Niña, with SST in

890-439: A number of climate models of differing complexity to simulate rainfall bands in the southwest Pacific and see how the magnitude and areal extent was affected by the SPCZ and ENSO . During El Niño or warm-phase conditions, the SPCZ typically shifted northeastward with dryer conditions on islands to the southwest, in agreement with observations. Conversely, a southwestward shift in rainfall accompanied La Niña or cold-phase events in

979-517: A quarter of the planet, and particularly in the form of temperature at the ocean surface, can have a significant effect on weather across the entire planet. Tropical instability waves visible on sea surface temperature maps, showing a tongue of colder water, are often present during neutral or La Niña conditions. La Niña is a complex weather pattern that occurs every few years, often persisting for longer than five months. El Niño and La Niña can be indicators of weather changes across

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1068-425: A secondary peak in sea surface temperature across the far eastern equatorial Pacific Ocean sometimes follows the initial peak. An especially strong Walker circulation causes La Niña, which is considered to be the cold oceanic and positive atmospheric phase of the broader El Niño–Southern Oscillation (ENSO) weather phenomenon, as well as the opposite of El Niño weather pattern, where sea surface temperature across

1157-516: A wet monsoon season for Asia . The subtropical ridge position is linked to how far northward monsoon moisture and thunderstorms extend into the United States . Typically, the subtropical ridge across North America migrates far enough northward to begin monsoon conditions across the Desert Southwest from July to September. When the subtropical ridge is farther north than normal towards

1246-587: Is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure , clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above (opposite to a cyclone ). Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as

1335-416: Is a significant problem in large urban centers during summer months such as Los Angeles, California and Mexico City . The existence of upper-level (altitude) high pressure allows upper level divergence which leads to surface convergence . If a capping mid-level ridge does not exist, this leads to free convection and the development of showers and thunderstorms if the lower atmosphere is humid. Because

1424-583: 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 (in the Pacific) and Darwin, Australia (on the Indian Ocean). El Niño episodes have negative SOI, meaning there

1513-684: Is associated with higher than normal air sea level pressure over Indonesia, Australia and across the Indian Ocean to the Atlantic . La Niña has roughly the reverse pattern: high pressure over the central and eastern Pacific and lower pressure through much of the rest of the tropics and subtropics. The two phenomena last a year or so each and typically occur every two to seven years with varying intensity, with neutral periods of lower intensity interspersed. El Niño events can be more intense but La Niña events may repeat and last longer. A key mechanism of ENSO

1602-405: Is known as Ekman transport . Colder water from deeper in the ocean rises along the continental margin to replace the near-surface water. This process cools the East Pacific because the thermocline is closer to the ocean surface, leaving relatively little separation between the deeper cold water and the ocean surface. Additionally, the northward-flowing Humboldt Current carries colder water from

1691-520: Is longer, it is classified as an El Niño "episode". It is thought that there have been at least 30 El Niño events between 1900 and 2024, with the 1982–83 , 1997–98 and 2014–16 events among the strongest on record. Since 2000, El Niño events have been observed in 2002–03, 2004–05, 2006–07, 2009–10, 2014–16 , 2018–19, and 2023–24 . Major ENSO events were recorded in the years 1790–93, 1828, 1876–78, 1891, 1925–26, 1972–73, 1982–83, 1997–98, 2014–16, and 2023–24. During strong El Niño episodes,

1780-553: Is lower pressure over Tahiti and higher pressure in Darwin. La Niña episodes on the other hand have positive SOI, meaning there is higher pressure in Tahiti and lower in Darwin. Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water, in part because of deep convection over the warm water. El Niño episodes are defined as sustained warming of the central and eastern tropical Pacific Ocean, thus resulting in

1869-406: Is not predictable. It affects the climate of much of the tropics and subtropics , and has links ( teleconnections ) to higher-latitude regions of the world. The warming phase of the sea surface temperature is known as " El Niño " and the cooling phase as " La Niña ". The Southern Oscillation is the accompanying atmospheric oscillation , which is coupled with the sea temperature change. El Niño

