The North Atlantic Oscillation NAO

The NAO dictates much of the Climate variability from the eastern seaboard of North America to Siberia and from the Arctic to the subtropical Atlantic, especially during the boreal winter (Hurrett et al. 2003).

NAO refers to a redistribution of atmospheric mass between the Arctic and subtropical Atlantic. The NAO is traditionally described by two weather maps showing the distribution of sea level pressure (SLP) over the North Atlantic in two typical "educational" situations: NAO+ and NAO- when the pressure difference between Lisbon and Reykjavik is respectively positive and negative (Figs. 1, 2).

The NAO is understood to swing from one phase to another to produce large changes in the mean wind speed and direction over the Atlantic, the heat and moisture transport between the Atlantic and the neighbouring continents and the intensity and number of storms, their paths, and the associated weather. Such variations have a significant impact on the wind and buoyancy-driven ocean circulation as well as on the site and intensity of water mass transformation (Fig. 3).

Fig. 1. A schematic of the Atlantic-Arctic sector under NAO positive conditions (Stenseth et al. 2004).

In NAO winter situations, enhanced westerly flow across the North Atlantic moves warm and moist maritime air over Europe, northerlies over Greenland and northeastern Canada carry cold air southwards, decreasing SST and land temperatures over the North-West Atlantic, the Labrador Sea ice extends further south

Arctic Oscillation Impact
Fig. 2. A schematic of the Atlantic-Arctic sector under NAO negative conditions (Stenseth et al. 2004).

while the Greenland Sea ice boundary is found to the North of its climatological mean extent. NAO+ winters, associated with chill, dry, northwesterlies across the Labrador Sea are characterized by deep-reaching convective renewal of LSW and widespread distribution of chilled SST across the Northwest Atlantic.

NAO Index (December-March) 1864 -2000

NAO Index (December-March) 1864 -2000

Fig. 3. Winter (December-March) index of the NAO based on the difference of normalized SLP between Lisbon, Portugal, and Stykkisholmur/Reykjavik, Iceland form 1864 through 2000. The heavy solid line represents the index smoothed to remove fluctuations with periods less than 4 years (Stenseth et al. 2004).

Fig. 3. Winter (December-March) index of the NAO based on the difference of normalized SLP between Lisbon, Portugal, and Stykkisholmur/Reykjavik, Iceland form 1864 through 2000. The heavy solid line represents the index smoothed to remove fluctuations with periods less than 4 years (Stenseth et al. 2004).

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