Arctic oscillation

Arctic oscillation
Name	Arctic oscillation
Measurement	Sea level pressure anomalies
Area	Northern Hemisphere
Related	North Atlantic Oscillation, Antarctic oscillation

Arctic oscillation. The Arctic oscillation is a prominent pattern of climate variability characterized by opposing atmospheric pressure anomalies between the polar region and the mid-latitudes of the Northern Hemisphere. It is a dominant mode of atmospheric circulation that influences weather patterns across continents, with its positive and negative phases dictating the strength of the polar vortex and the westerly winds. The index, defined by fluctuations in sea level pressure, is closely related to other major oscillations like the North Atlantic Oscillation and has significant implications for seasonal forecasting and understanding climate change.

Definition and Terminology

The concept was formally identified and named in the late 1990s by climate scientists including David W. J. Thompson and John Michael Wallace. It is defined statistically as the leading empirical orthogonal function of sea level pressure anomalies north of 20°N. The terminology distinguishes it from regional patterns, describing a hemisphere-wide seesaw in mass between the Arctic and surrounding zones. While closely linked, it is considered a broader index than the North Atlantic Oscillation, which is its most prominent regional manifestation over the Atlantic Ocean. The related Southern Hemisphere phenomenon is termed the Antarctic oscillation or Southern Annular Mode.

Characteristics and Measurements

The primary characteristic is its dipolar structure, with one center over the Arctic Ocean and a ring of opposite sign centered around 45°N latitude. It is measured by an index, typically derived from principal component analysis of pressure data from sources like the National Centers for Environmental Prediction reanalysis. A positive phase is marked by lower-than-average pressure over the North Pole and stronger mid-latitude westerlies, confining cold air. A negative phase features higher polar pressure, a weaker polar vortex, and increased meridional flow that allows frigid air to plunge into regions like North America and Eurasia. The phase can change rapidly, influencing weekly weather.

Effects on Climate and Weather

The phase profoundly affects temperature and precipitation patterns across the Northern Hemisphere. During its positive phase, places like Alaska, Scandinavia, and Siberia often experience milder, wetter winters, while the Mediterranean Basin can be drier. In its negative phase, severe winter weather becomes more likely in the eastern United States, Western Europe, and East Asia, as seen during infamous cold outbreaks. It modulates storm tracks, impacting snowfall in the Rocky Mountains and rainfall in the Pacific Northwest. Its influence extends to ocean conditions, affecting sea ice drift in the Fram Strait and surface temperatures in the North Atlantic.

Causes and Mechanisms

The dynamics are driven by complex interactions between the atmosphere, ocean, and sea ice. A key mechanism involves wave-mean flow interactions and the transfer of energy from mid-latitude Rossby waves into the polar stratosphere, which can weaken or strengthen the polar vortex. Forcings from tropical convection, such as that related to the Madden–Julian oscillation, and sea surface temperature patterns in the Pacific Ocean, including those from El Niño–Southern Oscillation, can trigger phase changes. Recent research also investigates links to Arctic amplification and reduced sea ice cover in the Barents Sea, which may alter energy fluxes and pressure gradients.

Observations and Trends

Long-term observations from datasets like the Twentieth Century Reanalysis show significant variability on timescales from weeks to decades. A notable trend toward a more positive phase occurred from the 1970s to the 1990s, contributing to warming in high-latitude regions like Greenland. Since the early 2000s, there has been an increase in frequent negative phases in winter, coinciding with years of severe cold snaps in the Midwestern United States and Tokyo. Monitoring is conducted by institutions such as NASA and the National Oceanic and Atmospheric Administration, using instruments on satellites like Aqua and a network of weather stations.

Impacts on Environment and Society

The environmental impacts are extensive, affecting ecosystem phenology, permafrost stability, and the migration patterns of species like the caribou. For society, its phases have major economic consequences, influencing energy demand for heating, the viability of shipping routes through the Northern Sea Route, and agricultural yields. Persistent negative phases can strain infrastructure, as seen during the Winter of 2009–10 in Europe. It is a critical factor in seasonal forecasts used by entities like the European Centre for Medium-Range Weather Forecasts and the UK Met Office to prepare for extreme weather, influencing sectors from aviation to public health.

Category:Climate patterns Category:Atmospheric dynamics Category:Arctic