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1958-517: Is the Bjerknes feedback (named after Jacob Bjerknes in 1969) in which the atmospheric changes alter the sea temperatures that in turn alter the atmospheric winds in a positive feedback. Weaker easterly trade winds result in a surge of warm surface waters to the east and reduced ocean upwelling on the equator . In turn, this leads to warmer sea surface temperatures (called El Niño), a weaker Walker circulation (an east-west overturning circulation in

2047-530: Is the case of other storms that include Anne's Spot on Saturn and the Great Dark Spot on Neptune . Anticyclones had also been detected near the poles of Venus . ENSO El Niño–Southern Oscillation ( ENSO ) is a global climate phenomenon that emerges from variations in winds and sea surface temperatures over the tropical Pacific Ocean . Those variations have an irregular pattern but do have some semblance of cycles. The occurrence of ENSO

2136-406: Is typically around 0.5 m (1.5 ft) higher than near Peru because of the buildup of water in the West Pacific. The thermocline , or the transitional zone between the warmer waters near the ocean surface and the cooler waters of the deep ocean , is pushed downwards in the West Pacific due to this water accumulation. The total weight of a column of ocean water is almost the same in

2225-678: The Four Corners , thunderstorms of the New Mexican Monsoon can spread northward into Arizona and New Mexico . When suppressed to the south, the atmosphere dries out across the Desert Southwest, causing a break in the monsoon regime. On weather maps, high-pressure centers are associated with the letter H in English, within the isobar with the highest pressure value. On constant-pressure upper-level charts, anticyclones are located within

2314-518: The International Date Line and 120°W ), including the area off the west coast of South America , as upwelling of cold water occurs less or not at all offshore. This warming causes a shift in the atmospheric circulation, leading to higher air pressure in the western Pacific and lower in the eastern Pacific, with rainfall reducing over Indonesia, India and northern Australia, while rainfall and tropical cyclone formation increases over

2403-600: The Solomon Islands through Vanuatu , Fiji , Samoa , and Tonga . Low-level convergence along this band forms cloudiness as well as showers and thunderstorms . Thunderstorm activity, or convection, within the band is dependent upon the season, as the more equatorward portion is most active in the Southern Hemisphere summer, and the more poleward portion is most active during transition seasons of fall and spring. The convergence zone shifts east or west depending on

2492-578: The Southern Ocean to the tropics in the East Pacific . The combination of the Humboldt Current and upwelling maintains an area of cooler ocean waters off the coast of Peru. The West Pacific lacks a cold ocean current and has less upwelling as the trade winds are usually weaker than in the East Pacific, allowing the West Pacific to reach warmer temperatures. These warmer waters provide energy for

2581-411: The subtropical ridge , deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm-core lows such as tropical cyclones , due to descending cool air from the backside of upper troughs such as polar highs , or from large-scale sinking such as

2670-474: The upward movement of air . As a result, the warm West Pacific has on average more cloudiness and rainfall than the cool East Pacific. ENSO describes a quasi-periodic change of both oceanic and atmospheric conditions over the tropical Pacific Ocean. These changes affect weather patterns across much of the Earth. The tropical Pacific is said to be in one of three states of ENSO (also called "phases") depending on

2759-461: The Azores high pressure may bring about anticyclonic gloom during the winter because they pick up moisture as they move over the warmer oceans. High pressures that build to the north and move southwards often bring clear weather because they are cooled at the base (as opposed to warmed) which helps prevent clouds from forming. Once arctic air moves over an unfrozen ocean, the air mass modifies greatly over

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2848-569: The Bjerknes feedback naturally triggers negative feedbacks that end and reverse the abnormal state of the tropical Pacific. This perspective implies that the processes that lead to El Niño and La Niña also eventually bring about their end, making ENSO a self-sustaining process. Other theories view the state of ENSO as being changed by irregular and external phenomena such as the Madden–Julian oscillation , tropical instability waves , and westerly wind bursts . The three phases of ENSO relate to

