North Atlantic oscillation

The North Atlantic Oscillation (NAO) is a weather pattern over the North Atlantic Ocean. It shows changes in the difference of atmospheric pressure at sea level between the Icelandic Low and the Azores High. These changes affect how strong and where the westerly winds blow and where storm tracks go across the North Atlantic.

Scientists discovered the NAO through many studies in the late 1800s and early 1900s. Unlike the El Niño–Southern Oscillation in the Pacific Ocean, the NAO is mostly an air pattern. It is one of the most important ways the climate changes in the North Atlantic and nearby wet areas.

The NAO is closely related to the Arctic oscillation, but it is different from the Atlantic multidecadal oscillation.

Definition

The North Atlantic Oscillation (NAO) is a weather pattern over the North Atlantic Ocean. It shows changes in air pressure between two main areas: the Azores High in the south and the Icelandic Low in the north. Scientists measure this by looking at the pressure difference between weather stations, like Lisbon in the south and Reykjavík in the north, or Ponta Delgada and Reykjavík.

There are also more complex ways to define the NAO using modern weather data, but they all try to describe the same pattern of pressure changes.

Description

Westerly winds blowing across the Atlantic bring moist air into Europe. When these winds are strong, summers are cool, winters are mild and rain is frequent. When the winds are weak, temperatures can be very hot or very cold and there is less rain.

There are two important pressure systems that control these winds: a low-pressure system over Iceland (called the Icelandic Low) and a high-pressure system over the Azores (called the Azores High). The strength and position of these systems change from year to year, and this change is what we call the North Atlantic Oscillation (NAO). When the pressure difference between these two systems is large, the westerly winds are strong. This brings cool summers and mild, wet winters to Central Europe. When the pressure difference is small, the westerly winds are weak. This leads to cold, dry winters in northern Europe and more storms and rain in southern Europe and North Africa.

The NAO has a big effect on weather in the North Atlantic region from November to April. It influences wind speed and direction, temperature, moisture, and the number and path of storms. Scientists think the NAO might be easier to predict than we used to believe, which could help us make better winter weather forecasts.

The NAO may also affect weather in North America, though not as much as it does in Europe. In winter, a strong NAO can keep very cold air from moving into the southern United States. Together with El Niño, this can make winters warmer in places like the upper Midwest and New England. When the NAO is weak, parts of the northern United States can experience unusually cold winters with heavy snow. In summer, a weak NAO might help create long-lasting heat waves over Europe, but this is still being studied.

Effects on North Atlantic sea level

When the North Atlantic Oscillation is strong, the air pressure drops in some areas. This change in pressure can cause the sea level to go up a little bit in those places. This is important for understanding past sea levels and guessing what might happen to sea levels in the future. Small changes in air pressure can make the sea level change by a few centimeters.

North Atlantic hurricanes

The North Atlantic Oscillation affects where big storms, called hurricanes, travel. When a part of the weather system called the Azores High is farther south, it pushes storms toward the Gulf of Mexico. When it is farther north, storms can move up the North American Atlantic Coast.

Studies of very old weather patterns show that during some times, like 3000–1400 BC and the last 1000 years, fewer big hurricanes hit the Gulf coast. But between 1400 BC and AD 1000, many powerful hurricanes struck that area more often.

Ecological effects

The North Atlantic Oscillation has an impact on nature and wildlife. When it is stronger, it can bring colder weather to the North-West Atlantic. This colder weather helps some animals, like snow crabs, to thrive.

In other places, like the North Sea, the warmer weather caused by the North Atlantic Oscillation makes it harder for baby fish, such as cod, to survive. This has affected fishing in some areas, like Newfoundland. In the United States, the North Atlantic Oscillation can bring warmer weather and more rain, which changes the water and affects plants and animals. Even far away, like on the Tibetan Plateau, the North Atlantic Oscillation can influence the weather and affect forests and dust storms.

Winter of 2009–10 in Europe

The winter of 2009–10 in Europe was unusually cold. Scientists think this happened because of a few things happening together: low activity from the sun, a special kind of weather pattern in the Pacific Ocean, and a strong change in air pressure over the North Atlantic. This made the winds weaker and allowed cold air to move into Europe.

During the winter of 2010–11 in Northern and Western Europe, the usual weather patterns changed again. The area of low pressure near Iceland moved, letting very cold air from the Arctic come into Europe. This brought more cold weather to places like the UK and parts of Europe. In some areas of Canada, though, these winters were warmer than usual.

Scientists have found that when there is less ice on the Arctic Ocean in the summer, it can lead to colder winters in Europe. Less ice means the ocean absorbs more heat, which changes the air pressure and allows cold Arctic air to move further south into Europe during winter.

Winter of 2015–16 in Europe

During the winter of 2015–2016, Europe experienced mostly positive effects from the North Atlantic Oscillation. This happened even though there was a very strong El Niño event in the Pacific Ocean, which usually has the opposite effect. The specific type of El Niño, called an Eastern Pacific El Niño, helped create stronger westerly winds and milder weather over Europe.

Some places felt these changes more than others. For example, Cumbria in England had one of its wettest months ever recorded. Meanwhile, the Maltese Islands in the Mediterranean had one of the driest years on record up until March, with only about 235 mm of rain on average, and some areas getting less than 200 mm.