Arctic Oscillation Index [AO]

  Teleconnections  
AAO (Antarctic Oscillation) MJO (Madden Julian Oscillation) Polar/Eurasia SST (Sea Surface Temperature)
AO (Arctic Oscillation) NAO (North Atlantic Oscillation) QBO (Quasi-Biennial Oscillation) Temp 10 hPa Polo Nord
EA (East Atlantic Index) Oscillazioni intrastagionali SCAND (Scandinavia pattern) Temp e geopotenziali 10 hPa
EA/WR (East Atl./Western Russia) PNA (Pacific North-America pattern) SOI (Southern Oscillation Index) Temp e geopotenziali 100 hPa
Stratosphere Analysis NAM (North Annular Mode)


Teleconnective indices: Arctic Oscillation Index [AO]

The Arctic Oscillation Index (AO) is a large-scale index of climate variability. It is an important mode of climate variability for the Northern Hemisphere. It coincides with the North Annular Mode (NAM), when the latter is calculated at 1000 hPa (the NAM is calculated up to stratospheric altitudes). For more, please refer to the dedicated page to NAM.
When the AO is in its positive phase (AO+), a ring of strong winds circulating counterclockwise around the North Pole confines colder air to the polar regions. In this phase, the pressure at sea level at high latitudes is extremely low (strengthening of the polar vortex, that is the area of ​​low pressure that is stationed semi-permanently high above the North Pole), while at mid-latitudes there is a strengthening of anticyclones. With a deep polar vortex, particularly strong western currents are observed, not very wavy and therefore with consequent reduced heat exchange with the lower latitudes.
When the wind belt becomes weaker and more distorted we are in the negative phase of the AO (AO-); in this phase, characterized by low pressures in the mid-latitudes, an easier penetration towards the south of the cold arctic air masses is observed and an increase in storminess in the mid-latitudes. The intensity of the phenomenon can also lead to the splitting of the polar vortex, with powerful irruptions of cold air in the mid-latitudes.
The AO is calculated by projecting the daily geopotential anomalies at 1000 hPa between latitudes between 20° N and 90° N with respect to the Arctic oscillation model defined by an empirical function of the monthly geopotential averages at 1000 hPa during the period 1979- 2000 (see the map in the figure). The time series is then normalized with the standard deviation of the monthly average.

Loading pattern of the AO index
The loading pattern of the AO is defined as the leading mode of Empirical Orthogonal Function (EOF) analysis of monthly mean 1000mb height during 1979-2000 period
Arctic Oscillation: negative and positive phase
Arctic oscillation: negative and positive phase (credits: wikipedia.org).

Arctic oscillation - AO index: Observed & GFS Forecasts

The daily AO indices are shown for the previous 120 days. The indices are standardized by standard deviation of the observed monthly AO index from 1979-2000.
The values at the upper left and right corner of each figure indicate the mean value of the AO index and the correlation coefficients between the observation and the forecasts, respectively.

Arctic oscillation - AO index: Observed & GFS Forecasts
The daily AO indices are shown for the previous 120 days. Credits: cpc.ncep.noaa.gov

AO index: Observed and Ensemble forecast

The daily AO indices are shown for the previous 120 days, and the ensemble forecasts of the daily AO index at selected lead times are appended onto the time series. The indices are standardized by standard deviation of the observed monthly AO index from 1979-2000. A 3-day running mean is applied to the forecast time series.
The values at the upper left and right corners of each figure indicate the mean value of the AO index and the correlation coefficients between the observations and the forecasts, respectively.
The first panel shows the observed AO index (black line) plus forecasted AO indices from each of the 11 GFS ensemble members starting from the last day of the observations (red lines).
The ensemble mean forecasts of the AO index are obtained by averaging the 11 GFS ensemble members (blue lines), and the observed AO index (black line) is superimposed on each panel for comparison. For the forecasted indices (lower 3 panels), the yellow shading shows the ensemble mean plus and minus one standard deviation among the ensemble members, while the upper and lower red lines show the range of the forecasted indices, respectively.

AO index: Observed and Ensemble forecast
Daily AO indices are shown for the previous 120 days. The values at the upper left and right corners of each figure indicate the mean value of the AO index and the correlation coefficients between the observations and the forecasts, respectively. Credits: cpc.ncep.noaa.gov

Daily AO index since January 1950

In the following table all the monthly average values of AO index from 1950 to today. With a red scale, values higher than +0.5 are highlighted, with a blue scale those lower than -0.5.
Below the table, a diagram again shows the AO values recorded.

