3.6.4 The North Atlantic Oscillation and Northern Annular Mode

The only teleconnection pattern prominent throughout the year in the NH is the NAO (Barnston and Livezey, 1987). It is primarily a north-south dipole in sea level pressure characterised by simultaneous out-of-phase pressure and height anomalies between temperate and high latitudes over the Atlantic sector, and therefore corresponds to changes in the westerlies across the North Atlantic into Europe (Figure 3.30). The NAO has the strongest signature in the winter months (December to March) when its positive (negative) phase exhibits an enhanced (diminished) Iceland Low and Azores High (Hurrell et al., 2003). The NAO is the dominant pattern of near-surface atmospheric circulation variability over the North Atlantic, accounting for one third of the total variance in monthly MSLP in winter. It is closely related to the NAM, which has similar structure over the Atlantic but is more zonally symmetric. The leading winter pattern of variability in the lower stratosphere is also annular, but the MSLP anomaly pattern that is associated with it is confined almost entirely to the Arctic and Atlantic sectors and coincides with the spatial structure of the NAO (Deser, 2000; see also Section 3.5.5 and Box 3.3).

Figure 3.30. Changes in winter (December–March) surface pressure, temperature, and precipitation corresponding to a unit deviation of the NAO index over 1900 to 2005. (Top left) Mean sea level pressure (0.1 hPa). Values greater than 0.5 hPa are stippled and values less than −0.5 hPa are hatched. (Top right) Land-surface air and sea surface temperatures (0.1°C; contour increment 0.2°C). Temperature changes greater than 0.1°C are indicated by stippling, those less than –0.1°C are indicated by hatching, and regions of insufficient data (e.g., over much of the Arctic) are not contoured. (Bottom left) Precipitation for 1979 to 2003 based on GPCP (0.1 mm per day; contour interval 0.6 mm per day). Stippling indicates values greater than 0.3 mm per day and hatching values less than –0.3 mm per day. Adapted and updated from Hurrell et al. (2003).

There is considerable debate over whether the NAO or the NAM is more physically relevant to the winter circulation (Deser, 2000; Ambaum et al., 2001, 2002), but the time series are highly correlated in winter (Figure 3.31). As Quadrelli and Wallace (2004) showed, they are near neighbours in terms of their spatial patterns and their temporal evolution. The annular modes are intimately linked to the configuration of the extratropical storm tracks and jet streams. Changes in the phase of the annular modes appear to occur as a result of interactions between the eddies and the mean flow, and external forcing is not required to sustain them (De Weaver and Nigam, 2000). In the NH, stationary waves provide most of the eddy momentum fluxes, although transient eddies are also important. To the extent that the intrinsic excitation of the NAO/NAM pattern is limited to a period less than a few days (Feldstein, 2002), it should not exhibit year-to-year autocorrelation in conditions of constant forcing. Proxy and instrumental data, however, show evidence for intervals with prolonged positive and negative NAO index values in the last few centuries (Cook et al., 2002; Jones et al., 2003). In winter, a reversal occurred from the minimum index values in the late 1960s to strongly positive NAO index values in the mid-1990s. Since then, NAO values have declined to near the long-term mean (Figure 3.31). In summer, Hurrell et al. (2001, 2002) identified significant interannual to multi-decadal fluctuations in the NAO pattern, and the trend towards persistent anticyclonic flow over northern Europe has contributed to anomalously warm and dry conditions in recent decades (Rodwell, 2003).

Figure 3.31. Normalised indices (units of standard deviation) of the mean winter (December–March) NAO developed from sea level pressure data. In the top panel, the index is based on the difference of normalised sea level pressure between Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland from 1864 to 2005. The average winter sea level pressure data at each station were normalised by dividing each seasonal pressure anomaly by the long-term (1864 to 1983) standard deviation. In the middle panel, the index is the principal component time series of the leading EOF of Atlantic-sector sea level pressure. In the lower panel, the index is the principal component time series of the leading EOF of NH sea level pressure. The smooth black curves show decadal variations (see Appendix 3.A). The individual bar corresponds to the January of the winter season (e.g., 1990 is the winter of 1989/1990). Updated from Hurrell et al. (2003); see http://www.cgd.ucar.edu/cas/jhurrell/indices.html for updated time series.

