The Southern Oscillation is in a negative phase when pressures are higher in Darwin than in Tahiti.
The Southern Oscillation Index (SOI) is a standardized index that is derived from the observed changes in sea level pressure (SLP) between Tahiti and Darwin, Australia. This index is used to predict future weather patterns. During El Nio and La Nia events, the SOI is one measurement of the large-scale fluctuations in air pressure that occur between the eastern and western tropical Pacific. These changes can be attributed to the Southern Oscillation. In general, when time series of the SOI are smoothed, a very good correlation with variations is seen in the temperatures of the oceans throughout the eastern tropical Pacific. The negative phase of the SOI indicates that the air pressure in Tahiti is lower than normal while the air pressure in Darwin is higher than normal. Extended periods of negative Southern oscillation index values coincide with ocean temperatures over the eastern tropical Pacific that are abnormally warm (cold), which is typical of El Nio (La Nia) events.