2937-474: The Coastal Niño Index (ICEN), strong El Niño Costero events include 1957, 1982–83, 1997–98 and 2015–16, and La Niña Costera ones include 1950, 1954–56, 1962, 1964, 1966, 1967–68, 1970–71, 1975–76 and 2013. Currently, each country has a different threshold for what constitutes an El Niño event, which is tailored to their specific interests, for example: In climate change science, ENSO is known as one of

3026-603: The EP and CP types, and some scientists argue that ENSO exists as a continuum, often with hybrid types. The effects of the CP ENSO are different from those of the EP ENSO. The El Niño Modoki is associated with more hurricanes more frequently making landfall in the Atlantic. La Niña Modoki leads to a rainfall increase over northwestern Australia and northern Murray–Darling basin , rather than over

3115-492: The El Niño state. This process is known as Bjerknes feedback . Although these associated changes in the ocean and atmosphere often occur together, the state of the atmosphere may resemble a different ENSO phase than the state of the ocean or vice versa. Because their states are closely linked, the variations of ENSO may arise from changes in both the ocean and atmosphere and not necessarily from an initial change of exclusively one or

3204-491: The El Niños of 2006-07 and 2014-16 were also Central Pacific El Niños. Recent years when La Niña Modoki events occurred include 1973–1974, 1975–1976, 1983–1984, 1988–1989, 1998–1999, 2000–2001, 2008–2009, 2010–2011, and 2016–2017. The recent discovery of ENSO Modoki has some scientists believing it to be linked to global warming. However, comprehensive satellite data go back only to 1979. More research must be done to find

3293-541: The Equator, were defined. The western region is located over Indonesia and the eastern one over the equatorial Pacific, close to the South American coast. However, data on EQSOI goes back only to 1949. Sea surface height (SSH) changes up or down by several centimeters in Pacific equatorial region with the ESNO: El Niño causes a positive SSH anomaly (raised sea level) because of thermal expansion while La Niña causes

3382-520: The Pacific Ocean and are dependent on agriculture and fishing. In climate change science, ENSO is known as one of the internal climate variability phenomena. Future trends in ENSO due to climate change are uncertain, although climate change exacerbates the effects of droughts and floods. The IPCC Sixth Assessment Report summarized the scientific knowledge in 2021 for the future of ENSO as follows: "In

3471-563: The SPCZ axis. Figure 1 shows qualitative agreement between all of these SPCZ indicators. The position of the SPCZ can change on seasonal, interannual, and possibly longer timescales. Research into SPCZ movements of the 20th century are linked to changes in the IPO and ENSO. Folland et al., 2002 defined an index to describe the Interdecadal Pacific oscillation (IPO) with sea surface temperature and night marine air temperature to determine how

3560-472: The SPCZ varies with the IPO. When the IPO index has negative temperature anomalies, the SPCZ is displaced southwest and moves northeastward when the IPO index has positive temperature anomalies. The Southern Oscillation Index (SOI) is a metric for describing warm- and cold-phase conditions associated with the El Niño–Southern Oscillation (ENSO) and can also describe movements of the position of

3649-411: The SPCZ. Negative SOI index values are associated with warm-phase or El Niño-like conditions and a northeastward displacement of the SPCZ. Positive SOI index values, on the other hand, describe cold-phase or La Niña-like conditions and a southwestward displacement of the SPCZ. Determining the position of the SPCZ over longer timescales in the past (pre-20th century) has been studied using coral records of

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3738-453: The Walker circulation, which was named after Gilbert Walker who discovered the Southern Oscillation during the early twentieth century. The Walker circulation is an east-west overturning circulation in the vicinity of the equator in the Pacific. Upward air is associated with high sea temperatures, convection and rainfall, while the downward branch occurs over cooler sea surface temperatures in

3827-454: The West Pacific northeast of Australia averages around 28–30 °C (82–86 °F). SSTs in the East Pacific off the western coast of South America are closer to 20 °C (68 °F). Strong trade winds near the equator push water away from the East Pacific and towards the West Pacific. This water is slowly warmed by the Sun as it moves west along the equator. The ocean surface near Indonesia