Feldstein (2002) suggested that the trend and increase in the variance of the NAO/NAM index from 1968 through 1997 was greater than would be expected from internal variability alone, while NAO behaviour during the first 60 years of the 20th century was consistent with atmospheric internal variability. However, the results are not so clear if based on just the period 1975 to 2004 (Overland and Wang, 2005). Although monthly-scale NAO variability is strong (Czaja et al., 2003; Thompson et al., 2003), there may be predictability from stratospheric influences (Thompson et al., 2002; Scaife et al., 2005; see Box 3.3). There is mounting evidence that the recent observed inter-decadal NAO variability comes from tropical and extratropical ocean influences (Hurrell et al., 2003, 2004), land surface forcing (Gong et al., 2003; Bojariu and Gimeno, 2003) and from other external factors (Gillett et al., 2003).

The NAO exerts a dominant influence on winter surface temperatures across much of the NH (Figure 3.30), and on storminess and precipitation over Europe and North Africa. When the NAO index is positive, enhanced westerly flow across the North Atlantic in winter moves warm moist maritime air over much of Europe and far downstream, with dry conditions over southern Europe and northern Africa and wet conditions in northern Europe, while stronger northerly winds over Greenland and northeastern Canada carry cold air southward and decrease land temperatures and SST over the northwest Atlantic. Temperature variations over North Africa and the Middle East (cooling) and the southeastern USA (warming), associated with the stronger clockwise flow around the subtropical Atlantic high-pressure centre, are also notable. Following on from Hurrell (1996), Thompson et al. (2000) showed that for JFM from 1968 to 1997, the NAM accounted for 1.6°C of the 3.0°C warming in Eurasian surface temperatures, 4.9 hPa of the 5.7 hPa decrease in sea level pressure from 60°N to 90°N; 37% out of the 45% increase in Norwegian-area precipitation (55°N–65°N, 5°E–10°E), and 33% out of the 49% decrease in Spanish-region rainfall (35°N–45°N, 10°W–0°W). There were also significant effects on ocean heat content, sea ice, ocean currents and ocean heat transport.

Positive NAO index winters are associated with a northeastward shift in the Atlantic storm activity, with enhanced activity from Newfoundland into northern Europe and a modest decrease to the south (Hurrell and van Loon, 1997; Alexandersson et al., 1998). Positive NAO index winters are also typified by more intense and frequent storms in the vicinity of Iceland and the Norwegian Sea (Serreze et al., 1997; Deser et al., 2000). The correlation between the NAO index and cyclone activity is highly negative in eastern Canada and positive in western Canada (Wang et al., 2006b). The upward trend towards more positive NAO index winters from the mid-1960s to the mid-1990s has been associated with increased wave heights over the northeast Atlantic and decreased wave heights south of 40°N (Carter, 1999; Wang and Swail, 2001; see also Section 3.5.6).

The NAO/NAM modulates the transport and convergence of atmospheric moisture and the distribution of evaporation and precipitation (Dickson et al., 2000). Evaporation exceeds precipitation over much of Greenland and the Canadian Arctic and more precipitation than normal falls from Iceland through Scandinavia during winters with a high NAO index, while the reverse occurs over much of central and southern Europe, the Mediterranean and parts of the Middle East (Dickson et al., 2000). Severe drought has persisted throughout parts of Spain and Portugal as well (Hurrell et al., 2003). As far eastward as Turkey, river runoff is significantly correlated with NAO variability (Cullen and deMenocal, 2000). There are many NAO-related effects on ocean circulation, such as the freshwater balance of the Atlantic Ocean (see Chapter 5), on the cryosphere (see Chapter 4), and on many aspects of the north Atlantic/European biosphere (see the Working Group II contribution to the IPCC Fourth Assessment Report).