The Arctic Oscillation (AO), also known as the Northern Annular Mode or Northern Hemisphere Annular Mode (NAM), is a weather pattern near the Arctic pole, located north of 55 degrees latitude. It is a key factor in how the climate changes in the Northern Hemisphere. In the Southern Hemisphere, a similar pattern is called the Antarctic Oscillation or Southern Annular Mode (SAM). The AO index changes over time without a set pattern and is marked by sea-level pressure changes that are not tied to seasons. These changes include areas of high pressure in the Arctic balanced by areas of low pressure near 37–45° N.
The North Atlantic Oscillation (NAO) is closely related to the Arctic Oscillation. Scientists debate whether the AO or NAO better represents the atmosphere's behavior. Some argue that the NAO can be described in a way that connects more directly to measurable changes in the atmosphere.
Description
The Arctic oscillation forms a ring-shaped pattern of changes in air pressure near the North and South Poles. In the Arctic, the presence of landmasses makes this pattern less clear, while the pattern around the Antarctic Pole is nearly circular.
Scientists believe the Arctic oscillation is connected to global weather patterns. NASA scientist James E. Hansen described how this pattern influences weather far from the Arctic, though his explanation contains an error: during the positive phase of the Arctic oscillation, air pressure in the Arctic is low, which strengthens the jet stream.
The Arctic oscillation index is calculated using changes in the height of the atmosphere at 1000 hPa (a measure of air pressure) from 20°N to 90°N. These changes are compared to a specific pattern, called the Arctic oscillation loading pattern. This pattern is based on the most common changes in air pressure observed between 1979 and 2000. The final index is adjusted to a standard scale using the average monthly changes in pressure.
Periodicity
Over most of the past 100 years, the Arctic oscillation switched between positive and negative states. Data using a 60-day average shows the oscillation has been moving toward a more positive state since the 1970s, though it has become more neutral in the last 10 years. The oscillation still changes randomly between negative and positive values on daily, monthly, seasonal, and yearly time scales. However, scientists can predict short-term changes with high accuracy. The match between real data and the 7-day average forecasts from the Global Forecast System is about 0.9.
A back-and-forth pattern in air pressure between the poles and mid-latitudes was first discovered by Edward Lorenz. This pattern was named the Arctic oscillation in 1998 by David W.J. Thompson and John Michael Wallace.
Impacts
The National Snow and Ice Data Center explains how the Arctic oscillation affects weather. During the positive phase, higher pressure in the middle regions of the Earth pushes ocean storms farther north. This causes wetter weather in Alaska, Scotland, and Scandinavia, while making the western United States and the Mediterranean drier. In this phase, very cold winter air does not reach as far into the center of North America as it does during the negative phase. This keeps much of the United States east of the Rocky Mountains warmer than usual but makes Greenland and Newfoundland colder than normal. Weather patterns during the negative phase are generally the opposite of those during the positive phase.
Scientists now often use the Arctic oscillation to explain unusual weather events. For example, a report from the National Oceanic and Atmospheric Administration’s National Climatic Data Center, titled State of the Climate December 2010, mentions the phrase "negative Arctic Oscillation" four times. This example shows how frequently scientists are using this term in their official reports.
Events of 2010
In February 2010, the Arctic Oscillation reached its lowest monthly average value since record-keeping began in 1950, at about −4.266. This month saw three major snowstorms in the mid-Atlantic region of the United States. The first storm brought 25 inches (640 mm) of snow to Baltimore, Maryland, on February 5–6. A second storm delivered 19.5 inches (500 mm) to the same area on February 9–10. In New York City, a separate storm added 20.9 inches (530 mm) of snow on February 25–26. Another storm hit Catalonia and nearby French regions, including Languedoc-Roussillon and Midi-Pyrenées, on March 8, leaving 60 cm of snow in Girona. These extreme snow events are considered very unusual and as severe as the negative Arctic Oscillation value itself. The negative Arctic Oscillation values in 2010 and the following winter allowed colder air to move farther south than usual into subtropical South Florida, causing record low temperatures. In Cancún, a tropical beach area, February, March, and December had the coolest average monthly minimum temperatures for that year, with temperatures up to over 4°C below normal.
The lowest Arctic Oscillation value in January since 1950 was −3.767 in 1977. This occurred at the same time as the coldest average January temperature in New York City, Washington, D.C., Baltimore, and other mid-Atlantic locations during that period. Between 1950 and 2010, the January Arctic Oscillation was negative 60.6% of the time. Nine of the ten coldest Januarys in New York City since 1950 happened during periods when the Arctic Oscillation was negative.