Rain-causing negative IOD ends, La Nina persists

An Indian Ocean climate driver that has contributed to this year’s record-breaking flooding has returned to neutral territory, but Australia’s third-consecutive La Nina is likely to persist into next month or February, the Bureau of Meteorology says in its latest update.

A combination of a negative Indian Ocean Dipole (IOD), a La Nina and positive phase of the Southern Annual Mode (SAM) have combined to bring drenching conditions to eastern Australia this year.

The bureau says sea surface temperatures (SST) are now generally close to average across the northern half of the Indian Ocean basin, with areas of warm and cool anomalies in the east and the west of the basin dissipating over recent weeks.

“In the absence of a gradient in SST anomalies across the tropical Indian Ocean, the influence of the IOD on Australian rainfall patterns has faded,” it says.

Nevertheless, all seven models monitored by the bureau say the La Nina will continue through this month before returning to neutral levels over the remainder of summer. The events typically decay during autumn.

The Southern Ocean SAM is “weakly positive” and likely to remain neutral to positive through this month, boosted by La Nina and a strong polar vortex over Antarctica.

“In summer months, a positive SAM increases the chance of above average rainfall for parts of north-east Tasmania, eastern Victoria, eastern New South Wales and south-eastern Queensland, and increases the chance of below average rainfall for western Tasmania,” the bureau says.

The summer outlook released by the bureau late last month says it’s likely to be wetter than usual along Australia’s east coast – particularly coastal NSW, most of Victoria, large areas of Queensland, and northern and eastern Tasmania. There’s also an increased chance of an above average number of tropical cyclones.

Any further significant rainfall has the potential to lead to widespread flooding, given high rivers, full dams, and saturated catchments.

Communities are still facing risks flowing from the severe weather that affected inland NSW last month, as waters move through swollen river systems.

Minor to major flood warnings were issued today for communities, including Hay and Balranald, on the Murrumbidgee and for towns on the Lachlan River. Bourke and Wilcannia, on the Darling, are also among towns affected.

“Rainfall over recent weeks and months has caused prolonged flooding along the Barwon and Darling Rivers with flood peaks flowing into the Barwon-Darling River System from multiple tributaries,” one of the warnings says.

The SA Government has advised property owners to track river conditions and to prepare their properties, as the Murray River floods as a result of high flows upstream.