In summary , the upward vertical motion on the eastern flank of the cut-off low combined with easterly moisture advection from the Coral Sea produced several days of rainfall over the region .

The synoptic-dynamics of this event are typical for heavy rainfall along the east coast of Australia . Several historical events associated with flooding along the east coast show similar structures , such as the February-April 2022 floods ( Barnes et al ., 2023 ), heavy rainfall in Sydney in March 2021 ( Reid et al ., 2021 ), and many others . However , these events typically occur throughout the warm season and autumn months , meaning that this event brought unusual ( although not unprecedented ) heavier falls to the region .

September rainfall event

The heavy rainfall event that impacted Western Australia and the Northern Territory in September is similar to the one in August despite occurring on the opposite side of the continent . Up to 100 mm fell over the northwestern parts of the continent over several days . This event was also associated with Rossby wave breaking generating a cut-off low over the area of interest ( Figure 3a ). In this case , a deeper surface cyclonic structure promoted moisture advection from the north-west ( Figure 3c ), which together with the vertical motion induced by the cutoff low ( Figure 3a ), produced dry-season rainfall for the northwestern parts of the country .

Were these events unusual ?

Several dry-season rainfall events ( of varying intensity and length ) were observed during these two months . Although they differ in some aspects , one feature remains similar in each . Amplified upper-level flow , cut-off lows and Rossby wave breaking were present in the majority of cases , promoting vertical motion and rainfall over the usually dry northern parts of the country . Rossby wave breaking events tend to cluster over a particular region for several days or weeks . This was certainly the case over the usually dry months of August and September . Was the frequency and amplitude of these events unusual ? This is still very much an open question . There is much uncertainty about what causes and contributes to this clustering and repetition . Although the impacts during the wet August and September were limited to mostly minor flooding , repeated Rossby wave breaking can drive extreme weather such as major flooding ( e . g . March 2022 , Barnes et al ., 2023 ) or extended heatwaves ( e . g . Black Saturday heatwave in 2009 , Parker et al ., 2014 ). This is a major source of uncertainty about our changing climate , and a problem that the atmospheric science community will no doubt need to tackle .

Figure 3 : Synoptic dynamics of rainfall over northwestern Australia between 21-25 September 2024 . Key components of the rainfall event are highlighted with arrows . Data source : European Centre for Medium-Range Weather Forecasts ( ECMWF ) reanalysis ( ERA5 )