3916-410: The West Pacific to a depth of about 30 m (90 ft) in the East Pacific. Cooler deep ocean water takes the place of the outgoing surface waters in the East Pacific, rising to the ocean surface in a process called upwelling . Along the western coast of South America, water near the ocean surface is pushed westward due to the combination of the trade winds and the Coriolis effect . This process

4005-436: The air subsidence at their center, act to steer tropical cyclones around and out their periphery. Due to the subsidence within this type of system, a cap can develop which inhibits free convection and hence mixing of the lower with the middle level troposphere. This limits thunderstorms and other low-pressure weather activity near their centers and traps low-level pollutants such as ozone as haze under their base, which

4094-429: The asymmetric nature of the warm and cold phases of ENSO, some studies could not identify similar variations for La Niña, both in observations and in the climate models, but some sources could identify variations on La Niña with cooler waters on central Pacific and average or warmer water temperatures on both eastern and western Pacific, also showing eastern Pacific Ocean currents going to the opposite direction compared to

4183-438: The atmosphere) and even weaker trade winds. Ultimately the warm waters in the western tropical Pacific are depleted enough so that conditions return to normal. The exact mechanisms that cause the oscillation are unclear and are being studied. Each country that monitors the ENSO has a different threshold for what constitutes an El Niño or La Niña event, which is tailored to their specific interests. El Niño and La Niña affect

4272-436: The atmospheric and oceanic conditions. When the tropical Pacific roughly reflects the average conditions, the state of ENSO is said to be in the neutral phase. However, the tropical Pacific experiences occasional shifts away from these average conditions. If trade winds are weaker than average, the effect of upwelling in the East Pacific and the flow of warmer ocean surface waters towards the West Pacific lessen. This results in

4361-591: The correlation and study past El Niño episodes. More generally, there is no scientific consensus on how/if climate change might affect ENSO. There is also a scientific debate on the very existence of this "new" ENSO. A number of studies dispute the reality of this statistical distinction or its increasing occurrence, or both, either arguing the reliable record is too short to detect such a distinction, finding no distinction or trend using other statistical approaches, or that other types should be distinguished, such as standard and extreme ENSO. Likewise, following

4450-543: The currents in traditional La Niñas. Coined by the Peruvian Comité Multisectorial Encargado del Estudio Nacional del Fenómeno El Niño (ENFEN), ENSO Costero, or ENSO Oriental, is the name given to the phenomenon where the sea-surface temperature anomalies are mostly focused on the South American coastline, especially from Peru and Ecuador. Studies point many factors that can lead to its occurrence, sometimes accompanying, or being accompanied, by

4539-431: The day, there is more incoming solar radiation and heating so temperatures rise rapidly near the surface. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not blocked, allowing the escape of heat and giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to

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4628-573: The descending portion of the Hadley cell circulation. Upper-level high-pressure areas lie over tropical cyclones due to their warm core nature. Surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence , or converging height lines near or above

4717-461: The east. During El Niño, as the sea surface temperatures change so does the Walker Circulation. Warming in the eastern tropical Pacific weakens or reverses the downward branch, while cooler conditions in the west lead to less rain and downward air, so the Walker Circulation first weakens and may reverse.   The Southern Oscillation is the atmospheric component of ENSO. This component

4806-614: The eastern Pacific below average, and air pressure high in the eastern Pacific and low in the western Pacific. The ENSO cycle, including both El Niño and La Niña, causes global changes in temperature and rainfall. If the temperature variation from climatology is within 0.5 °C (0.9 °F), ENSO conditions are described as neutral. Neutral conditions are the transition between warm and cold phases of ENSO. Sea surface temperatures (by definition), tropical precipitation, and wind patterns are near average conditions during this phase. Close to half of all years are within neutral periods. During

4895-479: The eastern Pacific. However, in the 1990s and 2000s, variations of ENSO conditions were observed, in which the usual place of the temperature anomaly (Niño 1 and 2) is not affected, but an anomaly also arises in the central Pacific (Niño 3.4). The phenomenon is called Central Pacific (CP) ENSO, "dateline" ENSO (because the anomaly arises near the dateline ), or ENSO "Modoki" (Modoki is Japanese for "similar, but different"). There are variations of ENSO additional to

