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109-716: The North Atlantic Oscillation ( NAO ) is a weather phenomenon over the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic Low and the Azores High . Through fluctuations in the strength of the Icelandic Low and the Azores High, it controls the strength and direction of westerly winds and location of storm tracks across

218-412: A force or "weight" of about 10.1 newtons , resulting in a pressure of 10.1 N/cm or 101   kN /m (101 kilopascals, kPa). A column of air with a cross-sectional area of 1   in would have a weight of about 14.7   lbf , resulting in a pressure of 14.7   lbf/in . Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and

327-568: 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

436-602: 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

545-415: A diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides . This effect is strongest in tropical zones, with an amplitude of a few hectopascals, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle, and a semi-circadian (12 h) cycle. The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters)

654-436: A function of altitude can be approximated as constant and contributes little to this fall-off. Pressure measures force per unit area, with SI units of pascals (1 pascal = 1 newton per square metre , 1   N/m ). On average, a column of air with a cross-sectional area of 1 square centimetre (cm ), measured from the mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1.03 kilogram and exerts

763-489: 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

872-410: A location on Earth 's surface ( terrain and oceans ). It is directly proportional to the mass of air over that location. For numerical reasons, atmospheric models such as general circulation models (GCMs) usually predict the nondimensional logarithm of surface pressure . The average value of surface pressure on Earth is 985 hPa. This is in contrast to mean sea-level pressure, which involves

981-553: 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

1090-463: A mechanism in which a shrinking summertime sea ice cover changes the air pressure zones in the Arctic atmosphere and effects on European winter weather. If there is a particularly large-scale melt of Arctic sea ice in summer, as observed in recent years, two important effects are intensified. Firstly, the retreat of the light ice surface reveals the darker ocean, causing it to warm up more in summer from

1199-411: 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

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1308-632: A negative phase of the NAO. This is thought to be the first study showing a link between NAO and terrestrial insects in North America. The NAO's ecological effects extend as far as the Tibetan Plateau , where increases in aridity resulting in significant forest mortality and intensification of dust storms have been linked to NAO- events. The winter of 2009–10 in Europe was unusually cold. It

1417-427: A pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). Conversely, 10.3 m is the maximum height to which water can be raised using suction under standard atmospheric conditions. Low pressures, such as natural gas lines, are sometimes specified in inches of water , typically written as w.c. (water column) gauge or w.g. (inches water) gauge. A typical gas-using residential appliance in

1526-517: 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

1635-469: 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

1744-425: 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

1853-424: A standard lapse rate) associated with reduction of sea level from high elevations. The Dead Sea , the lowest place on Earth at 430 metres (1,410 ft) below sea level, has a correspondingly high typical atmospheric pressure of 1,065   hPa. A below-sea-level surface pressure record of 1,081.8 hPa (31.95 inHg) was set on 21 February 1961. The lowest non-tornadic atmospheric pressure ever measured

1962-409: Is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere. It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition. The mean sea-level pressure (MSLP) is the atmospheric pressure at mean sea level . This

2071-422: 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 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

2180-603: 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

2289-457: Is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so atmospheric pressure decreases with increasing elevation. Because the atmosphere is thin relative to the Earth's radius—especially the dense atmospheric layer at low altitudes—the Earth's gravitational acceleration as

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2398-628: Is hypothesized that this may be due to a combination of low solar activity, a warm phase of the El Niño–Southern Oscillation and a strong easterly phase of the Quasi-Biennial Oscillation all occurring simultaneously. The Met Office reported that the UK, for example, had experienced its coldest winter for 30 years. This coincided with an exceptionally negative phase of the NAO. Analysis published in mid-2010 confirmed that

2507-511: Is instead reported in kilopascals. In the US weather code remarks, three digits are all that are transmitted; decimal points and the one or two most significant digits are omitted: 1,013.2 hPa (14.695 psi) is transmitted as 132; 1,000 hPa (100 kPa) is transmitted as 000; 998.7   hPa is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia , where

2616-405: 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

2725-520: 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,

2834-561: 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

2943-490: Is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet . The altimeter setting in aviation is an atmospheric pressure adjustment. Average sea-level pressure is 1,013.25 hPa (29.921 inHg; 760.00 mmHg). In aviation weather reports ( METAR ), QNH is transmitted around the world in hectopascals or millibars (1 hectopascal = 1 millibar), except in

3052-406: 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

3161-521: The Arctic oscillation (AO) (or Northern Annular Mode (NAM)), but should not be confused with the Atlantic multidecadal oscillation (AMO). The NAO has multiple possible definitions. The easiest to understand are those based on measuring the seasonal average air pressure difference between stations, such as: These definitions all have in common the same northern point (because this is the only station in

