The Bureau of Meteorology has officially declared that La Niña is underway, echoing similar statements made earlier this month by climate agencies in the USA and Japan.

-- What is La Niña? --

La Niña refers to a change in the sea surface temperature and wind patterns across the equatorial Pacific Ocean. It is a prolonged (multiple months) departure from the normal conditions that we would expect to see in this region at any given time of the year.

Image: Sea surface temperature anomaly map showing a distinctive La Niña pattern in the Pacific Ocean this week. Source: NOAA / CPC

In Australia, La Niña typically causes above-average rain, cooler daytime temperatures, less likelihood of drought, more tropical cyclones and an earlier start to the northern wet season. These outcomes aren’t guaranteed during every La Niña, although they become more likely when it is present.

-- False start? --

The Bureau of Meteorology’s declaration of La Niña comes about three weeks after similar statements were released by the U.S. Climate Prediction Centre (CPC) and the Japan Meteorological Agency (JMA).

This discrepancy simply comes down to the fact that each organisation uses slightly different thresholds to classify La Niña. Australia’s thresholds are a bit harder to satisfy, so the Bureau usually declares La Niña (and El Niño) several weeks after the US.

-- Active end to the year --

Regardless of its official start date, the La Niña pattern in the Pacific Ocean is likely to influence Australia’s weather during the remainder of 2020 and possibly into the start of 2021 as well. In contrast to last year’s prolific fires and devastating drought, this spring and summer are likely to feature more rain, storms and floods.