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.
Melbourne has chilled through its coldest pair of days this late in the year since 1983, reaching a footy-season like 13.2 degrees on Saturday and 14.2 on Sunday.
Following a nice Sunday to end the weekend, the mercury is set to plunge on Monday as a cold front welcomes the working week.
Hail storms, heavy rain and gusty winds have lashed Victoria and hundreds of people have called for help from emergency services as a cold front moved across the state.