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== North Atlantic Oscillation ==

<div class="definition"><div class="short_definition">The North Atlantic Oscillation (NAO) refers to a redistribution of atmospheric mass between the Arctic and subtropical Atlantic, manifesting as a north–south seesaw in the North Atlantic [[sea level pressure]] and [[geopotential]] height fields. The NAO is one of the dominant patterns of Northern Hemisphere [[climate variability]], with significant impacts across much of North America, North Africa, Eurasia, and the Arctic.


The name “North Atlantic Oscillation” was coined by Sir Gilbert Walker in 1924, and Walker and Bliss constructed the first NAO index in 1932 as a linear combination of wintertime [[pressure]] and [[temperature]] at several stations. 


The positive phase of the NAO is characterized by an anomalously strong Icelandic low and Azores high, which coincides with a strengthened and northerly displaced westerly North Atlantic [[jet stream]]. The positive NAO phase generally is accompanied by above average temperatures throughout northern Eurasia and eastern North America, below average temperatures over northeast Canada and North Africa, enhanced storminess and [[precipitation]] across the North Atlantic and northern Eurasia, and reduced precipitation in southern Europe. Conversely, the negative phase of the NAO is characterized by an anomalously weak Icelandic low and Azores high and a weakened, southerly displaced North Atlantic jet stream, with opposite [[climate]] impacts to that of the positive phase. The NAO has an intrinsic time scale of about 10 days, but substantial NAO variability occurs at longer time scales as well.</div><br/> </div>

References:

Hurrell, J. W., Y. Kushnir, M. Visbeck, and G. Ottersen, 2003: An overview of the North Atlantic Oscillation. ''The North Atlantic Oscillation: Climate Significance and Environmental Impact'', ''Geophys. Monogr''., Vol. 134, Amer. Geophys. Union, 1–35.

Walker, G. T., 1924: Correlation in seasonal variations in weather IX: A further study of world weather. ''Mem. Indian Meteor. Dep.'', '''24''', 275–332.


''Term added 22 October 2014''

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