MOIST air, combined with an increasingly negative Indian Ocean Dipole (IOD) and a positive Southern Annular Mode (SAM) have conspired to create the record-breaking floods through NSW and Queensland over the past week.
While the Bureau of Meteorology's latest climate driver update confirms the 2021-22 La Nina event has ended, there are several other climate influences associated with above-average rainfall in eastern Australia, including an Indian Ocean now at close to negative IOD ranges, correlated with above average rainfall.
Warmer than usual waters around Australia are leading to more moisture-filled air eastern Australia which has been a driver of the heavy rain, which has delivered isolated tallies over 600mm and widespread tallies in excess of 400mm in an area stretching from Taree south to Wolllongong.
During this recent rainfall event, very warm waters off the Australian coast, up to 23 degrees, provided extra energy and moisture contributing to the deep trough and east coast low, leading to the relative concentration of the heavy rainfall to one 24-hour period. Along with the IOD negative, the positive SAM is also correlated with higher rainfall.
The SAM refers to the non-seasonal, north-south movement of the strong westerly winds that blow almost continuously in the mid- to high-latitudes of the southern hemisphere.
In the positive phase, the SAM directs more moisture-filled air than usual into eastern Australia, driving above average rainfall and more east coast lows in winter.
The BOM reported its Campbelltown gauge, with more than 30 years of data, recorded its highest ever daily total rainfall amount for any month, while six gauges with more than 100 years of observations have set a record 4-day total for July.
Dozens of gauges from Nowra on the South Coast to Taree and the Manning River catchment have set daily rainfall records for July.
The BOM also reported this week that the IOD index has been very close to or exceeded negative IOD thresholds over the past 4 weeks, meaning it is very likely an official negative IOD event will be declared soon.
BOM officials said all climate model outlooks it used indicated a negative IOD event was likely for the coming months.
In the Pacific, while La Nina has just ended, the BOM said climate models suggested another La Nina may reform later in the year.