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.
A cold front will cross Tasmania on Tuesday morning bringing a wintry blast and low level snow.
No rain fell in Perth at all during the month of April for only the fourth time in the past 140 years, according to the Bureau of Meteorology.
There were two decent rounds of alpine snow last month and much of Australia's southeast shivered through their coldest temperatures in at least six months last week.