Climate models suggest this La Niña will be weak and short-lived, expected to persist until early autumn 2018.
The Bureau says signs of La Niña in the equatorial Pacific increased during spring, with the central to eastern tropical Pacific Ocean cooling since late winter.
It is now at La Niña thresholds (0.8 °C below average).
Atmospheric indicators, including the Southern Oscillation Index (SOI), trade winds and cloud patterns, also show clear La Niña signals.
In order for 2017–18 to be classed as a La Niña year, the event needs to last for at least three months.
Climate models surveyed by the Bureau suggest that while this event is likely to persist over the southern summer, it will be weaker than the strong La Nina of 2010–12.
La Niña typically brings above-average rainfall to eastern Australia during late spring and summer. However, sea surface temperature patterns in the Indian Ocean and closer to Australia are not typical of a La Niña event, reducing the likelihood of widespread above-average summer rainfall. La Niña can also increase the chance of prolonged warm spells for southeast Australia.
The Indian Ocean Dipole (IOD) is currently neutral. IOD events are typically unable to form between December and April.
Source: Bureau of Meteorology