Focus Areas E2- Oceanography

What is the Southern Oscillation Index?

The Southern Oscillation Index (SOI) is a term used by weather scientists. It refers to a measure of the broad atmospheric pressure conditions observed over the Pacific Ocean during a particular month or season and is associated with the differences in sea surface temperatures across the Ocean.

The calculation of the SOI value for any particular 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.

Negative values for the SOI, if observed over a period of a few months, are taken as signs of El Niño conditions. If the negative values persist for more than five months then the conditions are classified as an El Niño episode. During an El Niño it is usual to observe:

  • a reduction in the strength of the Pacific Trade Winds;
  • a decrease in rainfall over eastern and northern Australia;
  • cooler sea surface temperatures to the north of Australia; and
  • warmer sea surface temperatures in the central and eastern tropical Pacific Ocean

Positive values of the SOI are taken as signs of a La Niña event and are usually associated with the opposite weather pattern.
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.

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

The following graph, from CSIRO Australia, shows an overall picture of the SOI from 1860 until 2004.  El Niño (red) and La Niña (blue) events are clearly seen throughout the decades with the extended 1997 event clearly longer than the rest.

Figure 2:  The Southern Oscillation Index overview between 1860 and 2004 (Source: CSIRO Australia)

As can be seen from the above graph, El Niño is not new. El Niño related atmospheric and oceanic changes can be traced back for at least 400 years.  In fact in 1791 an El Niño event almost wiped out Australia’s first European settlement at Sydney Cove.  El Niño events tend to occur every few years and last for many months (and sometimes much longer with devastating results). The table below records the El Niño years between 1900 and 1998.

Table 1:  El Niño years between 1900 and 1998 (Source: CSIRO)

























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