Southern Oscillation Index
El Niño conditions prevailed in 2015 and at the start of 2016, followed by ENSO-neutral conditions for much of 2017.
The Southern Oscillation Index (SOI) is based on pressure differences between Tahiti and Darwin. It is the key atmospheric indicator of the El Niño-Southern Oscillation(ENSO). ENSO fluctuates between its El Niño and La Niña phases and has wide-ranging effects on seasonal conditions in Queensland. The SOI and Pacific Ocean sea-surface temperature anomalies are used to assess the presence of El Niño and La Niña and the likelihood of drier or wetter than normal conditions.
El Niño is characterised by warmer than average waters in the central tropical Pacific, and sustained negative values of the SOI. For Queensland, El Niño is often, but not always, associated with below average rainfall. During the 2015-2016 El Niño, the 6-month average value of the SOI fell to -16 and spring through summer rainfall was below average in northern and central parts of the state.
La Niña is characterised by the reverse conditions, often shifting tropical weather systems closer to Queensland, which increases the likelihood of above average summer rainfall, floods and tropical cyclones. During the strong La Niña of 2010-2011, the 6-month average value of the SOI rose to +22 and spring through summer rainfall was extremely high across most of the state.
A monthly summary of the Southern Oscillation Index (SOI), included as 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 2017.