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La Nina strength may not indicate its effect: BOM

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Weather models generally indicate the La Nina event currently underway will be short-lived and of weak to moderate strength, though the Bureau of Meteorology (BOM) says the “strength of an event doesn't necessarily equal the strength of its effect”.

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, though there is some variation between the models in predicted peak strength.

For 2021/22 to be considered a La Nina year, the event must be sustained for at least three months.

“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,” BOM’s latest BOM Climate Driver Update report says.

Climate models suggest the current La Nina weather pattern will persist until late summer, and possibly early autumn. Three of the seven models continue the event into February but by March only one model continues to meet the threshold.

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.