6.6.5.2 The Record of Past Atlantic Variability

Climate variations over the North Atlantic are related to changes in the NAO (Hurrell, 1995) and the Atlantic Multidecadal Oscillation (Delworth and Mann, 2000; Sutton and Hodson, 2005). From 1980 to 1995, the NAO tended to remain in one extreme phase and accounted for a substantial part of the winter warming over Europe and northern Eurasia. The North Atlantic region has a unique combination of long instrumental observations, many documentary records and multiple sources of proxy records. However, it remains difficult to document past variations in the dominant modes of climate variability in the region, including the NAO, due to problems of establishing proxies for atmospheric pressure, as well as the lack of stationarity in the NAO frequency and in storm tracks. Several reconstructions of NAO have been proposed (Cook et al., 2002b; Cullen et al., 2002; Luterbacher et al., 2002). Although the reconstructions differ in many aspects, there is a general tendency for more negative NAO indices during the 17th and 18th centuries than in the 20th century, thus indicating that the colder mean climate was characterised by a more zonal atmospheric pattern than in the 20th century. The coldest reconstructed European winter in 1708/1709, and the strong warming trend between 1684 and 1738 (+0.32°C per decade), have been related to a negative NAO index and the NAO response to increasing radiative forcing, respectively (Luterbacher et al., 2004). Some spatially resolved simulations employing GCMs indicate that solar and volcanic forcings lead to continental warming associated with a shift towards a high NAO index (Shindell et al., 2001, 2003, 2004; Stendel et al., 2006). Increased solar irradiance at the end of the 17th century and through the first half of the 18th century might have induced such a shift towards a high NAO index (Shindell et al., 2001; Luterbacher et al., 2004; Xoplanki et al., 2005).

It is well known that the NAO exerts a dominant influence on winter temperature and precipitation over Europe, but the strength of the relationship can change over time and region (Jones et al., 2003). The strong trend towards a more positive NAO index in the early part of the 18th century in the Luterbacher et al. (2002) NAO reconstruction appears connected with positive winter precipitation anomalies over northwest Europe and marked expansions of maritime glaciers in a manner similar to the effect of positive winter precipitation anomalies over recent decades for the same glaciers (Nesje and Dahl, 2003; Pauling et al., 2006).