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.
Sydneysiders have endured their hottest summer since records began, with official data revealing records across the city were smashed during the 2016-17 season.
Western Australia's biggest wet season on record is coming to an end, with just one more weak tropical low forecast for next week.
Sydney just endured its hottest summer in more than 150 years, setting a number of new records relating to heat.