The NAO was first described in the diaries of the missionary Hans Egede Saabye as a see-saw in temperature between Greenland and Denmark (Saabye, 1942). The NAO was later defined by Sir Gilbert Walker as "the tendency for pressure to be low near Iceland in winter when it is high near the Azores and south-west Europe" (Walker, 1924;Walker and Bliss, 1932). Accounting for more than 1/3 of the total variance of the sea level pressure (SLP) field over the North Atlantic, the NAO is most pronounced during the winter months (December through March)due to an increased sea air temperature contrast (Barnston and Livezey, 1987).