Southern oscillation index

The index tracks fluctuations in air pressure between Tahiti and Darwin, and gives an indication of the development and intensity of El Nino or La Nina events in the Pacific Ocean. El Nino episodes are often indicated by sustained negative values of the SOI below -8

El Nino episodes

-8 SOI

Sources: NOAA; Australia Bureau of Meteorology
By Wen Foo and Travis Hartman | REUTERS GRAPHICS