Editing Arctic oscillation (section)

== Description ==

[[File:New.ao.loading.gif|thumb|upright=1.3|The loading pattern of the Arctic Oscillation<ref>{{Cite web | url=https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.loading.shtml | title=CPC - Teleconnections: Arctic Oscillation Loading Pattern}}</ref>]]

The Arctic oscillation appears as a ringlike (or "annular") pattern of sea-level pressure anomalies centered at the poles. The presence of continents and large landmasses disrupts the ringlike structure at the Arctic pole, while anomalies surrounding the Antarctic pole are nearly circular.

The Arctic oscillation is believed by climatologists to be causally related to (and thus partially predictive of) [[Climate oscillation|global weather patterns]].  NASA climatologist [[James E. Hansen]] explained the mechanism by which the Arctic oscillation affects weather at points so{{Clarify|reason=phrase introduced by "that" expected, so that the "so ... that ..." would be complete|date=December 2021}} distant from the Arctic, as follows (note, however, that Hansen's explanation is erroneous: pressure in the Arctic is low in the positive AO phase, which configuration also enhances the jet stream): 
{{quote|The degree to which Arctic air penetrates into middle latitudes is related to the AO index, which is defined by surface atmospheric pressure patterns.  When the AO index is positive, surface pressure is high 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 low pressure in the polar region, weaker zonal winds, and greater movement of frigid polar air into middle latitudes."<ref name="Hansen 2009">{{cite journal |last=Hansen |first=James |author2=Reto Ruedy|author3=Makiko Sato|author4=Ken Lo|year=2009|title=If It's That Warm, How Come It's So Damned Cold?|url=http://www.columbia.edu/~jeh1/mailings/2010/20100115_Temperature2009.pdf|access-date=2013-07-17 }}</ref>}}

The Arctic oscillation index is defined using the daily or monthly 1000 hPa [[geopotential height]] anomalies from latitudes 20°&nbsp;N to 90°&nbsp;N. The anomalies are projected onto the Arctic oscillation loading pattern,<ref>{{Cite web | url=https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.loading.shtml | title=CPC - Teleconnections: Arctic Oscillation Loading Pattern}}</ref> which is defined as the first [[Empirical orthogonal functions|empirical orthogonal function]] (EOF) of monthly mean 1000&nbsp;hPa geopotential height during the 1979-2000 period. The time series is then normalized with the monthly mean index's [[standard deviation]].

[[File:Arctic Oscillation.svg|thumb|upright=1.3|Arctic Oscillation time series for the extended, December to March (DJFM), winter season 1899–2011.]]