Most relevant to the present study is the effect of NAO-related changes in mean atmospheric circulation on the strength and direction of precipitation-laden mid-latitude cyclones. The NAO is generally conceptualized as existing in one of two extreme states, positive or negative, a positive (negative) NAO refers to an intensified (weakened) poleward pressure gradient resulting from a synchronous strengthening (weakening) of the Azores High (AH) and deepening (shallowing) of the Icelandic Low (IL) on the order of 15 mb (Hurrell, 1995) . Resultant changes in mean circulation show meridionally (zonally) oriented westerlies moving onto Europe, increased (decreased) moisture transport, and a winter of higher (lower) than average precipitation in northern Europe and Scandinavia, and lower (higher) than average precipitation in western Greenland, southern Europe, and the Mediterranean.

Because the signature of the NAO is strongly regional, a simple NAO index was defined as the difference between the normalized mean winter sea-level pressure (SLP) anomalies at locations representative of the relative strengths of the AH and IL. The first NAO index was defined by Walker and Bliss (1932) and simplified by Rogers (1984), who constructed an NAO index starting in 1894, using SLP anomalies from Ponta Delgados, Azores and Akuyreyri, Iceland. Hurrell (1995) selected Lisbon, Portugal and Stykkisholmur, Iceland in order to extend the record another 30 years (Fig. 1a). Investigation of the power spectrum of the NAO index for the 130 winters (1864-1995) reveals dominant periods of NAO-related North Atlantic sector variability, centered on 6-10 years and 2-3 years (Hurrell, 1996a) . Also significant is the red portion of the NAO power spectrum, which suggests NAO-related interdecadal-centennial scale variability may play a principle role in Holocene climate variability. The tree-ring based NAO index, extending from 1701-1980, confirms this low frequency power and suggests it is on the order of 70 years (Cook et al., in press) .