For example, one of the most recent results

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,^[58] the amplitude of the ENSO variability in the observed data still increases, by as much as 60% in the last 50 years.^{[59] Sony VAIO VPCS115FG Battery} It is not certain what exact changes will happen to ENSO in the future ^[60]: Different models make different predictions.^[61] 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), ^{Sony VAIO VPCS117GG Battery} El Niño will become weaker than it was.^[62] It may also be that the stabilizing and destabilizing forces influencing the phenomenon will eventually compensate for each other.^[63] More research is needed to provide a better answer to that question. The ENSO is considered to be a potentialtipping element in Earth's climate. ^{Sony VAIO VPCS117GGB Battery} The traditional Niño, also called Eastern Pacific (EP) El Niño,^[65] 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). ^{Sony VAIO VPCS118EC Battery} The phenomenon is called Central Pacific (CP) El Niño,^[65] "dateline" El Niño (because the anomaly arises near the dateline), or El Niño "Modoki" (Modoki isJapanese 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.^{[68] Sony VAIO VPCS119FJ/B Battery} The recent discovery of El Niño Modoki has some scientists believing it to be linked to global warming.^[69] However, Satellite data goes back only to 1979. More research must be done to find the correlation and study past El Niño episodes. There is also a scientific debate on the very existence of this "new" ENSO. ^{Sony VAIO VPCS119GC Battery} Indeed, 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 El Niños. ^{Sony VAIO VPCS11AFJ Battery} Following the asymmetric nature of the warm and cold phases of ENSO, a study could not identify such distinctions for La Niña, both in observations and in the climate models.The first recorded El Niño that originated in the central Pacific and moved toward the east was in 1986.There is no scientific consensus on how/if climate change may affect ENSO. ^{Sony VAIO VPCS11AGJ Battery} Extreme weather conditions related to the El Niño cycle correlate 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. ^{Sony VAIO VPCS11AHJ Battery} 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 Niña events. ^{Sony VAIO VPCS11AVJ Battery} A severe outbreak of Rift Valley fever occurred after extreme rainfall in north-eastern Kenya and southern Somalia during the 1997–98 El Niño.ENSO may be linked to civil conflicts. Scientists at The Earth Institute of Columbia University] have analyzed data from 1950 to 2004 and suggest that ENSO may have had a role in 21% of all civil conflicts since 1950, ^{Sony VAIO VPCS11J7E/B Battery} with the risk of annual civil conflict doubling from 3% to 6% in countries affected by ENSO during El Niño years relative to La Niña years. ENSO conditions have occurred at two- to seven year intervals for at least the past 300 years, but most of them have been weak. ^{Sony VAIO VPCS11M1E/W Battery} There is also evidence for strong El Niño events during the early Holocene epoch 10,000 years ago.El Niño affected pre-Columbian Incas ^[86] and may have led to the demise of the Moche and other pre-Columbian Peruvian cultures.^[87] A recent study suggests that a strong El-Niño effect between 1789–93 caused poor crop yields in Europe, ^{Sony VAIO VPCS11V9E Battery} which in turn helped touch off the French Revolution.^[88] The extreme weather produced by El Niño in 1876–77 gave rise to the most deadly famines of the 19th century.^[89] The 1876 famine alone in northern China killed up to 13 million people.^[90] An early recorded mention of the term "El Niño" to refer to climate occurs in 1892, ^{Sony VAIO VPCS11V9E/B Battery} 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. ^{Sony VAIO VPCS11X9E/B Battery} 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".^{Sony VAIO VPCS123FGB Battery} The major 1982–83 El Niño led to an upsurge of interest from the scientific community. The period 1990–1994 was unusual in that El Niños have rarely occurred in such rapid succession.^[91] An especially intense El Niño event in 1998 caused an estimated 16% of the world's reef systems to die. ^{Sony VAIO VPCS125EC Battery} The event temporarily warmed air temperature by 1.5 °C, compared to the usual increase of 0.25 °C associated with El Niño events.