Monthly values of the The Southern Oscillation Index (SOI) during 
1950-1995. This series consists of 540 observations on the SOI, 
computed as the ``difference of the departure from the long-term 
monthly mean sea level pressures'' at Tahiti in the South Pacific 
and Darwin in Northern Australia. The index is one measure of the 
so-called "El Nino-Southern Oscillation" -- an event of critical 
importance and interest in climatological studies in recent decades. 
The fact that most of the observations in the last part of the series 
take negative values is related to a recent warming in the tropical 
Pacific, and this pattern has been of key interest in recent studies. 
A key question of interest is to determine just how unusual this event 
is, and if it can reasonably be explained by standard "stationary" 
time series models, or requires models that include drifts/trends 
that may be related to global climatic change. 


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