The study of hemispheric and global-scale oscillations in sea-level pressure has a centurylong history. Inverse pressure variations over southeastern Australia and southern South America were first noted by Hildebransson (1897) in his studies of centers of action. Low-frequency pressure seesaws were confirmed by Lockyer (1906) and, undoubtedly, these results provided the basis for the extensive investigations of Sir Gilbert Walker between 1909 and the 1930s. Several large-scale pressure patterns were distinguished by Walker in an attempt to isolate predictors useful in long-range forecasting. Through studies of the temporal correlation of monthly mean sea-level pressure at various locations around the world he discovered three large-scale oscillations of pressure and associated temperature and precipitation anomalies. The circulation modes are identified according to the strongest simultaneous negative correlations with a given location at some remote distance, 3,000-6,000 km away. The three patterns identified by Walker (1924) and Walker and Bliss (1932) were:

1 The North Atlantic Oscillation (NAO), involving the Icelandic low and Azores high. 2 The North Pacific Oscillation (NPO), involving the Aleutian low and North Pacific

high. 3 The Southern Oscillation (SO) between the southeast Pacific high and the equatorial

trough in the Indian Ocean-Indonesian region (Figure 5.1).