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North Atlantic oscillation

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   The North Atlantic oscillation (NAO) is a climatic phenomenon in the
   North Atlantic Ocean of fluctuations in the difference of sea-level
   pressure between the Icelandic Low and the Azores high. Through
   east-west rocking motions of the Icelandic Low and the Azores high, it
   controls the strength and direction of westerly winds and storm tracks
   across the North Atlantic. It is related to and highly correlated with
   the Arctic oscillation.

   The NAO was discovered in the 1920s by Sir Gilbert Walker. Similar to
   the El Niño phenomenon in the Pacific Ocean, the NAO is one of the most
   important drivers of climate fluctuations in the North Atlantic and
   surrounding continents.

Description

   Yearly NAO indexes
   Yearly NAO indexes

   Westerly winds blowing across 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 heatwaves,
   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 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 façade. In contrast, if the
   index is low (NAO-), westerlies are suppressed, these areas suffer cold
   winters and storms track southerly 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 the intensity,
   number and track of storms.

   Although having a less direct influence than for Western Europe, the
   NAO is also believed to have an impact on the weather over much of in
   eastern North America. During the winter, when the index is high
   (NAO+), the Icelandic low draws a stronger southwesterly circulation
   over the eastern half of the North American continent which prevents
   Arctic air from plunging southward. In combination with the El Niño,
   this effect can produce significantly warmer winters over much of the
   United States and southern Canada.

Related material

     * Anticyclone
     * El Niño / Southern Oscillation Index
     * Arctic oscillation
     * Global warming
     * Pacific decadal oscillation
     * North Atlantic Current

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