4984-404: The eastern equatorial part of the central Pacific Ocean will be lower than normal by 3–5 °C (5.4–9 °F). The phenomenon occurs as strong winds blow warm water at the ocean's surface away from South America, across the Pacific Ocean towards Indonesia. As this warm water moves west, cold water from the deep sea rises to the surface near South America. The movement of so much heat across

5073-567: The eastern portion of the country as in a conventional EP La Niña. Also, La Niña Modoki increases the frequency of cyclonic storms over Bay of Bengal , but decreases the occurrence of severe storms in the Indian Ocean overall. The first recorded El Niño that originated in the central Pacific and moved toward the east was in 1986. Recent Central Pacific El Niños happened in 1986–87, 1991–92, 1994–95, 2002–03, 2004–05 and 2009–10. Furthermore, there were "Modoki" events in 1957–59, 1963–64, 1965–66, 1968–70, 1977–78 and 1979–80. Some sources say that

5162-485: The ensemble of coupled models. At its southeast edge, the circulation around the feature forces a salinity gradient in the ocean, with fresher and warmer waters of the western Pacific lying to its west. Cooler and saltier waters lie to its east. Tropical textbook : from trade winds to cyclone (2 vol) Archived 2012-12-16 at archive.today , 897 pp., Florent Beucher, 25 mai 2010, Météo-France, ISBN   978-2-11-099391-5 Anticyclone An anticyclone

5251-535: The equator and to the poles aloft. As air moves towards the mid-latitudes, it cools and sinks leading to subsidence near the 30° parallel of both hemispheres. This circulation known as the Hadley cell forms the subtropical ridge. Many of the world's deserts are caused by these climatological high-pressure areas . Because these anticyclones strengthen with height, they are known as warm core ridges. The development of anticyclones aloft occurs in warm core cyclones such as tropical cyclones when latent heat caused by

5340-400: The existence of El Niño, or the phase of ENSO . The climatological position can be estimated by computing its mean position over 30 or more years. There are several metrics to measure the position of the SPCZ. The location of maximum rainfall, maximum of low level convergence , maxima of the 500 hPa vertical motion, and the minimum in outgoing longwave radiation (OLR) are four indicators of

5429-718: The following years: Transitional phases at the onset or departure of El Niño or La Niña can also be important factors on global weather by affecting teleconnections . Significant episodes, known as Trans-Niño, are measured by the Trans-Niño index (TNI). Examples of affected short-time climate in North America include precipitation in the Northwest US and intense tornado activity in the contiguous US. The first ENSO pattern to be recognised, called Eastern Pacific (EP) ENSO, to distinguish if from others, involves temperature anomalies in

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5518-413: The formation of clouds is released aloft increasing the air temperature; the resultant thickness of the atmospheric layer increases high pressure aloft which evacuates their outflow. In the absence of rotation, the wind tends to blow from areas of high pressure to areas of low pressure . The stronger the pressure difference (pressure gradient) between a high-pressure system and a low-pressure system,

5607-534: The global climate and disrupt normal weather patterns, which as a result can lead to intense storms in some places and droughts in others. El Niño events cause short-term (approximately 1 year in length) spikes in global average surface temperature while La Niña events cause short term surface cooling. Therefore, the relative frequency of El Niño compared to La Niña events can affect global temperature trends on timescales of around ten years. The countries most affected by ENSO are developing countries that are bordering

5696-409: The globe. Atlantic and Pacific hurricanes can have different characteristics due to lower or higher wind shear and cooler or warmer sea surface temperatures. La Niña events have been observed for hundreds of years, and occurred on a regular basis during the early parts of both the 17th and 19th centuries. Since the start of the 20th century, La Niña events have occurred during

5785-455: The highest height line contour. On Jupiter , there are two examples of an extraterrestrial anticyclonic storm; the Great Red Spot and the recently formed Oval BA on Jupiter. They are powered by smaller storms merging unlike any typical anticyclonic storm that happens on Earth where water powers them. Another theory is that warmer gases rise in a column of cold air, creating a vortex as