JANFEBMARAPRMAYJUN JULAUGSEPOCTNOVDEC
1950-0.060+0.627-0.008+0.555+0.072+0.539-0.802-0.851+0.358-0.379-0.515-1.928
1951-0.085-0.400-1.934-0.776-0.863-0.918+0.090-0.377-0.818-0.213-0.069+1.987
1952+0.368-1.747-1.859+0.539-0.774-0.441+0.383-0.030-0.383-0.437-1.891-1.827
1953-1.036-0.249+1.068-1.256-0.562+0.023+0.333+0.085+0.662-0.194+0.354+0.575
1954-0.148-0.181+0.476+0.512-1.656-0.268+0.341-0.122+0.301+0.513-0.328+0.553
1955-1.163-1.542-1.568+0.194+0.242-0.266+0.332+0.760+0.357+0.099-1.342-0.444
1956-1.204-2.029+0.470-0.868+1.391+0.280-0.215-0.652-0.202+1.139-0.066+0.001
1957+2.062-1.513-2.013+0.238-0.966-0.760-0.646+0.097-0.956+0.903-1.380+0.828
1958-1.438-2.228-2.522-0.360-0.336-1.149-0.684-0.755-0.012+0.770-0.011-1.687
1959-2.013+2.545+1.432+0.119-0.341-0.033+0.105-0.745-0.281-0.249-1.411-0.042
1960-2.484-2.212-1.625-0.297-0.857+0.055-0.619-1.008-0.382-1.187-0.553-0.343
1961-1.506+0.621+0.341-0.237+0.157+0.837-0.108+0.013+0.815+0.203-0.010-1.668
1962+1.645-0.358-2.848+1.169+0.068+0.287-0.927+0.152-0.056-0.016-1.112-0.711
1963-3.311-1.721+0.724-0.348+0.771-0.585-0.303-0.625+0.083+1.069-0.419-1.178
1964+0.385-0.575-0.558+0.663+1.174+0.142+0.734-1.207-0.227+0.342-0.344-0.246
1965-1.046-2.084-0.905+0.568-0.153+0.038-0.510-0.255-0.698+0.394-1.341+0.163
1966-3.232-1.438-0.911-1.837+1.124+0.408+0.011-0.945+0.011-1.077+0.111-1.401
1967-0.576+1.180+1.967+1.700+0.127+0.647+0.259-0.293+0.133+1.299+0.334-0.347
1968-0.409-2.154+1.741+0.328-0.241+0.420-0.836-0.671-1.009-1.013-2.183-0.783
1969-2.967-3.114-1.582+0.438-0.720-0.348+0.410-0.782-0.083+0.098+0.326-1.856
1970-2.412-1.325-2.084+0.302+0.531+0.875+0.139-0.263+0.030+0.098+0.378-0.399
1971-0.163-0.922-1.091-0.583+0.679-0.668-0.578+0.818+0.153+1.185+0.419+0.824
1972+0.167-0.195-0.141+1.007+0.140-0.049-0.553-0.082-0.920+0.392-0.380+1.238
1973+1.232+0.786+0.537-1.126+0.073+0.531+0.270+0.313+0.114+0.337+0.002-0.181
1974+0.232-0.489-0.746+0.309-0.507-0.048+0.390-0.533-0.136-1.024-0.435+0.556
1975+1.595+0.194+0.151+0.409-0.614-0.323+0.345+0.130+1.278+0.138+0.619+1.290
1976+0.034+1.656+0.587+0.440+0.060+0.328-0.325+0.559-0.743-0.804-0.087-2.074
1977-3.767-2.010+0.344+1.329+0.104-0.226-0.492-1.412+0.586-0.009+0.605-0.240
1978-0.347-3.014+0.502-0.967+0.059+0.635-0.604-0.354-0.099+0.895+2.470-0.980
1979-2.233-0.697-0.814-1.157-0.250+0.933+0.039-0.684-0.046-1.243+0.475+1.295
1980-2.066-0.934-1.433-0.419-1.155+0.721-0.622-0.185+0.313-0.521-1.361-0.057
1981-0.116-0.332-1.645+0.430+0.180-0.438+0.561-0.244-1.040-1.167-0.188-1.216
1982-0.883+0.974+1.074+1.454-0.209-1.180+0.005+0.362+0.558-0.211+0.661+0.967
1983+1.359-1.806-0.567-0.738-0.441+0.313+0.131+1.098+0.167+1.369-0.688+0.186
1984+0.905-0.303-2.386-0.284+0.479+0.007+0.019+0.466-0.413-0.270-0.966+0.446
1985-2.806-1.440+0.551+0.652-0.432-0.347-0.390-0.001+0.114+1.035-1.218-1.948
1986-0.568-2.904+1.931+0.103+0.367+0.535-0.008-0.826-0.023+1.425+0.926+0.060
1987-1.148-1.473-1.746+0.387+0.325-0.710-0.466-0.836+0.287-0.080-0.536-0.534
1988+0.265-1.066-0.197-0.561-0.846+0.061-0.143+0.255+1.039+0.032-0.035+1.679
1989+3.106+3.279+1.530-0.250+0.889+0.345+0.866+0.551+0.703+0.991+0.034-0.644
1990+1.001+3.402+2.990+1.879+0.943+0.304-0.296-0.180-0.210+0.660+0.521+1.277
1991+0.723-0.876-0.527+0.530+0.486-0.115-0.188+0.797-0.112-0.252+0.285+1.613
1992+0.550+1.122+0.984-0.521+1.341-0.302+0.191+0.535-0.640-0.366+0.717+1.627
