The North Atlantic Oscillation: Past, present, and future

Abstract

The climate of the Atlantic sector exhibits considerable variability on a wide range of time scales. A substantial portion is associated with the North Atlantic Oscillation (NAO), a hemispheric meridional oscillation in atmospheric mass with centers of action near Iceland and over the subtropical Atlantic. NAO-related impacts on winter climate extend from Florida to Greenland and from northwestern Africa over Europe far into northern Asia. Over the last 3 decades, the phase of the NAO has been shifting from mostly negative to mostly positive index values. Much remains to be learned about the mechanisms that produce such low frequency changes in the North Atlantic climate, but it seems increasingly likely that human activities are playing a significant role. W hen the North Atlantic Oscillation (NAO) is in its positive phase, low-pressure anomalies over the Icelandic region and throughout the Arctic combine with high-pressure anomalies across the subtropical Atlantic to produce stronger-thanaverage westerlies across the midlatitudes. During a positive NAO, conditions are colder and drier than average over the northwestern Atlantic and Mediterranean regions, whereas conditions are warmer and wetter than average in northern Europe, the eastern United States, and parts of Scandinavia (Fig. 1 top). Walker and Bliss (1) were among the first to recognize and study this pattern of climate anomalies, which is most pronounced during boreal winter (December through March).A remarkable feature of the NAO is its trend toward a more positive phase over the past 30 years, with a magnitude that seems to be unprecedented in the observational record (2). Some of the most pronounced anomalies have occurred since the winter of 1989, when record positive values of the NAO index have been documented (Fig. 1 Lower). Moreover, the trend in the NAO accounts for a myriad of remarkable changes in the climate over the middle and high latitudes of the Northern Hemisphere, as well as in marine and terrestrial ecosystems. Among these changes are:Milder winters in Europe downstream and across Asia, juxtaposed against more severe winters over eastern Canada and the northwest Atlantic (2); Pronounced regional changes in precipitation patterns, resulting in the advance of some northern European glaciers and the retreat of Alpine glaciers (3, 4); Changes in sea-ice cover in both the Labrador and Greenland Seas as well as over the Arctic (5); Pronounced decreases in mean sea level pressure (SLP) over the Arctic and changes in the physical properties of Arctic sea water (6, 7); Changes in the intensity of convection in the Labrador and the Greenland-Iceland Seas, which in turn influence the strength and character of the Atlantic meridional overturning circulation (8);Stratospheric cooling over the polar cap and total column ozone losses poleward of 40°N (9); Changes in the production of zooplankton and the distribution of fish (e.g., ref. 10), and changes in the length of the growing season over Europe (11).All of these changes seem to be strongly r...

A seasonally lagged signal of the North Atlantic Oscillation (NAO) in the North Pacific

“…The North Atlantic Oscillation (NAO), a regional seesaw in the atmospheric mass between Iceland and the Azores, is an important mode characterizing tropospheric climate variability from interannual to multidecadal timescales especially during the boreal winter (Visbeck et al, 2001;Wanner et al, 2001). Accordingly, the NAO has an important influence on the wintertime climate of the Northern Hemisphere from eastern North America to Siberia (Hurrell and VanLoon, 1997;Wanner et al, 2001).…”

“…The resulting basin-scale cyclonic circulation brings warm air over much of northern Europe resulting in warmer surface temperatures, while the same circulation advects cold Arctic air over northeastern Canada and the adjoining ocean leading to colder land and ocean surface temperatures (van Loon and Rogers, 1978;Wanner et al, 2001). Interest in the NAO has heightened recently as a result of its tendency to remain in its positive phase over the latter half of the twentieth century (Hurrell, 1996;Visbeck et al, 2001). Given its strong surface temperature expression, this trend can explain 31% of the winter interannual variability in northern extratropical temperatures over this period (Hurrell, 1996).…”

Non‐negative Matrix Factorization for the identification of patterns of atmospheric pressure and geopotential for the Northern Hemisphere

“…The sixth identified pattern ( Figure 7) displays one of the most prominent teleconnection patterns, the NAO, in fact, the negative phase of this low-frequency mode (Visbeck et al, 2001). It shows a north-south dipole pattern with high pressure over Greenland/Iceland and low pressure over the central Atlantic Ocean.…”

“…In summer, both areas have high pressure connected with a northwards movement of the low-pressure Intertropical Convergence Zone (ITCZ) in Asia. The second REOF, which displays parts of the North Atlantic Oscillation (NAO) (Visbeck et al, 2001), does not show equally exact annual fluctuations and also consists of shorter as well as longer variations (not plotted). Hannachi et al (2007) mention three main patterns that were uncovered, namely the North Atlantic Oscillation Pattern, the Pacific/North American Pattern (PNA) and the Scandinavian Pattern (SCAND, previously Eurasia-1) referring to the classification of low-frequency atmospheric circulation patterns by Barnston and Livezey (1987).…”

“…The NAO's positive phase (Visbeck et al, 2001), which is characterized by low pressure over Greenland/Iceland and high pressure over the subtropical Atlantic (known as Icelandic Low and Azores High), seems to be split up to NMF-2 and NMF-7 (Figure 9). …”