The Southern Oscillation Index (SOI) is the critical atmospheric indicator of the El Niño-Southern Oscillation (ENSO) phenomenon. ENSO is a coupled atmospheric and oceanic phenomenon which fluctuates between what are known as the El Niño and La Niña phases. These phases have wide-ranging effects on seasonal conditions in Queensland.
The SOI is calculated based on pressure differences between Tahiti and Darwin and, along with Pacific Ocean sea-surface temperature (SST) anomalies, it is used to assess the presence of El Niño and La Niña and the likelihood of drier or wetter than normal conditions.
El Niño
El Niño is characterised by warmer-than-average waters in the central tropical Pacific Ocean and sustained negative values of the SOI. For Queensland, El Niño is often, but not always, associated with below-average rainfall.
- A strong El Niño prevailed from mid-2015 through early 2016 when the six-month average value of the SOI fell to –16, and seasonal rainfall was below average in parts of northern and central Queensland. Over 80% of Queensland was drought-declared during this period under state government processes.
- The most recent El Niño prevailed from late 2023 through early 2024 when the six-month average value of the SOI fell to -9. While rainfall was below average at the start of this El Niño period, wetter-than-average conditions prevailed throughout most of the period.
La Niña
La Niña is characterised by the reverse conditions to El Niño and often shifts tropical weather systems closer to Queensland. The shift in weather systems increases the likelihood of above-average summer rainfall, floods and tropical cyclones.
- A strong La Niña occurred from late 2010 through early 2011 when the six-month average value of the SOI rose to +22, and spring and summer rainfall was extremely high across most of the state.
- More recently, La Niña conditions, with generally average to above-average rainfall, predominated from late 2020 through early 2023. During this period, the six-month average value of the SOI peaked at +15.
