Antarctic Oscillation (AAO)
The daily AAO index is constructed by projecting the daily (00Z) 700mb height anomalies poleward of 20°S onto the loading pattern of the AAO. Please note that year-round monthly mean anomaly data has been used to obtain the loading pattern of the AAO (Methodology). Since the AAO has the largest variability during the cold sesaon, the loading pattern primarily captures characteristics of the cold season AAO pattern.
On a page set up by Todd Mitchell found here is a general description of the AAO, as follows:
The Antarctic Oscillation (AAO) is the dominant pattern of non-seasonal tropospheric circulation variations south of 20S, and it is characterized by pressure anomalies of one sign centered in the Antarctic and anomalies of the opposite sign centered about 40-50S. The AAO is also referred to as the Southern Annular Mode (SAM). There is a Northern Hemisphere analog to the AAO, and it is called the Arctic Oscillation (or Northern Annular Mode). More information is available on annular modes through this link.
The AAO is defined as the leading principal component (PC) of 850 hPa geopotential height anomalies south of 20S (Thompson and Wallace 2000). (An AAO index calculated from SLP anomalies is available through this link.) I have calculated the PC (time series) of this pattern from the NCEP / NCAR reanalysis, and all calendar months are used. There are concerns about the usefulness of the reanalysis in the Antarctic (see articles in J. Climate). The AAO time series for 1968-98 can be compared with the index calculated by Thompson and Solomon (2002) from 500 hPa geopotential height from radiosondes.
Plots of PC1 of 850 hPa Z and a 500 hPa index. Both series are standardized with respect to 1968-98, and are correlated at 0.81 . The trend in the reanalysis-based index (the 850 hPa time series) is primarily before 1970. Qualitatively, the 500 radiosonde-based index exhibits less of a trend than the reanalysis-based index.
850 hPa geopotential height anomalies regressed onto the (normalized) AAO time series. Typical fluctuations in the AAO are associated with anomalies in excess of 30m over the pole and height anomalies of opposite sign and 5-10 m in magnitude at the latitude of New Zealand.
EOF1 plotted as a correlation map. Fluctuations in the AAO explain several percent of tropical variability.
Antarctic Oscillation & ENSM Forecasts