The breakdown of the La Niña weather pattern in the Pacific has stalled while a key Indian Ocean climate driver is tilting towards its wetter phase, making it more likely that eastern Australia will face more heavy rain and floods.
Just as the Bureau of Meteorology released a special climate report on the extreme rainfall and flooding that hit parts of south-eastern Queensland, northern New South Wales and the region around Sydney in February and March, its fortnightly report on climate influences pointed to the big wet extending for months to come.
The La Niña event, already in its second year, could yet persist into a third. The expected dissipation of the pattern has not progressed in the past two weeks, and two of the seven models used by the bureau project that the La Niña will last through winter.
Meanwhile climate models see the La Nina dissipating (though 2/7 of them see the event persisting through the winter). Will we get a 'three-peat'? @BOM_au pic.twitter.com/df3HYroe7l
— Peter Hannam (@p_hannam) May 24, 2022
Out west, the Indian Ocean dipole is forecast by all climate models to enter its negative phase in coming months.
That phase of the dipole – which gauges the relative differences of sea-surface temperatures across the ocean – increases the chances of above-average winter-spring rainfall for much of Australia. It also lifts the odds of warmer days and nights for northern Australia, according to the bureau.
Models are becoming more confident that we'll get a negative Indian Ocean Dipole in coming months. That typically results in above-average rainfall for central and eastern Australia. @BOM_au pic.twitter.com/w9Xlp0HYUi
— Peter Hannam (@p_hannam) May 24, 2022
The prospect of wetter than normal conditions for the east coast in particular will prompt fears of further floods. Catchments remain damp and dams are full, so it won’t require significant bursts of rain to cause more flash flooding and damage.
Spot the difference: June-August (left) and July-September show the odds strongly favour above-average rainfall across most of the country in coming months. @BOM_au pic.twitter.com/lBCXkD1PP0
— Peter Hannam (@p_hannam) May 25, 2022
The bureau’s special climate report detailed how a series of low-pressure systems this year caused severe flooding in Lismore, Brisbane and other regions.
More than 50 sites in south-eastern Queensland and north-east NSW collected more than one metre of rain in the week to 1 March alone, the report said.
The Hawkesbury-Nepean catchment to Sydney’s west and north recorded its wettest nine- and 14-day stints in records going back to 1900 for those periods up to 9 March.
Karl Braganza, national manager of the bureau’s climate services, said the events were exceptional because of their duration.
To get such large multi-day rainfall totals, “you either have to have very slow-moving systems or quasi-stationary systems”, Braganza said. “Particularly for the Brisbane event, that system just stayed in place for a very extended period of time.”
The report described the intense rainfall as “some of the most significant on record”.
Near Gympie, north of Brisbane, one rain gauge collected 424mm in the 24 hours to 9am on 23 February. The Brisbane city gauge recorded 792.9mm in the six days to 28 February, equivalent to almost four-fifths of the annual average at the current site of just over one metre.
The seven-day rainfall totals in the week to March 2 smashed many records, @BOM_au says in a special climate report. pic.twitter.com/cG8jO186Mp
— Peter Hannam (@p_hannam) May 24, 2022
The highest daily total was 228.4mm on 27 February, a record for any month at the current Brisbane site. Each of the three days from 26 to 28 February topped 200mm.
Rainfall in the Brisbane River catchment exceeded all records over one to seven days for the last week of February, including during the devastating January 1974 floods. The difference this time, though, was the construction of the Wivenhoe dam, which partly serves as a flood mitigation reservoir, in 1984.
North-eastern NSW copped similarly biblical amounts of rain as the system shifted southwards. For the three-day period ending at 9am on 1 March, five flood-warning sites exceeded one metre of rainfall.
“The catchments of the Tweed, Brunswick, Richmond and Wilsons rivers had seven-day average rainfalls that were 37% to 61% above previous records,” the report said. “Rainfall in this region was especially intense at one- and two-day timescales, with the Wilsons River catchment average rainfall exceeding previous records at both timescales by more than 200mm.”
Lismore, located on the Wilsons, was inundated when the river peaked at a record 14.4 metres on 28 February, overtopping the riverbank levee (10.6 metres) with flood waters. The previous record flood level was 12.27 metres in February 1954.
Rainfall totals around Sydney in the week to March 9 were also significant, prompting the evacuation of thousands of people. @BOM_au pic.twitter.com/ZdxvnhisSZ
— Peter Hannam (@p_hannam) May 24, 2022
For the Sydney region, heavy rainfall landed on already saturated soils, full reservoirs and swollen rivers, particularly during the week of 3-9 March. That quickly led to flash flooding as local stormwater and drainage systems were overwhelmed, with thousands of residents evacuated.
“Observations show that there has been an increase in the intensity of heavy rainfall events in Australia,” the report said. “The intensity of short-duration (hourly) extreme rainfall events has increased by around 10% or more in some regions in recent decades, with larger increases typically observed in the north of the country.”