3270-465: The Gulf of Maine are affected by this reduced cod catch. The strength of the NAO is also a determinant in the population fluctuations of the intensively studied Soay sheep . Strangely enough, Jonas and Joern (2007) found a strong signal between NAO and grasshopper species composition in the tall grass prairies of the midwestern United States. They found that, even though NAO does not significantly affect

3379-528: 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

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3488-472: The Siberian High often attains a sea-level pressure above 1,050 hPa (15.2 psi; 31 inHg), with record highs close to 1,085 hPa (15.74 psi; 32.0 inHg). The lowest measurable sea-level pressure is found at the centres of tropical cyclones and tornadoes , with a record low of 870 hPa (12.6 psi; 26 inHg). Surface pressure is the atmospheric pressure at

3597-599: 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

3706-523: The United States , Canada , and Japan where it is reported in inches of mercury (to two decimal places). The United States and Canada also report sea-level pressure SLP, which is adjusted to sea level by a different method, in the remarks section, not in the internationally transmitted part of the code, in hectopascals or millibars. However, in Canada's public weather reports, sea level pressure

3815-1339: The troposphere , the following equation (the barometric formula ) relates atmospheric pressure p to altitude h : p = p 0 ⋅ ( 1 − L ⋅ h T 0 ) g ⋅ M R 0 ⋅ L = p 0 ⋅ ( 1 − g ⋅ h c p ⋅ T 0 ) c p ⋅ M R 0 ≈ p 0 ⋅ exp ⁡ ( − g ⋅ h ⋅ M T 0 ⋅ R 0 ) {\displaystyle {\begin{aligned}p&=p_{0}\cdot \left(1-{\frac {L\cdot h}{T_{0}}}\right)^{\frac {g\cdot M}{R_{0}\cdot L}}\\&=p_{0}\cdot \left(1-{\frac {g\cdot h}{c_{\text{p}}\cdot T_{0}}}\right)^{\frac {c_{\text{p}}\cdot M}{R_{0}}}\approx p_{0}\cdot \exp \left(-{\frac {g\cdot h\cdot M}{T_{0}\cdot R_{0}}}\right)\end{aligned}}} The values in these equations are: Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate . Atmospheric pressure shows

3924-468: The upper Midwest and New England , but the impact to the south of these areas is debatable. Conversely, when the NAO index is low (NAO-), the upper central and northeastern portions of the United States can incur winter cold outbreaks more than the norm with associated heavy snowstorms. In summer, a strong NAO- is thought to contribute to a weakened jet stream that normally pulls zonal systems into

4033-426: 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

4142-416: The vapour pressure is equal to the atmospheric pressure around the liquid. Because of this, the boiling point of liquids is lower at lower pressure and higher at higher pressure. Cooking at high elevations, therefore, requires adjustments to recipes or pressure cooking . A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method

4251-471: The Atlantic Basin contributing significantly to excessively long-lasting heat waves over Europe, however, recent studies do not show the evidence of these associations. More recent studies have shown that the components (pressure centers strength, and locations) of the NAO are more powerful to investigate the relationships to seasonal and sub-seasonal climate variability over Europe, North America and

4360-542: The Atlantic bring moist air into Europe. In years when westerlies are strong, summers are cool, winters are mild and rain is frequent. If westerlies are suppressed, the temperature is more extreme in summer and winter leading to heat waves , deep freezes and reduced rainfall. A permanent low-pressure system over Iceland (the Icelandic Low ) and a permanent high-pressure system over the Azores (the Azores High ) control

4469-625: 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

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4578-482: 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

4687-565: 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

4796-497: 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

4905-496: 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

5014-555: 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

5123-469: The Gulf coast during 3000–1400 BC and again during the most recent millennium. These quiescent intervals were separated by a hyperactive period during 1400 BC – 1000 AD, when the Gulf coast was struck frequently by catastrophic hurricanes and their landfall probabilities increased by 3–5 times. Until recently, the NAO had been in an overall more positive regime since the late 1970s, bringing colder conditions to

5232-467: The Mediterranean region. Under a positive NAO index (NAO+), regional reduction in atmospheric pressure results in a regional rise in sea level due to the 'inverse barometer effect'. This effect is important to both the interpretation of historic sea level records and predictions of future sea level trends, as mean pressure fluctuations of the order of millibars can lead to sea level fluctuations of

5341-596: The NAO+ peak in the early 1990s may have contributed to the collapse of the Newfoundland cod fishery . In southwestern Europe, NAO- events are associated with increased aeolian activity. On the East Coast of the United States an NAO+ causes warmer temperatures and increased rainfall, and thus warmer, less saline surface water. This prevents nutrient-rich upwelling which has reduced productivity. Georges Bank and

5450-602: The North Atlantic. The NAO was discovered through several studies in the late 19th and early 20th centuries. Unlike the El Niño–Southern Oscillation phenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode. It is one of the most important manifestations of climate fluctuations in the North Atlantic and surrounding humid climates. The North Atlantic Oscillation is closely related to

