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All summer long: La Nina may persist until autumn

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Climate models suggest the current La Nina weather pattern will persist until late summer or early autumn, the Bureau of Meteorology (BOM) says.

Last month, BOM declared the flood-inducing weather pattern was underway, potentially bringing more rainfall than usual this summer and above average numbers of tropical cyclones over the next few months.

The latest BOM Climate Driver Update report says La Nina is now firmly established in the tropical Pacific and it will possibly persist until March, bringing higher rainfall across much of northern and eastern Australia during summer.

Sea surface temperatures in the tropical Pacific have cooled to La Nina thresholds, with climate model outlooks expecting them to cool further and cloud and wind patterns indicating the atmosphere is responding to these changes in ocean temperatures.

“This feedback process is known as ‘coupling’, and it means La Nina conditions are now expected to be locked in until at least the end of summer,” BOM says.

Most El Nino–Southern Oscillation (ENSO) indicators show clear La Nina patterns, BOM says, though for 2021–22 to be considered a La Nina year the event must be sustained for at least three months.

BOM says in general, the models anticipate that this La Nina will be short-lived and of weak to moderate strength, though there is some variation between models in predicted peak strength.

“The strength of an event doesn't necessarily equal the strength of its effect,” BOM says. “Regardless of whether La Nina thresholds are sustained for three months or for a shorter period, the presence of La Nina-like patterns in the Pacific increases the chances of above-average rainfall for northern and eastern Australia during spring and the coming summer.”

All seven international climate models surveyed by BOM anticipate further cooling of tropical Pacific sea surface temperatures and indicate La Nina thresholds are likely to be met during December and January. Three models continue the event into February but by March only one model continues to meet the threshold.

The Madden–Julian Oscillation (MJO) is forecast to progress eastwards across the western Pacific over the coming fortnight, increasing the chance that the monsoon will develop in the Australian region in the next week or two - a little earlier than normal.

BOM says Australia's climate has warmed by around 1.44°C since 1910 and in recent decades there has been a trend towards a greater proportion of rainfall from high intensity, short duration rainfall events, especially across northern Australia.

The next update is to be published on December 21.