| Summary: | The climate over the North Atlantic region exhibits considerable variability on a wide range of timescales, manifested as coherent fluctuations in ocean and land temperature, rainfall and surface pressure (Hurrell et al. 2002). Without doubt, the North Atlantic Oscillation (NAO) is the most widely known example of such variability. The NAO is a major source of interannual variability in the atmospheric circulation, and is associated with changes in the westerlies across the North Atlantic Sector (Hurrell 1995). The NAO can be described as an oscillation of atmospheric mass between the Arctic and the subtropical parts of the Atlantic, usually defined through changes in surface pressure. These oscillations produce changes in wind speed (and direction) over the North Atlantic, which affects heat and moisture transport over land adjacent to the ocean, as well as frequency and intensity of storms (Hurrell et al. 2003). Furthermore, the NAO strongly affects the heat and freshwater exchange at the ocean surface of the Atlantic Ocean itself by inducing changes in surface wind patterns (Hurrell et al. 2001). Such changes affect the strength and character of the overturning in the North Atlantic and could thus have influence on the thermohaline circulation (Delworth & Dixon 2000). Perhaps the main reason for the recent focus on the NAO is the strong positive trend in the index of the winter NAO from the 1980s and onwards. Hurrell & van Loon (1997) noted that the recent cooling over the northwest Atlantic and the warming across Europe since the early 1980s was related to the decadal variability of the NAO. Furthermore, they showed that since the early 1980s, circulation changes over the Atlantic was linked to coherent large-scale anomalies in precipitation (e.g. dry conditions over southern Europe and the Mediterranean and wetter-than-normal conditions over northern Europe and parts of Scandinavia). Also, Hurrell et al. (2001) suggested a link between the Northern Hemisphere warming and the positive trend in the ...
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