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Rainy days may be forerunner to a La Niña

A below-average run of consecutive dry days in autumn across much of Australia could be a forerunner to a return of La Nina conditions, according to the latest Australian Actuaries Climate Index.

During a La Niña event the northern rainfall onset typically occurs earlier than usual.

The index’s “consecutive dry days” category is below the 1981 to 2010 reference period in most regions in autumn, the Actuaries Institute says.

Multiple regions experienced the first below-average extreme high temperature value in many years as cooler weather was observed in southeastern parts of the country, including the Central Slopes, the South & Southern Western Flatlands and Tasmania’s Southern Slopes.

Parts of the southeast experienced extreme rainfall in March as ex-tropical cyclone Esther struck Victoria, bringing with it the largest amount of rainfall for that month since 1929 and creating the highest extreme rainfall value ever for the Murray Basin region.

The Actuaries Institute says a return to relative normality in autumn after an extreme summer is influenced by the El Niño– Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) remaining neutral.

The Bureau of Meteorology’s latest update says the chance of the first rains from September 1 for northern Australia arriving early in 2020-21 is higher than average over much of inland northern Australia, with the highest likelihood across central and some coastal parts of Queensland and the southern Northern Territory.

“Warmer than average Indian Ocean temperatures and an increased likelihood of La Niña in 2020 are influencing this outlook,” the bureau says. “This is due to increased moisture in the atmosphere which can enhance rainfall over the north of the country when weather patterns are favourable to rain.”

The ENSO Outlook is at La Niña watch status, indicating that there is a 50% chance a La Niña could develop in the coming months – twice the normal likelihood.

This would result in above-average winter and spring rainfall in Australia, particularly in the eastern and northern parts of the country, which may continue into summer, the Actuaries Institute says.