5874-471: The internal climate variability phenomena. The other two main ones are Pacific decadal oscillation and Atlantic multidecadal oscillation . La Niña impacts the global climate and disrupts normal weather patterns, which can lead to intense storms in some places and droughts in others. El Niño events cause short-term (approximately 1 year in length) spikes in global average surface temperature while La Niña events cause short term cooling. Therefore,

5963-427: The last several decades, the number of El Niño events increased, and the number of La Niña events decreased, although observation of ENSO for much longer is needed to detect robust changes. Studies of historical data show the recent El Niño variation is most likely linked to global warming. For example, some results, even after subtracting the positive influence of decadal variation, are shown to be possibly present in

6052-406: The level of non-divergence, which is near the 500 hPa pressure surface about midway up the troposphere. Because they weaken with height, these high-pressure systems are cold. Heating of the earth near the equator forces upward motion and convection along the monsoon trough or Intertropical Convergence Zone . The divergence over the near-equatorial trough leads to air rising and moving away from

6141-528: The long term, it is very likely that the precipitation variance related to El Niño–Southern Oscillation will increase". The scientific consensus is also that "it is very likely that rainfall variability related to changes in the strength and spatial extent of ENSO teleconnections will lead to significant changes at regional scale". The El Niño–Southern Oscillation is a single climate phenomenon that periodically fluctuates between three phases: Neutral, La Niña or El Niño. La Niña and El Niño are opposite phases in

6230-581: The neutral ENSO phase, other climate anomalies/patterns such as the sign of the North Atlantic Oscillation or the Pacific–North American teleconnection pattern exert more influence. El Niño conditions are established when the Walker circulation weakens or reverses and the Hadley circulation strengthens, leading to the development of a band of warm ocean water in the central and east-central equatorial Pacific (approximately between

6319-400: 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. It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other. The consequences of ENSO in terms of

6408-636: The oscillation which are deemed to occur when specific ocean and atmospheric conditions are reached or exceeded. An early recorded mention of the term "El Niño" ("The Boy" in Spanish) to refer to climate occurred in 1892, when Captain Camilo Carrillo told the geographical society congress in Lima that Peruvian sailors named the warm south-flowing current "El Niño" because it was most noticeable around Christmas. Although pre-Columbian societies were certainly aware of

6497-415: The other. Conceptual models explaining how ENSO operates generally accept the Bjerknes feedback hypothesis. However, ENSO would perpetually remain in one phase if Bjerknes feedback were the only process occurring. Several theories have been proposed to explain how ENSO can change from one state to the next, despite the positive feedback. These explanations broadly fall under two categories. In one view,

6586-417: The peruvian coast, and increased rainfall and decreased temperatures on its mountainous and jungle regions. Because they don't influence the global climate as much as the other types, these events present lesser and weaker correlations to other significant ENSO features, neither always being triggered by Kelvin waves , nor always being accompanied by proportional Southern Oscillation responses. According to

6675-483: The phenomenon, the indigenous names for it have been lost to history. The capitalized term El Niño refers to the Christ Child , Jesus , because periodic warming in the Pacific near South America is usually noticed around Christmas . Originally, the term El Niño applied to an annual weak warm ocean current that ran southwards along the coast of Peru and Ecuador at about Christmas time. However, over time

6764-545: The position of the SPCZ. Their coral oxygen isotope index indicated an eastward shift of the decadal mean position of the SPCZ since the mid 1800s. A shift of the SPCZ in this direction suggests there were more La Niña-like or cold-phase conditions in the Pacific, during this period, often called the Little Ice Age . Additional paleoclimate studies are still needed in order to test the reliability of these coral results. The IPO and ENSO can interact together to produce changes in

6853-419: The position of the SPCZ. West of about 140 W, both ENSO (measured with Southern Oscillation Index ) and IPO strongly influence the SPCZ latitude, but farther east only ENSO is a significant factor. Only near 170 W is there any indication of an interaction between the two factors. Besides observations of the SPCZ and movement in its position, there have been modelling studies as well. Widlansky et al. (2012) used