^[92] Since then, mass coral bleaching has become common worldwide, with all regions having suffered "severe bleaching".^[93] Major ENSO events were recorded in the years 1790–93, 1828, 1876–78, 1891, ^{Sony VAIO VPCS128EC Battery} 1925–26, 1972–73, 1982–83, 1997–98 and 2009–2010,^[55] with 1997-1998 being one of the strongest ever.The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is an index (which varies over time with no particular periodicity) of the dominant pattern of non-seasonal sea-level pressure variations north of 20N latitude, ^{Sony VAIO VPCS129GC Battery} and it is characterized by pressure anomalies of one sign in the Arctic with the opposite anomalies centered about 37–45N.^[1] The AO is believed by climatologists to be causally related to, and thus partially predictive of, weather patterns in locations many thousands of miles away, including many of the major population centers of Europe and North America. ^{Sony VAIO VPCS12C7E/B Battery} NASA climatologist Dr. James E. Hansen explains the mechanism by which the AO affects weather at points so distant from the Arctic: "The degree to which Arctic air penetrates into middle latitudes is related to the AO index, which is defined by surface atmospheric pressure patterns. ^{Sony VAIO VPCS12L9E/B Battery} When the AO index is positive, surface pressure is low in the polar region. This helps the middle latitude jet stream to blow strongly and consistently from west to east, thus keeping cold Arctic air locked in the polar region. When the AO index is negative, there tends to be high pressure in the polar region, weaker zonal winds, and greater movement of frigid polar air into middle latitudes." ^{Sony VAIO VPCS12V9E/B Battery} This zonally symmetric seesaw between sea level pressures in polar and temperate latitudes was first identified by Edward Lorenz ^[2] and named in 1998 by David W.J. Thompson and John Michael Wallace.^{[3] Sony VAIO VPCY115FGS Battery\} The North Atlantic oscillation (NAO) is a close relative of the AO and there exist arguments about whether one or the other is more fundamentally representative of the atmosphere's dynamics; Ambaum et al. argue that the NAO can be identified in a more physically meaningful way.^{[4] Sony VAIO VPCY115FX/BI Battery} Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s the oscillation has trended to more of a positive phase when averaged using a 60-day running mean, though it has trended to a more neutral state in the last decade. ^{Sony VAIO VPCY115FXBI Battery} The oscillation still fluctuates stochastically between negative and positive values on daily, monthly, seasonal and annual time scales, although, despite its stochastic nature, meteorologists have attained high levels of predictive accuracy in recent times, at least for the shorter term forecasts. ^{Sony VAIO VPCY118EC Battery} (The correlation between actual observations and the 7-day mean GFS ensemble AO forecasts is approximately 0.9, a figure at the high end for that statistic.)^[5] The more positive direction in recent decades has led to lower than normal arctic air pressure and higher than normal temperatures in much of the United States and northern Eurasia.^{[6] Sony VAIO VPCY118GX/BI Battery} The National Snow and Ice Data Center describes the effects of the AO in some detail: "In the positive phase, higher pressure atmidlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska,Scotland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. ^{Sony VAIO VPCY119FJ/S Battery} In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leavesGreenland and Newfoundland colder than usual. ^{Sony VAIO VPCY11AFJ Battery} Weather patterns in the negative phase are in general "opposite" to those of the positive phase."Climatologists are now routinely invoking the Arctic Oscillation in their official public explanations for extremes of weather. The following statement from the National Oceanic and Atmospheric Administration's National Climatic Data Center: ^{Sony VAIO VPCY11AGJ Battery} State of the Climate December 2010 which uses the phrase "negative Arctic Oscillation" four times, is very representative of this increasing tendency:"Cold arctic air gripped western Europe in the first three weeks of December. Two major snowstorms, icy conditions, and frigid temperatures wreaked havoc across much of the region... ^{Sony VAIO VPCY11AHJ Battery} The harsh winter weather was attributed to a negative Arctic Oscillation, which is a climate