The Southern Oscillation Index, or SOI, gives an indication of the development and intensity of El Niņo or La Niņa events in the Pacific Ocean. The SOI is calculated using the pressure differences between Tahiti and Darwin.

Sustained negative values of the SOI greater than −8 often indicate El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade Winds, and a reduction in winter and spring rainfall over much of eastern Australia and the Top End. You can read more about historical El Niño events and their effect on Australia in the Detailed analysis of past El Niño events.

Sustainted positive values of the SOI greater than +8 are typical of a La Niña episode. They are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern tropical Pacific Ocean become cooler during this time. Together these give an increased probability that eastern and northern Australia will be wetter than normal. You can read more about historical La Niña events and their effect on Australia in the Detailed analysis of past La Niña events.

The ENSO Wrap-Up includes the latest 30-day SOI value, as well as other information on indicators of El Niño and La Niña events.

The graph below shows monthly values of the SOI in recent years.

For further information, have a look at the Climate Variability and El Niño brochure.
Click here to find out more about El Niño and associated topics.

For the mathematically minded:

There are a few different methods of how to calculate the SOI. The method used by the Australian Bureau of Meteorology is the Troup SOI which is the standardised anomaly of the Mean Sea Level Pressure difference between Tahiti and Darwin. It is calculated as follows:

         [ Pdiff - Pdiffav ]
SOI = 10 -------------------
             SD(Pdiff)      

where

Pdiff   =   (average Tahiti MSLP for the month) - (average Darwin MSLP for the month),
Pdiffav   =   long term average of Pdiff for the month in question, and
SD(Pdiff)   =   long term standard deviation of Pdiff for the month in question.

The multiplication by 10 is a convention. Using this convention, the SOI ranges from about –35 to about +35, and the value of the SOI can be quoted as a whole number. The SOI is usually computed on a monthly basis, with values over longer periods such a year being sometimes used. Daily or weekly values of the SOI do not convey much in the way of useful information about the current state of the climate, and accordingly the Bureau of Meteorology does not issue them. Daily values in particular can fluctuate markedly because of daily weather patterns, and should not be used for climate purposes.