The NINO3.4 index is one of several El Niņo/Southern Oscillation (ENSO) indicators based on sea surface temperatures.
NINO3.4 is the average sea surface temperature anomaly in the region bounded by 5°N to 5°S, from 170°W to 120°W. This region has large variability on El Niņo time scales, and is close to the region where changes in local sea-surface temperature are important for shifting the large region of rainfall typically located in the far western Pacific.
An El Niņo or La Niņa event is identified if the 5-month running-average of the NINO3.4 index exceeds +0.4°C for El Niņo or -0.4°C for La Niņa for at least 6 consecutive months.
Power is gradually being turned back on across parts of South Australia but much of the state remains in darkness after a widespread blackout.
South Australia and its 1.677 million residents were left without power on Wednesday evening following severe storms.
Federal Energy Minister Josh Frydenberg says serious questions will be raised about how the entirety of South Australia could be left without electricity in the wake of a huge storm, dubbed the largest in 50 years.