Focus Areas E2- Oceanography

What is the Southern Oscillation Index?

The Southern Oscillation Index (SOI) is a term used by weather scientists to refer to the broad atmospheric pressure conditions experienced over the Pacific Ocean during a particular month or season.

The calculation of the SOI value for any specific month depends on regularly recorded measurements of air pressure at sea level in two convenient locations: Darwin in northern Australia and Tahiti an island in the central Pacific Ocean. The index is calculated from the difference in the average air pressure between Tahiti and Darwin for that particular month minus the long term average of the difference between the two locations and is expressed relative to a measure of the long term variability for the month in question.

Positive values for the SOI, if they continue over a few months, are taken as signs of La Niña conditions. If the positive values persist for more than five months then the conditions are classified as a La Niña episode which is usually characterised by:

  • an increase in the strength of the Pacific Trade Winds;
  • an increase in rainfall over eastern and northern Australia;
  • warmer sea surface temperatures to the north of Australia; and  
  • cooler sea surface temperatures in the central and eastern tropical Pacific ocean.

Negative values of the SOI are taken as signs of an El Niño event and are usually associated with the opposite weather pattern (See El Niño and Drought for more detail).

In the graph below, from the Australian Bureau of Meteorology, the actual SOI values are shown in the blue columns for each month from January 2002 up to mid 2007. Negative values for the SOI - those below the ‘zero’ line - are taken as signs of El Niño conditions. Positive SOI values - those above the ‘zero’ line - are taken as signs of La Niña conditions.  The swing between the two conditions is easily seen on this graph.

Graph 1:  The Southern Oscillation Index (SOI) between 2002 and 2007 (Source: Bureau of Meteorology)

The following graph, from CSIRO Australia, shows an overall picture of the SOI from 1860 until 2004.  La Niña (blue) and El Niño (red) events are clearly seen throughout the decades.

Graph 2:  Yearly values estimated for the Southern Oscillation Index between 1860 and 2004
(Source: CSIRO Australia)

As can be seen from the above graph, La Niña is not new.  Good data has been collected for well over a century, enabling us now to trace the occurrence of La Niñas in the past. On average La Niña episodes occur every 3 to 5 years. However their occurrence does not take place in a regular, predictable pattern and the interval between recorded La Niñas has varied from 2 to 7 years. During the twentieth century, significant La Niña episodes had their beginnings in 1903, 1906, 1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 1988, and 1995. (Source: NOAA ). Typically (but not in all cases), these El Niñas build up in one autumn and  gradually subside in the following autumn. Sometimes, as for example in the period between 1954 and 1956, shown in the above graph, the La Niña conditions persist for two or three years.

Students learn about El Niño and La Niña
Students learn to identify features of El Niño and La Niña