Key climate drivers behind record rainfall in New South Wales

BOM

Issued: Tuesday, 5 July 2022

The heavy rainfall along the eastern New South Wales coast that has led to major flooding in some catchments over the past week can be linked to changes to several climate drivers.

While the Bureau of Meteorology’s latest climate driver update confirms the 2021-22 La Niña has ended, there are several other climate influences associated with above-average rainfall in eastern Australia.

The Bureau’s 2022 winter outlook showed above average rainfall over the coming months, particularly for most of eastern and northern Australia, due to warmer than usual waters around the continent and more moisture-filled air being directed into eastern Australia.

The July rainfall event was influenced by the Indian Ocean Dipole, likely to become negative in the coming months, and the positive Southern Annular Mode (SAM).

The Indian Ocean Dipole (IOD) refers to the difference in sea surface temperatures between the western and eastern Indian Ocean. In the negative phase, warmer waters concentrate near Australia, leading to above average winter–spring rainfall as more moisture is available to weather systems crossing the continent.

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.

During this recent rainfall event, very warm waters off the Australian coast (21-23°C) 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.

The 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 Newcastle and the Hunter Valley have set daily rainfall records for July.

The highest four-day totals, which are over 400 mm at many sites, have been observed across the Hawkesbury-Nepean catchment including at; Darkes Forest; Beaumont; Robertson; Audley; Albion Park; and Fitzroy Falls.

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