5559-600: The North-West Atlantic, which has been linked with the thriving populations of Labrador Sea snow crabs , which have a low temperature optimum. The NAO+ warming of the North Sea reduces survival of cod larvae which are at the upper limits of their temperature tolerance, as does the cooling in the Labrador Sea, where the cod larvae are at their lower temperature limits. Though not the critical factor,

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5668-468: 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

5777-568: The Pacific Ocean, a largely positive North Atlantic Oscillation prevailed over Europe during the winter of 2015–2016. For example, Cumbria in England registered one of the wettest months on record. The Maltese Islands in the Mediterranean registered one of the driest years ever recorded up to beginning of March, with a national average of only 235 mm and some areas registering less than 200 mm. Atmospheric pressure#Mean sea-level pressure In most circumstances, atmospheric pressure

5886-400: The US is rated for a maximum of 1 ⁄ 2  psi (3.4 kPa; 34 mbar), which is approximately 14 w.g. Similar metric units with a wide variety of names and notation based on millimetres , centimetres or metres are now less commonly used. Pure water boils at 100 °C (212 °F) at earth's standard atmospheric pressure. The boiling point is the temperature at which

5995-457: 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

6104-457: 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

6213-465: 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

6322-479: The air near to the ground leads to rising movements and the atmosphere becomes less stable. One of these patterns is the air pressure difference between the Arctic and mid-latitudes: the Arctic oscillation with the Azores highs and Iceland lows known from the weather reports. If this difference is high, a strong westerly wind will result which in winter carries warm and humid Atlantic air masses right down to Europe. In

6431-429: 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

6540-436: 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

6649-454: 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 the atmosphere) and even weaker trade winds. Ultimately

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6758-503: The atmospheric pressure at a given altitude. Temperature and humidity also affect the atmospheric pressure. Pressure is proportional to temperature and inversely related to humidity, and both of these are necessary to compute an accurate figure. The graph on the right above was developed for a temperature of 15 °C and a relative humidity of 0%. At low altitudes above sea level, the pressure decreases by about 1.2 kPa (12 hPa) for every 100 metres. For higher altitudes within

6867-455: 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 is the Bjerknes feedback (named after Jacob Bjerknes in 1969) in which

6976-502: The concurrent ' El Niño ' event and the rare occurrence of an extremely negative NAO were involved. However, during the winter of 2010–11 in Northern and Western Europe , the Icelandic Low , typically positioned west of Iceland and east of Greenland, appeared regularly to the east of Iceland and so allowed exceptionally cold air into Europe from the Arctic. A strong area of high pressure was initially situated over Greenland , reversing

7085-591: 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

7194-559: 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

7303-473: The direction and strength of westerly winds into Europe. The relative strengths and positions of these systems vary from year to year and this variation is known as the NAO. A large difference in the pressure at the two stations (a high index year, denoted NAO+) leads to increased westerlies and, consequently, cool summers and mild and wet winters in Central Europe and its Atlantic facade. In contrast, if

7412-469: 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

7521-614: 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

7630-479: 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

7739-404: 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

7848-627: 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

7957-505: The extrapolation of pressure to sea level for locations above or below sea level. The average pressure at mean sea level ( MSL ) in the International Standard Atmosphere ( ISA ) is 1,013.25 hPa, or 1 atmosphere (atm), or 29.92 inches of mercury. Pressure (P), mass (m), and acceleration due to gravity (g) are related by P = F/A = (m*g)/A, where A is the surface area. Atmospheric pressure is thus proportional to

8066-674: 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

8175-409: 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

8284-560: The index is low (NAO-), westerlies are suppressed, northern European areas suffer cold dry winters and storms track southwards toward the Mediterranean Sea . This brings increased storm activity and rainfall to southern Europe and North Africa. Especially during the months of November to April, the NAO is responsible for much of the variability of weather in the North Atlantic region, affecting wind speed and wind direction changes, changes in temperature and moisture distribution and

8393-418: The intensity, number and track of storms. Research now suggests that the NAO may be more predictable than previously assumed and skillful winter forecasts may be possible for the NAO. There is some debate as to how much the NAO impacts short term weather over North America. While most agree that the impact of the NAO is much less over the United States than for Western Europe, the NAO is also believed to affect

8502-471: 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,

8611-427: 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

8720-528: 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

8829-427: The negative phase when pressure differences are low, cold Arctic air can then easily penetrate southward through Europe without being interrupted by the usual westerlies. Model calculations show that the air pressure difference with decreased sea ice cover in the Arctic summer is weakened in the following winter, enabling Arctic cold to push down to mid-latitudes. Despite one of the strongest El Niño events recorded in

