Signal versus noise in the Southern Oscillation.

The utility of a simple index for monitoring the Southern Oscillation signal is explored in detail. Based upon sea level pressure data at the two stations Tahiti (T) and Darwin (D), an optimal index, in the sense that it combines the Southern Oscillation variance into one series, is the combination (T_n - D_n) where the subscript n denotes normalization by the overall standard deviation of each series. A direct measure of the noise due to small-scale or transient phenomena that are not a part of the large-scale coherent Southern Oscillation fluctuations is the index (T_n + D_n). It is recommended that this index of noise should also be monitored in order to determine the representativeness of the Southern Oscillation index. -from Author