What is the meaning of La Niña and how will the weather event affect Australia’s summer?

The weather pattern occurs every few years and typically brings increased rainfall to the country's north and east

• BoM declares 2021 La Nina weather event for Australia
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The Australian Bureau of Meteorology has declared a La Nina weather event is under way, with modelling predicting it will persist until the late southern hemisphere summer or early autumn 2022".

So what is it, and what will this mean for our summer?