The North Atlantic Oscillation (NAO) is a weather pattern over the North Atlantic Ocean that involves changes in air pressure at sea level between the Icelandic Low and the Azores High. These changes affect the strength and direction of westerly winds and the path of storms across the North Atlantic.
The NAO was identified through research in the late 1800s and early 1900s. Unlike the El Niño–Southern Oscillation in the Pacific Ocean, the NAO is mainly an atmospheric pattern. It is one of the most significant examples of climate changes in the North Atlantic and nearby areas with high humidity.
The North Atlantic Oscillation is closely connected to the Arctic Oscillation (AO) or Northern Annular Mode (NAM). However, it should not be confused with the Atlantic Multidecadal Oscillation (AMO).
Definition
The NAO has several possible definitions. The simplest ones are based on measuring the average air pressure difference between specific stations, such as:
- Lisbon and Stykkishólmur/Reykjavík
- Ponta Delgada, Azores and Stykkishólmur/Reykjavík
- Azores (1865–2002), Gibraltar (1821–2007), and Reykjavík
All these definitions share the same northern station in Iceland, as this is the only station in the region with a long record of data. They also use different southern stations. These definitions aim to measure the same pattern of air pressure changes by selecting stations in the "center" of two stable pressure areas: the Azores High and the Icelandic Low (shown in the graphic).
A more complex definition, made possible by modern computer models and detailed weather records, uses a special mathematical method called the principal empirical orthogonal function (EOF) of surface pressure. This method closely matches the results from the station-based definitions. This leads to a discussion among scientists about whether the NAO is different from the AO/NAM, and if not, which one better describes the structure of the atmosphere.
Description
Westerly winds that move across the Atlantic Ocean carry moist air toward Europe. In years when these winds are strong, summers tend to be cool, winters are mild, and rainfall is common. However, when these winds are weak, temperatures become more extreme in both summer and winter. This can lead to heat waves, very cold periods, and less rainfall.
A permanent low-pressure area over Iceland, called the Icelandic Low, and a permanent high-pressure area over the Azores, called the Azores High, influence the direction and strength of westerly winds in Europe. The strength and position of these pressure systems change from year to year. This change is called the North Atlantic Oscillation, or NAO. When the pressure difference between Iceland and the Azores is large (a high NAO index), westerly winds are stronger. This results in cooler summers and milder, wetter winters in Central Europe and along the Atlantic coast. In contrast, when the pressure difference is small (a low NAO index), westerly winds are weaker. This causes colder, drier winters in northern Europe and shifts storm paths southward toward the Mediterranean Sea. This increases storm activity and rainfall in southern Europe and North Africa.
From November to April, the NAO plays a major role in changing weather patterns in the North Atlantic region. It affects wind speed and direction, temperature changes, moisture distribution, and the number, strength, and movement of storms. Recent research suggests that the NAO may be easier to predict than previously thought, which could improve winter weather forecasts.
There is some debate about how much the NAO affects short-term weather in North America. Most scientists agree that its impact is smaller in the United States compared to Western Europe. However, the NAO is believed to influence weather in parts of the central and eastern United States. During winter, when the NAO index is high, the Azores High creates a stronger southwesterly wind pattern over the eastern half of the United States. This pattern prevents cold Arctic air from moving far south into the United States. When combined with El Niño, this can lead to warmer winters in the upper Midwest and New England. However, the effects on areas farther south are uncertain. When the NAO index is low, colder air can move farther south, causing more frequent cold outbreaks and heavy snowstorms in the central and northeastern United States. In summer, a low NAO index may weaken the jet stream, which usually moves weather systems into the Atlantic. This could lead to longer heat waves in Europe, but recent studies have not confirmed this connection.
Recent studies show that the pressure systems involved in the NAO—such as their strength and location—are important for understanding seasonal and shorter-term climate patterns in Europe, North America, and the Mediterranean region.
Effects on North Atlantic sea level
When the NAO index is positive (NAO+), lower air pressure in certain areas causes sea levels in those regions to rise. This happens because of a phenomenon called the "inverse barometer effect." This effect is important for understanding past sea level changes and predicting future trends, as small changes in air pressure, measured in millibars, can cause sea levels to rise or fall by several centimeters.
North Atlantic hurricanes
The North Atlantic Oscillation (NAO) affects the paths of major North Atlantic tropical cyclones by controlling the position of the Azores High. When the Azores High is located farther south, it directs storms toward the Gulf of Mexico. In contrast, when the Azores High is positioned farther north, it allows storms to move along the North American Atlantic Coast.
Research on ancient storms shows that major hurricanes rarely hit the Gulf Coast between 3000–1400 BC and again during the most recent 1,000 years. However, between 1400 BC and AD 1000, the Gulf Coast experienced frequent and severe hurricanes. During this time, the chance of hurricanes making landfall increased by 3 to 5 times compared to other periods.
Ecological effects
Until recently, the NAO had been in a more positive phase since the late 1970s, leading to colder conditions in the North-West Atlantic. These colder conditions have supported growing populations of Labrador Sea snow crabs, which thrive best in low-temperature environments.
