The NAO is the dominant pattern of wintertime atmospheric circulation variability over the extra-tropical North Atlantic (Hurrell, 1995), and has exhibited decadal variability and trends (see Chapter 2, Section 2.6). There is strong evidence indicating that much atmospheric circulation variability in the form of the NAO arises from internal atmospheric processes (Saravanan, 1998; Osborn et al., 1999). During winters when the stratospheric vortex is stronger than normal, the NAO (and AO) tends to be in a positive phase suggesting an interaction and perhaps even a downward influence from the stratosphere to the troposphere (see Sections 7.2.5 and 7.6.3; Baldwin and Dunkerton, 1999). The recent trend in the NAO/AO could possibly thus be related to processes which are known to affect the strength of the strato-spheric polar vortex such as tropical volcanic eruptions (Kodera, 1994; Kelly et al., 1996), ozone depletion, and changes in greenhouse gas concentrations resulting from anthropogenic forcing (Shindell et al., 1999b).

It has long been recognised that fluctuations in SST are related to the strength of the NAO, and Dickson et al. (1996, 2000) have shown a link to the ocean gyre and thermohaline circulations. The leading mode of SST variability over the North Atlantic during winter is associated with the NAO. During high NAO years anomalous SSTs form a tri-polar pattern with a cold anomaly in the sub-polar region, a warm anomaly in the middle latitudes, and a cold sub-tropical anomaly (e.g., Deser and Blackmon, 1993), consistent with the spatial form of the anomalous surface fluxes associated with the NAO pattern (Cayan, 1992). This indicates that SST is responding to atmospheric forcing on seasonal time-scales (Deser and Timlin, 1997). However, GCM simulations suggest that SST in the North Atlantic can, in turn, have a marked effect on NAO (see Section 7.6.2). Winter SST anomalies were observed to spread eastward along the path of the Gulf Stream and North Atlantic Current with a transit time-scale of a decade (Sutton and Allen, 1997). These SST anomalies reflect anomalies in the heat content of the deep winter mixed layers that when exposed to the atmosphere in winter (Alexander and Deser, 1995) could affect the NAO, imprinting the advective time-scale of the gyre on the atmosphere (McCartney et al., 1996). Moreover, similar processes were identified in coupled GCM integrations (Groetzner et al., 1998; Timmermann et al., 1998; Delworth and Mann, 2000) where changes in SST due to oceanic processes (gyre advection or thermohaline circulation) affected the NAO. This, in turn, leads to changes in heat and fresh water fluxes, and in wind stress forcing of the oceanic circulation. Oceanic response to such changes in the forcing produced a negative feedback loop, leading to decadal oscillations. However, the role of these mechanisms is yet to be established.

Watanabe and Nitta (1999) have suggested that high latitude snow cover on land is responsible for decadal changes in the NAO. Changes in sea-ice cover in both the Labrador and Greenland Seas as well as over the Arctic also appear to be well correlated with the NAO (Deser et al., 2000). Such changes may also affect the atmosphere because of the large changes in sensible and latent heat fluxes along the ice edge.

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