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==== El Niño - Southern Oscillation ====
Con il nome di "''[[El Niño Southern Oscillation]]''" (o più brevemente "''ENSO''") si denomina un importante fenomeno sia oceanico che atmosferico. El Niño (e il fenomeno opposto, La Niña) sono fluttuazioni della temperatura delle acque superficiali dell'[[Oceano Pacifico|Oceano Pacifico Orientale]], tra le più importanti e influenti teleconnessioni oceaniche-atmosferiche.
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The name El Niño, from the [[Spanish language|Spanish]] for "the little boy", refers to the [[Christ child]], because the phenomenon is usually noticed around [[Christmas]] time in the Pacific Ocean off the west coast of [[South America]].<ref>California Department of Fish and Game, Marine Region. [http://www.dfg.ca.gov/mrd/elnino.html El Niño Information.] Retrieved on [[2007-06-07]].</ref> La Niña means "the little girl".<ref>[http://library.advanced.org/20901/la_nina.htm La Niña.]</ref> Their effect on climate in the subtropics and the tropics are profound. The atmospheric signature, the Southern Oscillation (SO) reflects the monthly or seasonal fluctuations in the air pressure difference between [[Tahiti]] and [[Darwin, Northern Territory|Darwin]]. The most recent occurrence of El Niño started in [[September 2006]]<ref>[http://edition.cnn.com/2006/WEATHER/09/13/weather.nino.reut/index.html El Nino forms in Pacific Ocean], [[CNN]]</ref> and lasted until early 2007.<ref>{{cite web |url=http://www.cbsnews.com/stories/2007/02/28/tech/main2523483.shtml |title=There Goes El Nino, Here Comes La Nina |publisher=The Associated Press / CBS News |date=2007-02-28 |accessdate=2007-03-02}}</ref>
ENSO is a set of interacting parts of a single global system of coupled ocean-atmosphere climate fluctuations that come about as a consequence of oceanic and [[atmospheric circulation]]. ENSO is the most prominent known source of inter-annual variability in weather and climate around the world (~3 to 8 years), though not all areas are affected. ENSO has signatures in the Pacific, Atlantic and Indian Oceans. El Niño causes weather patterns which causes it to rain in specific places but not in others, this is one of many causes for the drought.
In the Pacific, during major warm events, El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SO intensity. While ENSO events are basically in phase between the Pacific and Indian Oceans, ENSO events in the Atlantic Ocean lag behind those in the Pacific by 12 to 18 months. Many of the countries most affected by ENSO events are developing countries within main continents (South America, Africa...), with economies that are largely dependent upon their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events in the three oceans can have global socio-economic impacts. While ENSO is a global and natural part of the Earth's climate, whether its intensity or frequency may change as a result of global warming is an important concern. Low-frequency variability has been evidenced: the quasi-decadal oscillation (QDO). Inter-decadal (ID) modulation of ENSO (from PDO or IPO) might exist. This could explain the so-called protracted ENSO of the early 90s.
====Madden-Julian Oscillation====
[[File:MJO 5-day running mean through 1 Oct 2006.png|thumb|120px|Note how the MJO moves eastward with time.]]
The Madden-Julian Oscillation (MJO) is an equatorial traveling pattern of anomalous rainfall that is planetary in scale. It is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the [[Indian Ocean]] and [[Pacific Ocean]]. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall then generally becomes very nondescript as it moves over the cooler ocean waters of the eastern Pacific but reappears over the tropical [[Atlantic Ocean|Atlantic]] and Indian Ocean. The wet phase of enhanced convection and precipitation is followed by a dry phase where convection is suppressed. Each cycle lasts approximately 30-60 days. The MJO is also known as the 30-60 day oscillation, 30-60 day wave, or intraseasonal oscillation.
====North Atlantic Oscillation (NAO)====
Indices of the NAO are based on the difference of normalized sea level pressure (SLP) between Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland. The SLP anomalies at each station were normalized by division of each seasonal mean pressure by the long-term mean (1865-1984) standard deviation. Normalization is done to avoid the series of being dominated by the greater variability of the northern of the two stations. Positive values of the index indicate stronger-than-average westerlies over the middle latitudes.<ref name="CLIINDEX">National Center for Atmospheric Research. [http://www.cgd.ucar.edu/cas/jhurrell/indices.info.html Climate Analysis Section.] Retrieved on [[2007-06-07]].</ref>
====Northern Annualar Mode (NAM) or Arctic Oscillation (AO)====
The NAM, or AO, is defined as the first EOF of northern hemisphere winter SLP data from the tropics and subtropics. It explains 23% of the average winter (December-March) variance, and it is dominated by the NAO structure in the Atlantic. Although there are some subtle differences from the regional pattern over the Atlantic and Arctic, the main difference is larger amplitude anomalies over the North Pacific of the same sign as those over the Atlantic. This feature gives the NAM a more annular (or zonally-symmetric) structure. <ref name="CLIINDEX">National Center for Atmospheric Research. [http://www.cgd.ucar.edu/cas/jhurrell/indices.info.html Climate Analysis Section.] Retrieved on [[2007-06-07]].</ref>
====Northern Pacific (NP) Index====
The NP Index is the area-weighted sea level pressure over the region 30N-65N, 160E-140W.<ref name="CLIINDEX">National Center for Atmospheric Research. [http://www.cgd.ucar.edu/cas/jhurrell/indices.info.html Climate Analysis Section.] Retrieved on [[2007-06-07]].</ref>
====Pacific Decadal Oscillation (PDO)====
The PDO is a pattern of [[Pacific]] [[climate variability]] that shifts phases on at least inter-decadal time scale, usually about 20 to 30 years. The PDO is detected as warm or cool surface waters in the [[Pacific Ocean]], north of 20° N. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs. The mechanism by which the pattern lasts over several years has not been identified; one suggestion is that a thin layer of warm water during summer may shield deeper cold waters. A PDO signal has been reconstructed to [[1661]] through tree-ring chronologies in the [[Baja California]] area.
====Interdecadal Pacific Oscillation (IPO)====
The Interdecadal Pacific Oscillation (IPO or ID) display similar sea surface temperature (SST) and sea level pressure patterns to the PDO, with a cycle of 15–30 years, but affects both the north and south Pacific. In the tropical Pacific, maximum SST anomalies are found away from the equator. This is quite different to the quasi-decadal oscillation (QDO) with a period of 8-to-12 years and maximum SST anomalies straddling the equator, thus resembling ENSO.
==Climate models==
{{Main article|Climate models}}
Climate models use quantitative methods to simulate the interactions of the [[Earth's atmosphere|atmosphere]], [[ocean]]s, land surface, and ice. They are used for a variety of purposes from study of the dynamics of the weather and climate system to projections of future climate. All climate models balance, or very nearly balance, incoming energy as short wave (including visible) electromagnetic radiation to the earth with outgoing energy as long wave (infrared) electromagnetic radiation from the earth. Any unbalance results in a change in the average temperature of the earth.
The most talked-about models of recent years have been those relating temperature to emissions of [[carbon dioxide]] (see [[greenhouse gas]]). These models predict an upward trend in the [[surface temperature record]], as well as a more rapid increase in temperature at higher altitudes.
Models can range from relatively simple to quite complex:
* A simple radiant heat transfer model that treats the earth as a single point and averages outgoing energy
* this can be expanded vertically (radiative-convective models), or horizontally
* finally, (coupled) atmosphere–ocean–[[sea ice]] '''[[global climate model]]s''' discretise and solve the full equations for mass and energy transfer and radiant exchange.
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=== Modelli climatici ===
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