Antarctic Oscillation Index [AAO index]

Predicting cold antarctic air penetrations with the Arctic Oscillation Index

The Southern Annular Mode (SAM), is a low-frequency mode of atmospheric variability of the southern hemisphere that is defined as a belt of strong westerly winds or low pressure surrounding Antarctica which moves north or south as its mode of variability.
It is a climate driver for Australia, influencing the country's weather conditions – It is associated with storms and cold fronts that move from west to east that bring precipitation to southern Australia.
In its positive phase (AAO+), the westerly wind belt that drives the Antarctic Circumpolar Current intensifies and contracts towards Antarctica. In winter, a positive phase increases rainfall (including East coast lows) in south-eastern Australia (above Victoria) due to higher onshore flows from the Pacific Ocean, decrease rain in the south-west, and decrease snow in the alpine areas. In spring and summer, a positive phase reduces the chance of extreme heat and increases humid onshore flows, therefore making spring and summer more wetter than normal. A positive phase would usually occur more frequently with a La Niña event.
Its negative phase involves the belt moving towards the equator, whereby decreasing rainfall in the southeast of Australia in the summer and as well as raising the possibility of spring heatwaves. Moreover, winters will usually be wetter than normal in the south and southwest with more snowfall in the alpine areas, but drier in the east coast due to less moist onshore flows from the east and blockage of cold fronts by the Great Dividing Range, which would act as a rain shadow. This phase will usually be more frequent with an El Niño event.

AAO index: observed & GFS forecasts

The daily AAO indices are shown for the previous 120 days, and the GFS forecasts of the daily AAO 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.

AAO index: observed & GFS Ensemble forecasts

The daily AAO indices are shown for the previous 120 days, and the ensemble forecasts of the daily AAO index at selected lead times are appended onto the time series. The indices are standardized by standard deviation of the observed monthly AAO 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 AAO index and the correlation coefficients between the observations and the forecasts, respectively.
The first panel shows the observed AAO index (black line) plus forecasted AAO indices from each of the 11 MRF ensemble members starting from the last day of the observations (red lines).
The ensemble mean forecasts of the AAO index are obtained by averaging the 11 MRF ensemble members (blue lines), and the observed AAO 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.

AAO index: GFS Ensemble mean forecasts differences from observed

Differences between 15-day running mean values of AAO index observation and the Ensemble mean outlooks.

Monthly mean AAO index since January 1979

In the following table all the average monthly values of the AAO 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 AAO values recorded so far.

Standardized 3-Month Running Mean AAP Index

Vertical cross-section of geopotential height anomalies

The daily geopotential height anomalies at 14 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°S.
The blue (red) colors represent a strong (weak) polar vortex. The black solid lines show the zero anomalies.
The lower diagram shows the AAO index calculated daily.