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 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.
Out west, the Indian Ocean dipole is forecast by all climate models to enter its negative phase in the 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. According to the bureau, it also lifts the odds of warmer days and nights for northern Australia.
Models are becoming more confident that we’ll get a negative Indian Ocean Dipole in the 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
In particular, the prospect of wetter-than-normal conditions for the east coast 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.
The bureau’s special climate report detailed how a series of low-pressure systems caused severe flooding in Lismore, Brisbane, and other regions this year.
More than 50 sites in south-eastern Queensland and north-east NSW collected more than one meter 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 extendeperiodme.”
The report described the intense rainfall as “some of the most significant on record”.
Near Gympie, north of Brisbane, one rain gauge collected 424mm from 24 hours to 9 am 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 meter.
The highest daily total was 228.4mm on 27 February, a monthly record 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.
Sign up to receive an email with the top stories from Guardian Australia every morning.
North-eastern NSW copped similarly biblical amounts of rain as the system shifted southwards. Five flood-warning sites exceeded one meter of rainfall for the three days ending at 9 am on 1 March.
“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 by more than 200mm.”
Lismore, located on the Wilsons, was inundated when the river peaked at 14.4 meters on 28 February, overtopping the riverbank levee (10.6 meters) with flood waters. The previous record flood level was 12.27 meters in February 1954.
“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.”