Southern Oscillation Index
You are viewing the archived 2015 report.
Key finding
A transition occurred, from a La Niña in early 2012 to a strong El Niño during the second half of 2015.
The Southern Oscillation Index (SOI) is an atmospheric indicator of the El Niño-Southern Oscillation (ENSO). ENSO, particularly its El Niño and La Niña phases, has wide-ranging effects on rainfall and temperature in the Pacific and surrounding areas, including Queensland.
In Queensland, the SOI, along with other indices like Pacific Ocean sea surface temperatures in the Niño 3.4 region (5°N–5°S, 170°W–120°W), is used to assess the presence of El Niño and La Niña conditions and the likelihood of dry or wet conditions over winter, spring and summer.
El Niño is characterised by warmer than average waters in the central tropical Pacific, and a negative SOI due to atmospheric pressure being lower than average at Tahiti and higher than average at Darwin. In Queensland, El Niño is often associated with below average rainfall in winter, spring and summer.
During La Niña the pattern is reversed, with cooler than average waters in the central tropical Pacific, and a positive SOI due to atmospheric pressures being higher than average at Tahiti and lower than average at Darwin. In Queensland, La Niña increases the risk of above average summer rainfall, floods and tropical cyclones, due to a westward shift of tropical weather systems. A particularly strong La Niña event prevailed from mid-2010 to early 2012, with a six-month average value of the SOI rising as high as +22 in February 2011. The La Niña broke down in the autumn of 2012, and was followed by a period of ENSO-neutral conditions until mid-2014.
In November 2014, the six-month average value of the SOI fell below -6, and has remained below this level (as of end of January 2016). During the current El Niño event, the six-month average value of the SOI reached its lowest value (-15.7) in October 2015.
More information:
Indicator: Southern Oscillation Index
A monthly summary of the Southern Oscillation Index (SOI), including a six-monthly mean. The SOI is an indicator of El Niño (below -8) and La Niña (above +8) conditions. Data is current as at December 2015.