2.1 Arctic Oscillation Index

Last Updated: 
May 27, 2015


The Arctic Oscillation (AO) is a pattern of variability in the atmospheric pressures of the Arctic and North Atlantic oceans, resulting is large changes in weather from year to year, and decade to decade. The North Atlantic Oscillation and AO are different ways of describing the same phenomenon.

The Arctic climate is highly variable. The AO index gives us information on the natural phases of this variation. It tells us about "normal" weather conditions that can greatly vary over decades.

The AO has the largest effect during winter (January, February and March), so the index is usually represented as patterns in winter climate in the Arctic1.

When the AO index is in a “positive phase” (left globe), high atmospheric pressure persists south of the North Pole, and lower pressures sit over the North Pole. In a positive phase, very cold winter air does not extend as far south into the middle of North America as it would during a negative phase. The AO positive phase is often called the “Warm” phase in North America.

Arctic oscillation positive and negative
Positive (left) and Negative (right) AO Phases © Figures courtesy of J. Wallace, University of Washington) from the National Snow and Ice Data Centre1.

When the AO index is in a "negative phase", relatively high atmospheric pressure sits over the Beaufort Sea (called the Beaufort High) and the North Pole, and low pressures stay further south, about 45o N. Cold winter air extends far to the south in North America. The AO negative phase is often called the “cold” phase in North America. Weather patterns in the negative phase are in general "opposite" to those of the positive phase. The AO phases also have effects on Western Europe and Africa as shown on the diagrams.

AO values are obtained from NOAA/ National Weather Service National Centers for Environmental Prediction, Climate Prediction Center2.

NWT focus

As weather and climate affect many aspects of northern ecosystems, understanding the AO is essential to understanding changes in northern ecosystems.

A positive AO index is related to a decade (about 10 years) weaker clockwise circulation in the Beaufort Sea (weaker Beaufort Gyre)3, 4, which results in changes in currents across the Arctic Ocean and a decrease in old thicker sea ice at the pole3. A positive AO is also linked to warmer winter temperatures on average, in northwestern North America3, but is linked to winters colder than average in Nunavut3, 5.

The effects of AO on weather patterns in the NWT are clearer in the north (Beaufort Sea) and northeast (tundra) part of the territory6, 7. The Pacific Decadal Oscillation appears to have a stronger effect on weather in the south and western (forested) part of the territory6, 8. There is evidence9that the AO, which has been associated with climatic changes in the Arctic and North Atlantic, may be a good predictor of shifts in the Pacific Decadal Oscillation.

Current view - status and trend

JFM Season standardized AO index 1950-2014
Source: NOAA/National Weather Service National Centers for Environmental Prediction, Climate Prediction Center.

The standardized seasonal mean AO index during cold season (blue line) is constructed by averaging the daily AO index for January, February and March for each year. The black line denotes the standardized five-year running mean of the index. 

Over most of the past century, the AO alternated rapidly between its positive and negative phases. However, in the 1970s, and then again from late 1980s to late 1990s, the index remained “stuck” in a strong positive (warm) phase, with a record high in 1990. This extended positive phase is being studied extensively10, 11. The current pattern (since about 2005) is more consistent with the rapid flip-fop patterns observed before this exceptionally long positive phase3.

Looking forward

The variability in the AO is a natural phenomenon that can reduce or amplify the effects on Arctic climate caused by increased greenhouse gas emissions10. In decades when the natural effects of AO are similar to the predicted effects of human-caused climate change, it is difficult to distinguish between the two9. The current pattern of fast changes between positive and negative phases in AO offers scientists a renewed opportunity to study the effects of human-caused climate change in the Arctic11.

Looking around

Many Pacific Arctic changes are continuing, despite the observation that climate indices such as the Arctic Oscillation were negative or neutral for six of the last nine years. The Pacific Arctic may be having a larger role in shaping the persistence of Arctic change than has been previously recognized. Quote From the NOAA’s Arctic Theme Page, Observations in the Pacific Arctic12.

For more information

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Contact us

Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


Ref. 1. National Snow and Ice Data Centre. Current. The Arctic Oscillation. Arctic Climatology and Meteorology, Education Centre. University of Colorado, Boulder.

Ref. 2. NOAA. 2008. Arctic Oscillation. US National Weather Centre, Climate Prediction Service NOAA.

Ref. 3. Richter-Menge, J., S. Nghiem, D. Perovich, and I. Rigor. 2008. Sea Ice Cover. Arctic Report Card 2007. www.arctic.noaa.gov/reportcard/seaice.html

Ref. 4. Lukovich, J.V. and D.G. Barber. 2007. On the spatiotemporal behaviour of sea ice concentration anomalies in the Northern Hemisphere. Journal of Geophysical Research 112:D13117

Ref. 5. Bonsal, B. and A. Shabbar. 2011. Large-scale climate oscillations influencing Canada, 1900-2008, Canadian Biodiversity: Ecosystem Status and Trends 2010. Tehcnical Thematic Report No. 4. Canadian Councils of Resource Ministers. Ottawa, ON.

Ref. 6. Déry, S.J. and E.F. Wood. 2005. Decreasing river discharge in northern Canada. Geophysical Research Letters, 32: CitelD L10401.

Ref. 7. Hogg, E.H., J.P. Brandt, and B. Kochtubajda. 2005. Factors affecting interannual variation in growth of western Canadian aspen forests during 1951-2000. Canadian Journal of forest research 35, 610-622.

Ref. 8. Fauria, M.M. and E.A. Johnson. 2006. Large-scale climatic patterns control large lightning fire occurrence in Canada and Alaska forest regions. Journal of Geophysical Research 111:1-17.

Ref. 9. Sun Jianqi, Wang Huijun. 2006. Relationship between Arctic Oscillation and Pacific Decadal Oscillation on decadal timescale. Chinese Science Bulletin 51:75-79.

Ref. 10. Cohen, J. and M. Barlow. 2005. The NAO, the AO, and global warming: how closely related? J. Climate 18:4498-4513.

Ref. 11. McGuire, A.D., F.S. Chapin, J.E. Walsh, and C. Wirth. 2006. Integrated Regional Changes in Arctic Climate Feedbacks: Implications for the Glocal Climate System*. Annual Review of Environment and Resources 31: 61-91.

Ref. 12. National Oceanic and Atmospheric Administration (US). 2008. Activities of NOAA that support the objectives of the international polar year (IPY) March 2007-March 2009. Arctic Theme Page (Web) Observation 2. Causes and Impacts of Recent Changes in the Pacific Arctic. National Oceanic and Atmospheric Administration (US).

Updated: May 27, 2015