North Atlantic Oscillation (NAO)

The North Atlantic Oscillation (NAO) is the most important mode of atmospheric variability over the North Atlantic Ocean, first identified by (Walker, 1924 and Walker and Bliss, 1932). The NAO is a measure of the strength of the westerly winds blowing across the North Atlantic Ocean within the 40ºN–60ºN latitude band. It has effects on the climate of North America, the North Atlantic, and the European continent (Loon and Rogers, 1978 and Wallace and Gutzler, 1981).

The NAO is based on the sea-level pressure difference between the Subtropical High (Azores) and the Subpolar Low. The positive phase reflects below-normal pressure over the high latitudes of the North Atlantic and above-normal pressure over the central North Atlantic, the eastern United States, and Western Europe. The negative phase reflects the opposite pattern of pressure anomalies over these regions. Both phases of the NAO are associated with changes in the intensity and location of the jet stream and storm frequency.

It can occur annually or persist over decades. It is termed an “oscillation” because the changes in atmospheric pressure essentially alternate between two predominant patterns or modes: a positive mode and a negative mode. Thus, these modes or phases are defined by above-normal air pressure in one of these regions and below-normal air pressure in the other.

Positive phase: During positive NAO phases, the increased pressure difference between the two regions results in a stronger jet stream over the Atlantic and a northward shift in the storm track. Consequently, northern Europe experiences increased storminess and precipitation, along with higher-than-average temperatures associated with air masses arriving from lower latitudes. At the same time, southern Europe experiences reduced storminess and below-average precipitation. In eastern North America, the positive NAO phase generally increases air pressure, a condition associated with fewer cold-air outbreaks and reduced storminess.

Negative phase: The NAO is in a negative phase when both the subpolar low and the subtropical high are weaker than average. During negative NAO phases, the Atlantic jet stream and storm track have a more west–east orientation, which leads to reduced storminess, below-average precipitation, and below-average temperatures in northern Europe. Conversely, southern Europe experiences increased storminess, above-average precipitation, and warmer-than-average temperatures. In eastern North America, the negative NAO phase generally brings lower air pressure, a condition associated with strong cold-air outbreaks and increased storminess.

Beyond these general characteristics, the NAO exhibits a complex dynamical structure that manifests most strongly during the Northern Hemisphere winter, when the interaction between midlatitude waves, large-scale circulation, and the thermal contrast of the North Atlantic favors amplification of pressure anomalies. The organization of these anomalies can be described using simple indices, which employ the normalized sea-level pressure difference between representative regions of the Azores and Iceland, as well as statistical methods that extract the dominant pattern of atmospheric variability in the basin, such as EOF analysis. These different methods consistently capture the alternation between strengthening and weakening of the westerly wind belt, an essential feature of the phenomenon (Hurrell, 1995).

From a historical and paleoclimatic perspective, long-term reconstructions based on early instrumental series and proxy data show that the NAO exhibits persistent phases on decadal scales, indicating that part of its variability is associated with internal ocean–atmosphere coupling processes. Studies using observed pressures since the nineteenth century, such as those based on the Gibraltar and Reykjavík records, confirm that prolonged phases occurred long before recent global warming, highlighting the importance of natural variability (Jones et al., 1997).

The physical mechanisms responsible for modulating the NAO include internal atmospheric processes, such as the action of midlatitude cyclonic vortices and momentum transport by transient waves, as well as the influence of remote forcings. Interactions with stratospheric circulation, especially during sudden stratospheric warming events, can trigger or prolong negative NAO phases by altering the structure of the polar vortex. Additionally, conditions in the tropical and subtropical Atlantic, such as sea surface temperature anomalies, can establish teleconnections that reinforce particular phases of the oscillation (Hurrell et al., 2003).

The consequences of the NAO extend to the ocean as well. Intensified positive phases tend to increase ocean mixing in the North Atlantic, cooling the surface and promoting greater ventilation of deep waters, whereas sustained negative phases may reduce this mixing and allow anomalous sea-surface warming. These effects influence the position of the sea-ice edge, salinity patterns, the strength of the North Atlantic Current, and even marine ecosystems, as observed in variations in zooplankton abundance and fluctuations in fish stocks (Hurrell et al., 2003).

Although analyses show prolonged positive NAO trends over parts of the twentieth century, there is still no consolidated consensus regarding how much of these changes can be attributed to anthropogenic warming. Internal variability across multiple scales, the impact of volcanic activity, natural tropical Pacific fluctuations, and stratospheric changes all add complexity to the problem. Climate models reproduce certain aspects of these trends, but distinguishing between external forcing and internal noise remains an active area of research (Hurrell, 1995).