The dominant mode of climate variability in the North Atlantic basin is the North Atlantic Oscillation (NAO). This meridional oscillation ofatmospheric pressure and winds is considered less pervasive than ENSO, but the variation of the westerly winds over the North Atlantic Ocean bears some similarity to the ENSO phenomenon in the equatorial Pacific Ocean. NAO is recognized as a factor influencing weather patterns throughout the Northern Hemisphere (Hurrell and van Loon, 1997), and it may be a more influential weather factor than ENSO in regions surrounding the North Atlantic Ocean.
NAO is characterized by atmospheric pressure variations related to north-south atmospheric mass oscillations. These oscillations are due to changing Arctic air masses near Iceland and subtropical air masses over the Atlantic Ocean from the Azores to the Iberian Peninsula. The NAO signature is strongly regional and is represented by an index defined as the difference between the normalized mean December to March sea-level pressure anomalies at Lisbon, Portugal, and Stykkisholmur, Iceland (Hurrell, 1995). The atmospheric pressure and wind variations associated with NAO alter heat and moisture transport between the Atlantic Ocean and surrounding continents by influencing the number and path ofwinter storms (Hurrell et al., 2001; Trigo et al., 2004). A strong Icelandic low and Azores high produce a strong south to north pressure gradient, strong westerly winds, and stronger winter storms following more northerly tracks. This positive phase of the NAO delivers warm, moist air and milder maritime winters over the European continent and above average temperatures in the eastern United States. The negative NAO phase is associated with a weak pressure gradient, weaker westerlies, fewer and weaker winter storms, and colder than normal winter temperatures in Europe and the United States.
The NAO is most pronounced during the winter when it accounts for more than one-third of the total variance of the sea-level pressure field over the North Atlantic Ocean (Hurrell and van Loon, 1997). However, both NAO phases are associated with changes in the location and intensity of the North Atlantic jet stream and storm tracks. The NAO varies at time scales of days to centuries without a clear cyclical pattern, but it exhibits a tendency to remain in one
1899 1915 1931 1947 1963 1979 1995 Year
Fig. 8.7. The North Atlantic Oscillation (NAO) index for 1899-2005 expressed as normalized December to March principal component values. (Data courtesy of NOAA and the Climate Prediction Center from their website at http://www.cpc.ncep.noaa. gov/products/precip/CWlink/pna/nao.shtml.)
phase for intervals lasting less than 15 years (Hurrell, 1995; Appenzeller et al.,
1998). The NAO was in a generally positive phase from about 1900 to 1930 (Fig. 8.7) resulting in warmer winter temperatures across much of Europe. A negative NAO phase from the early 1940s to the early 1970s was associated with markedly cold winters in Europe. Over the last 30 years of the twentieth century, the NAO abruptly changed to a highly positive phase containing the highest positive values since 1864 (Hurrell and van Loon, 1997; Hurrell et al., 2001). During this recent period, warmer winters returned to Europe accompanied by anomalously dry conditions over southern Europe and the Mediterranean and wetter than normal winters over northern Europe and parts of Scandinavia (Dai et al., 1997; Knippertz et al., 2003). However, correlations between NAO and precipitation are strongest in western and central Europe at locations near the coast and decrease inland (Trigo et al., 2004; Bouwer et al., 2006).
The NAO is most directly associated with changes in the surface westerlies across the Atlantic Ocean into Europe, but NAO plays a role in the Northern Hemisphere planetary wave system structure and is a factor in defining global atmospheric circulation. The planetary waves are important in defining temperature patterns, storm tracks, and time and space characteristics of precipitation. Therefore, precipitation patterns and winter temperatures in North America and North Africa, in addition to Europe, are attributed to NAO phases (Uppenbrink,
1999), and the NAO has been related to monsoon rainfall in India. A significant part of the winter variability in spatially averaged Northern Hemisphere mid-latitude precipitation is accounted for by NAO (New et al., 2001). Seasonal NAO variability is evident in streamflow variability in the eastern United States, Europe, and tropical South America and Africa (Dettinger and Diaz, 2000), and Peterson et al. (2002) report a positive correlation between the NAO and the discharge of northern Eurasian rivers. Although Bouwer et al. (2006) found a weak relationship between NAO and December-February river discharge in northwestern Europe, Hannaford and Marsh (2006) identified a strong relationship between annual runoff and NAO since the early 1960s in Scotland and maritime areas of western England and Wales. Trigo et al. (2004) found January-March streamflow in the Iberian Peninsula highly correlated with the December-February NAO index.
The process or processes responsible for the low-frequency variations of the NAO and its unprecedented trend over the past 30 years are poorly understood, but a complex ocean-atmosphere link involving ENSO is likely (Hoerling et al., 2001; Hurrell et al., 2001; Pozo-Vazquez et al., 2001). Identification of processes is hindered by the superimposition of decadal NAO variability on interannual variability. However, it is well established that much of the NAO atmospheric variability arises from processes internal to the atmosphere (Hoerling et al., 2001). The difficulty in defining the complex processes related to NAO is emphasized by emerging evidence that it might be a seesaw of atmospheric mass between the polar cap and the middle latitudes in both the Atlantic and Pacific ocean basins. This hemispheric high-latitude circulation with links to the stratosphere is called the Arctic Oscillation (Ambaum et al., 2001; Higgins et al., 2002), and Wang and Schimel (2003) suggest the NAO and the Arctic Oscillation may represent two paradigms of the same phenomenon.
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