1993+3.495+0.184+0.764-0.435-1.607-0.520-0.511-0.393-0.361-0.565+1.002-0.104
1994-0.288-0.862+1.881+0.225-0.115+1.606+0.351+0.828-0.084+0.174+1.779+0.894
1995-0.154+1.429+0.393-0.963-0.891-0.112-0.217+0.544-0.549+0.075-0.723-2.127
1996-1.200+0.163-1.483-1.525-0.226+0.497+0.715+0.125-1.140+0.183+0.136-1.721
1997-0.457+1.889+1.091+0.324-0.961-0.815-0.431+0.121+0.195-0.700-0.661-0.071
1998-2.081-0.183-0.254-0.038+0.429-0.711-0.212+0.650-1.050+0.294-1.449+1.353
1999+0.110+0.482-1.492+0.284+0.226+0.707-0.002-0.672+0.059-0.006+0.611+1.043
2000+1.270+1.076-0.451-0.279+0.969+0.586-0.649+0.144+0.395+0.317-1.581-2.354
2001-0.959-0.622-1.687+0.906+0.452-0.015-0.031+0.520-0.707+0.707+0.819-1.322
2002+1.381+1.304+0.902+0.748+0.401+0.573+0.328-0.229-0.043-1.488-1.425-1.592
2003-0.472+0.128+0.933-0.178+1.017-0.102+0.075-0.280+0.467-0.670+0.642+0.265
2004-1.686-1.528+0.318-0.409-0.094-0.236-0.201-0.720+0.855-0.515+0.678+1.230
2005+0.356-1.271-1.348-0.046-0.763-0.383-0.030+0.026+0.802+0.030+0.228-2.104
2006-0.170-0.156-1.604+0.138+0.156+1.071+0.103-0.265+0.606-1.029+0.521+2.282
2007+2.034-1.307+1.182+0.544+0.894-0.555-0.397-0.034+0.179+0.383-0.519+0.821
2008+0.819+0.938+0.586-0.455-1.205-0.090-0.480-0.080-0.327+1.676+0.092+0.648
2009+0.800-0.672+0.121+0.973+1.194-1.351-1.356-0.054+0.875-1.540+0.459-3.413
2010-2.587-4.266-0.432-0.275-0.919-0.013+0.435-0.117-0.865-0.467-0.376-2.631
2011-1.683+1.575+1.424+2.275-0.035-0.858-0.472-1.063+0.665+0.800+1.459+2.221
2012-0.220-0.036+1.037-0.035+0.168-0.672+0.168+0.014+0.772-1.514-0.111-1.749
2013-0.610-1.007-3.185+0.322+0.494+0.549-0.011+0.154-0.461+0.263+2.029+1.475
2014-0.969+0.044+1.206+0.972+0.464-0.507-0.489-0.372+0.102-1.134-0.530+0.413
2015+1.092+1.043+1.837+1.216+0.763+0.427-1.108-0.689-0.165-0.250+1.945+1.444
2016-1.449-0.024+0.280-1.051-0.036+0.313+0.085+0.472+0.781-1.917-0.611+1.786
2017+0.942+0.340+1.365-0.089-0.730+0.402+0.634+0.150-0.492+0.690-0.078-0.059
2018-0.281+0.113-0.941+0.544+1.180+0.380+0.612+0.836+0.585+0.413-1.116+0.110
2019-0.713+1.149+2.116-0.255-1.231-0.601-0.890-0.722+0.306-0.082-1.193+0.412
2020+2.419+3.417+2.641+0.928-0.027-0.122-0.412-0.381+0.631-0.072+2.086-1.736
2021-2.484-1.191+2.109-0.204-0.161+0.840+0.625-0.217-0.247-0.139+0.099+0.201
2022+0.854+1.558+0.245-0.606+1.218-0.084+0.018-0.180-0.661+1.352

Standardized 3-Month Running Mean AO Index. Credits: cpc.ncep.noaa.gov

Standardized 3-Month Running Mean AO Index. Credits: cpc.ncep.noaa.gov
Monthly average values of AO index from 1950 to today. Credits: cpc.ncep.noaa.gov

Vertical cross-section of Geopotential Height Anomalies

The daily geopotential height anomalies at 17 pressure levels are shown for the previous 120 days as indicated, and they are normalized by standard deviation using 1979-2000 base period. The anomalies are calculated by subtracting 1979-2000 daily climatology, and then averaged over the polar cap poleward of 65° N.
The blue (red) colors represent a strong (weak) polar vortex. The black solid lines show the zero anomalies.

Vertical cross-section of Geopotential Height Anomalies
Vertical cross-section of Geopotential Height Anomalies for latitude greater than 65° N. The blue (red) colors represent a strong (weak) polar vortex. The black solid lines show the zero anomalies.

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