Following Walker's work about his famous three oscillations published during the 1920–30s, many papers were written about atmospheric oscillations. A fourth atmospheric oscillation in the middle and high southern latitudes was found, and named the Antarctic Oscillation (AO). AO refers to a large scale alternation of atmospheric mass between the mid-latitudes and high latitudes surface pressure. In order to understand the spatial structure of sea level pressure variation in detail, empirical orthogonal function analysis is applied. An objective index of the Antarctic Oscillation Index (AOI) is defined as the difference of zonal mean sea level pressure between 40°S and 65°S. The AOI has the potential for clarifying climate regimes in the southern hemisphere, similar to how the NAO and the NPO has been used in the northern hemisphere.