When the NAO is in a positive phase (NAO+), warming in the North Sea reduces the survival of cod larvae, which are already near their maximum temperature tolerance. Similarly, cooling in the Labrador Sea, where cod larvae are near their minimum temperature tolerance, also harms their survival. While not the main cause, the NAO+ peak in the early 1990s may have contributed to the decline in the Newfoundland cod fishery.
In southwestern Europe, NAO- events are linked to stronger winds.
On the East Coast of the United States, NAO+ conditions bring warmer temperatures and more rainfall, creating warmer and less salty surface water. This prevents nutrient-rich water from rising to the surface, reducing fish populations. Areas like Georges Bank and the Gulf of Maine have seen lower cod catches due to this.
The strength of the NAO also influences the population changes of the well-studied Soay sheep.
Interestingly, Jonas and Joern (2007) discovered a clear connection between the NAO and grasshopper species in the tall grass prairies of the U.S. Midwest. Even though the NAO does not directly affect weather in this region, they found that more common grasshopper species (such as Hypochlora alba, Hesperotettix spp., Phoetaliotes nebrascensis, Melanoplus scudderi, Melanoplus keeleri, and Pseudopomala brachyptera) became more abundant after winters during NAO+ phases. Less common species (such as Campylacantha olivacea, Melanoplus sanguinipes, Mermiria picta, Melanoplus packardii, and Boopedon gracile) increased in number after winters during NAO- phases. This study was the first to show a link between the NAO and land-based insects in North America.
The NAO’s effects on ecosystems extend to the Tibetan Plateau, where NAO- events have been linked to drier conditions, increased forest deaths, and more frequent dust storms.
Winter of 2009–10 in Europe
The winter of 2009–10 in Europe was unusually cold. Some scientists think this may be because of several factors happening at the same time, including low solar activity, a type of El Niño event called a Modoki or Central Pacific (CP) El Niño, and a strong easterly phase of the Quasi-Biennial Oscillation. These events may have caused the jet stream to become more wavy, leading to a negative North Atlantic Oscillation (NAO) phase. This means the pressure differences between the Icelandic Low and Azores High were reduced. The Met Office reported that the UK had its coldest winter in 30 years during this time. This cold weather happened at the same time as an unusually negative NAO phase. A study published in mid-2010 confirmed that the El Niño event and the rare negative NAO phase were connected.
In the winter of 2010–11, Northern and Western Europe experienced very cold weather because the Icelandic Low, a low-pressure area usually west of Iceland, moved east of Iceland. This allowed cold Arctic air to flow into Europe. A strong high-pressure area over Greenland changed the usual wind patterns in the northwestern Atlantic, creating a blocking pattern. This pattern sent warm air to northeastern Canada and cold air to Western Europe, similar to the previous winter. This cold weather occurred during a La Niña season, which is an Eastern Pacific (EP) La Niña event. These events are often linked to a negative NAO phase, which brings cold and wet conditions to southern Europe and the eastern U.S. This event was also connected to a rare Arctic dipole anomaly.
In the northwestern part of the Atlantic, both winters were mild, especially 2009–2010, which was the warmest recorded in Canada. The winter of 2010–2011 was also above normal in northern Arctic regions of Canada.
Scientists have found that when there is less Arctic sea ice in summer, the chances of cold winters with heavy snow in Central Europe increase. Researchers from the Potsdam Research Unit of the Alfred Wegener Institute have discovered that reduced summer sea ice changes air pressure zones in the Arctic atmosphere, which affects European winter weather.
If there is a large-scale melting of Arctic sea ice in summer, two effects become stronger. First, the melting exposes darker ocean water, which absorbs more heat from the sun (this is called the ice–albedo feedback mechanism). Second, the reduced ice cover allows heat stored in the ocean to escape into the atmosphere (this is called the lid effect). These changes cause the air near the ground to warm more in autumn and winter because the ocean is warmer than the atmosphere during these seasons.
This warming causes air to rise, making the atmosphere less stable. One result is the pressure difference between the Arctic and mid-latitudes, known as the Arctic oscillation. This includes the Azores High and Icelandic Low, which are reported in weather patterns. When the pressure difference is large, strong westerly winds bring warm and humid air from the Atlantic to Europe in winter. However, when the pressure difference is small (a negative phase), cold Arctic air can move southward into Europe without being blocked by usual westerly winds. Models show that reduced Arctic sea ice in summer weakens the pressure difference, allowing Arctic cold to reach mid-latitudes in the following winter.
Winter of 2015–16 in Europe
During the winter of 2015–2016, despite one of the strongest El Niño events recorded in the Pacific Ocean, a strong North Atlantic Oscillation (NAO) remained over Europe. This occurred because of a Canonical or Eastern Pacific (EP) El Niño event, which have opposite effects on the NAO and the Arctic Oscillation (AO). These events often support a positive NAO, leading to stronger westerly winds and milder weather. For example, Cumbria in England experienced one of the wettest months ever recorded. In the Maltese Islands of the Mediterranean, one of the driest years ever recorded by early March occurred, with a national average rainfall of just 235 mm. Some areas received less than 200 mm of rain.