8938-644: 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

9047-402: The normal wind pattern in the northwestern Atlantic, creating a blocking pattern driving warm air into northeastern Canada and cold air into Western Europe, as was the case during the previous winter. This occurred during a La Niña season, and is connected to the rare Arctic dipole anomaly . In the north western part of the Atlantic, both of these winters were mild, especially 2009–2010, which

9156-400: 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

9265-531: The order of centimeters. By controlling the position of the Azores High, the NAO also influences the direction of general storm paths for major North Atlantic tropical cyclones : a position of the Azores High farther to the south tends to force storms into the Gulf of Mexico , whereas a northern position allows them to track up the North American Atlantic Coast. As paleotempestological research has shown, few major hurricanes struck

9374-652: 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

9483-415: 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,

9592-417: 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

9701-489: 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

9810-513: The principal empirical orthogonal function (EOF) of surface pressure. This definition has a high degree of correlation with the station-based definition. This then leads onto a debate as to whether the NAO is distinct from the AO/NAM, and if not, which of the two is to be considered the most physically based expression of atmospheric structure (as opposed to the one that most clearly falls out of mathematical expression). Westerly winds blowing across

9919-456: The region with a long record) in Iceland ; and various southern points. All are attempting to capture the same pattern of variation, by choosing stations in the "eye" of the two stable pressure areas, the Azores High and the Icelandic Low (shown in the graphic). A more complex definition, only possible with more complete modern records generated by numerical weather prediction , is based on

10028-406: 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

10137-422: 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 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

10246-403: The solar radiation ( ice–albedo feedback mechanism). Secondly, the diminished ice cover can no longer prevent the heat stored in the ocean being released into the atmosphere ( lid effect ). As a result of the decreased sea ice cover the air is warmed more greatly than it used to be particularly in autumn and winter because during this period the ocean is warmer than the atmosphere. The warming of

10355-431: 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

10464-521: 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

10573-547: 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

10682-452: 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

10791-442: 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 the global climate and disrupt normal weather patterns, which as

10900-491: The weather in the midwest, there was a significant increase in abundance of common grasshopper species (i.e. Hypochlora alba, Hesperotettix spp., Phoetaliotes nebrascensis, M. scudderi, M. keeleri, and Pseudopomala brachyptera ) following winters during the positive phase of NAO and a significant increase in the abundance of less common species (i.e. Campylacantha olivacea, Melanoplus sanguinipes, Mermiria picta, Melanoplus packardii, and Boopedon gracile ) following winters during

11009-546: The weather over much of upper central and eastern areas of North America. During the winter, when the index is high (NAO+), the Azores High draws a stronger south-westerly circulation over the eastern half of the North American continent which prevents Arctic air from plunging southward (into the United States south of 40 latitude). In combination with the El Niño , this effect can produce significantly warmer winters over

11118-483: The weight per unit area of the atmospheric mass above that location. Pressure on Earth varies with the altitude of the surface, so air pressure on mountains is usually lower than air pressure at sea level. Pressure varies smoothly from the Earth's surface to the top of the mesosphere . Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. As altitude increases, atmospheric pressure decreases. One can calculate

11227-466: 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

11336-441: 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

11445-511: Was confirming Newton's theory of gravitation at and on Schiehallion mountain in Scotland, and he needed to measure elevations on the mountain's sides accurately. William Roy , using barometric pressure, was able to confirm Maskelyne's height determinations; the agreement was within one meter (3.28 feet). This method became and continues to be useful for survey work and map making. El Ni%C3%B1o El Niño–Southern Oscillation ( ENSO )

11554-567: Was 1,084.8 hPa (32.03 inHg) measured in Tosontsengel, Mongolia on 19 December 2001. The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata in Evenk Autonomous Okrug , Russia (66°53'   N, 93°28'   E, elevation: 261 m, 856 ft) on 31 December 1968 of 1,083.8 hPa (32.005 inHg). The discrimination is due to the problematic assumptions (assuming

11663-439: Was 870 hPa (0.858 atm; 25.69 inHg), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft. One atmosphere (101.325 kPa or 14.7 psi) is also the pressure caused by the weight of a column of freshwater of approximately 10.3 m (33.8 ft). Thus, a diver 10.3 m under water experiences

11772-633: Was the warmest recorded in Canada. The winter of 2010-2011 was particularly above normal in the northern Arctic regions of that country. The probability of cold winters with much snow in Central Europe rises when the Arctic is covered by less sea ice in summer. Scientists of the Potsdam Research Unit of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association have decrypted

11881-444: Was used by explorers. Conversely, if one wishes to evaporate a liquid at a lower temperature, for example in distillation , the atmospheric pressure may be lowered by using a vacuum pump , as in a rotary evaporator . An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains, thanks to reliable pressure measurement devices. In 1774, Maskelyne

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