pattern that influences weather in the Northern Hemisphere. A very persistent, strong ridge of high pressure, or 'blocking system', near Greenland allowed cold Arctic air to slide south into Europe. ^{Sony VAIO VPCY11AVJ Battery} Europe was not the only region in the Northern Hemisphere affected by the Arctic Oscillation. A large snow storm and frigid temperatures affected much of the Midwest United States on December 10–13...."A further, quite graphic illustration of the effects of the negative phase of the oscillation occurred in February 2010. ^{Sony VAIO VPCY11M1E/S Battery} In that month, the Arctic Oscillation reached its most negative monthly mean value, −4.266, in the entire post-1950 era (the period of accurate record-keeping). That month was characterized by three separate historic snowstorms that occurred in the mid-Atlantic region of the United States. ^{Sony VAIO VPCY11S1E Battery} The first storm dumped 25 inches (640 mm) on Baltimore, Maryland, on February 5–6, and then a second storm dumped 19.5 inches (500 mm) on February 9–10. In New York City, a separate storm deposited 20.9 inches (530 mm) on February 25–26. This kind of snowstorm activity is as anomalous and extreme as the negative AO value itself. ^{Sony VAIO VPCY11V9E/S Battery} Similarly, the greatest negative value for the AO since 1950 in January was −3.767 in 1977, which coincided with the coldest mean January temperature in New York City, Washington, D.C., Baltimore, and many other mid-Atlantic locations in that span of time. And though the January AO has been negative only 60.6% of the time between 1950 and 2010, ^{Sony VAIO VPCY218EC/BI Battery} 9 of the 10 coldest Januarys in New York City since 1950 have coincided with negative AOs.^[8] However, the correlation between sharply negative Arctic Oscillations and excessive winter cold and snow in regions vulnerable in that way to these negative AOs should not be overstated. It is by no means a simple, one-to-one equivalence. ^{Sony VAIO VPCY218EC/G Battery} An extreme Arctic Oscillation does not necessarily mean extreme weather will occur. For example, since 1950, eight out of the 10 coldest Januarys in New York did not coincide with the 10 lowest January AO values. And the fourth warmest January there since 1950 coincided with one of those 10 most negative AOs.^{[8] Sony VAIO VPCY218EC/L Battery} So, although many climatologists believe that the Arctic Oscillation affects the probability of certain weather events occurring in certain places, the heightened chance of a phenomenon by no means assures it, nor does the lessened likelihood exclude it. Further, the precise value of the AO index only imperfectly reflects the severity of the weather associated with it. ^{Sony VAIO VPCY218EC/P Battery} The North Atlantic Oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference ofatmospheric pressure at sea level between the Icelandic low and the Azores high. Through east-west oscillation motions of the Icelandic low and the Azores high, ^{Sony VAIO VPCY21S1E/L Battery} it controls the strength and direction of westerly winds and storm tracks across the North Atlantic. It is part of the Arctic oscillation, and varies over time with no particular periodicity. The NAO was discovered in the 1920s by Sir Gilbert Walker. Unlike the El Niño-Southern Oscillation phenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode. ^{Sony VAIO VPCY21S1E/P Battery} 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 the Arctic oscillation (AO) or Northern Annular Mode (NAM), but should not be confused with the Atlantic Multidecadal Oscillation (AMO). ^{Sony VAIO VPCY21S1E/SI Battery} 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:Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland.Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland.Azores (1865–2002), Gibraltar (1821–2007), and Reykjavik, Iceland^[1] Sony VAIO VPCCW1S1E/P Battery These definitions all have in common the same northern point (because this is the only station in 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). Sony VAIO VPCCW1S1E/R Battery A more complex definition, only possible with more complete modern records generated by numerical weather prediction, is based on the principal empirical orthogonal function (EOF) of surface pressure.^[1] This definition has a high degree of correlation with the station-based definition. Sony VAIO VPCCW1S1E/W Battery