Assessment of Southern Oscillation Sea-Level Pressure Indices

Abstract

Monthly sea-level pressure anomalies at Easter Island, Rapa, Tahiti and Darwin, for the period 1951?79, are analysed to reveal their temporal characteristics and the coherence and time lead/lag relationships among them. For interannual scale oscillations, the pressure variations at Rapa and Easter Island lead those at Tahiti and those of opposite phase at Darwin by 2?8 months, where the large lead times are associated with lower frequency oscillations. Of the four stations, Rapa has the smallest percentage of its variance in the range of periods containing the Southern Oscillation; Darwin has the largest. Spatial smoothing by combining stations enhances the percentage of variance in the longer periods. Combining Tahiti and Darwin gives a substantial increase; combining Rapa and Easter Island, much less. Normalization of the time series through division by the monthly standard deviations is also treated. The combination of Tahiti and Darwin is recommended as a Southern Oscillation Index for diagnostic studies; the combination of Rapa and Easter Island may be found useful for prognostic applications.