6942-406: The relative frequency of El Niño compared to La Niña events can affect global temperature trends on decadal timescales. There is no sign that there are actual changes in the ENSO physical phenomenon due to climate change. Climate models do not simulate ENSO well enough to make reliable predictions. Future trends in ENSO are uncertain as different models make different predictions. It may be that

7031-438: The simulations. Widlanksy et al. (2012) argued the sea surface temperature biases in models created uncertainty in the rainfall projections and produce what has been named “the double ITCZ problem”. The impact of sea surface temperature bias was further investigated by using uncoupled atmospheric models with prescribed sea surface temperatures, and those 3 models each with differing complexity showed less severe double ITCZ bias than

7120-487: The southwest Pacific. Linsley et al. (2006) reconstructed sea-surface temperature and sea surface salinity in the southwest Pacific starting circa 1600CE by measuring the oxygen isotopic composition of four Porites coral records from Rarotonga and two from Fiji . Coral isotope measurements provide information on both sea surface temperature and sea surface salinity, so they can indicate times of increased or decreased temperature and/or precipitation associated with changes in

7209-431: The strength of the trade winds was first identified by Jacob Bjerknes in 1969. Bjerknes also hypothesized that ENSO was a positive feedback system where the associated changes in one component of the climate system (the ocean or atmosphere) tend to reinforce changes in the other. For example, during El Niño, the reduced contrast in ocean temperatures across the Pacific results in weaker trade winds, further reinforcing

7298-424: The stronger the wind. The coriolis force caused by Earth 's rotation gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and anticlockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down

7387-521: The temperature anomalies and precipitation and weather extremes around the world are clearly increasing and associated with climate change . For example, recent scholarship (since about 2019) has found that climate change is increasing the frequency of extreme El Niño events. Previously there was no consensus on whether climate change will have any influence on the strength or duration of El Niño events, as research alternately supported El Niño events becoming stronger and weaker, longer and shorter. Over

7476-618: The term has evolved and now refers to the warm and negative phase of the El Niño–Southern Oscillation (ENSO). The original phrase, El Niño de Navidad , arose centuries ago, when Peruvian fishermen named the weather phenomenon after the newborn Christ. La Niña ("The Girl" in Spanish) is the colder counterpart of El Niño, as part of the broader ENSO climate pattern . In the past, it was also called an anti-El Niño and El Viejo, meaning "the old man." A negative phase exists when atmospheric pressure over Indonesia and

7565-452: The tropical Pacific Ocean. The low-level surface trade winds , which normally blow from east to west along the equator, either weaken or start blowing from the other direction. El Niño phases are known to happen at irregular intervals of two to seven years, and lasts nine months to two years. The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Niño "conditions"; when its duration

7654-416: The warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system. When extremely cold air moves over relatively warm oceans, polar lows can develop. However, warm and moist (or maritime tropical) air masses which move poleward from tropical sources are slower to modify than arctic air masses. The circulation around mid-level (altitude) ridges, and

7743-466: The west Pacific is abnormally high and pressure over the east Pacific is abnormally low, during El Niño episodes, and a positive phase is when the opposite occurs during La Niña episodes, and pressure over Indonesia is low and over the west Pacific is high. On average, the temperature of the ocean surface in the tropical East Pacific is roughly 8–10 °C (14–18 °F) cooler than in the tropical West Pacific . The sea surface temperature (SST) of

7832-441: The western and east Pacific. Because the warmer waters of the upper ocean are slightly less dense than the cooler deep ocean, the thicker layer of warmer water in the western Pacific means the thermocline there must be deeper. The difference in weight must be enough to drive any deep water return flow. Consequently, the thermocline is tilted across the tropical Pacific, rising from an average depth of about 140 m (450 ft) in

7921-470: The wind flowing out of high-pressure systems and causes wind to flow more outward (more ageostrophically ) from the center. High-pressure systems are frequently associated with light winds at the surface and subsidence of air from higher portions of the troposphere . Subsidence will generally warm an air mass by adiabatic (compressional) heating. Thus, high pressure typically brings clear skies. Because no clouds are present to reflect sunlight during

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