Evidence for Changes in Southern Oscillation Relationships during the Last 100 Years

Abstract

Relationships are examined among records of atmospheric pressure at Darwin, Tahiti and Santiago and indices of the Southern Oscillation (SO) formed from these pressures for the period 1883–1984, and the sea surface temperature in the eastern equatorial Pacific (SST^*) as well as Indian summer-monsoon rainfall and continental and marine hemispheric temperatures. In general, the correlations of pressures and SO indices with SST^* have been greatest since World War II, but were also relatively high prior to World War I. An exception is Tahiti pressure which is poorly correlated with SST^* until about 1940. The correlations have tended to be higher when moderate or strong El Niño were most frequent. The question is discussed whether the decrease in correlations among the various datasets that is apparent in the middle third of the time period in question, roughly from 1920 to 1950, reflects a real change in atmospheric processes or is due to data problems. It is suggested that prior to World War I the pressure difference between the far eastern Pacific and Darwin was more representative of the Southern Oscillation, while between the two World Wars the Southern Oscillation was not quite so prominent, and since World War II the pressure difference between the central Pacific and Darwin has best reflected the Southern Oscillation.