Milton Speer

Big swell combined with a high tide caused damage to Narrabeen beach near Mactier and Wetherill Sts in northern Sydney , NSW . Photo : Nick Moir 28 July 2020 ; Sydney Morning Herald .

East Coast Low pressure systems ( ECLs ) that form near the southeast coast of Australia can cause heavy rain leading to flooding ; destructive winds leading to infrastructure damage ; and storm / tidal surges , leading to coastal erosion . However , they are also responsible for heavy coastal and adjacent inland catchment rainfall events , providing much-needed dam inflows .

With global warming accelerating in recent decades ( Bureau of Meteorology 2020 ), the observationally-based ECL database ( Speer et al . 2009 ) was in urgent need of updating to investigate trends in these systems . By extending the database from 2006 to 2019 for April – September and confirming consistency for the complete database 1970 – 2019 , new research ( Speer et al . 2021 ) indicates an increase in ECLs since the mid – 1990s with a trend for them to form further south and east away from the coast .

How do they form ?

Australian ECLs form mostly in the cooler months , April – September in the subtropical latitudes between 25 ^o S and 40 ^o S . However , their development is influenced primarily by changes in structure of mid-latitude frontal systems throughout the atmosphere that move from west to east , south of the Australian continent . A recent example in Figure 1 shows the 10am sea level pressure chart on 21 May 2020 ( left panel ) with a frontal system that has developed into a low pressure system ( an ECL ) 24 hours later ( right panel ), which is cut-off from the westerly winds south of the continent . When the centre of the low pressure drops 10 hPa or more in 24 hours at the latitudes of NSW , as in this case , ( 1013 to 1001 hPa ), it is referred to as a ‘ bomb ’ low pressure system in accordance with the criteria of Sanders and Gyakum ( 1980 ). The database of ECLs is defined by the area within the solid black lines in the right panel .

Once an ECL forms , atmospheric moisture evaporated from the Tasman Sea , especially from the warm East Australian Current ( Godfrey et al . 1980 ) adjacent to the southeast Queensland and NSW coasts , can fuel ECLs and provide much needed coastal rainfall while at the same time produce coastal flooding , destructive winds and coastal erosion from large waves . ECLs form in either westerly wind regimes , like the example in Figure 1 , or an easterly wind regime which is typically identified as an easterly trough ( Speer et al . 2009 ) close to the surface . The increase in frequency of ECLs recently is due to an increase in those that form in a westerly wind regime .

Figure 1 . 10am sea level pressure chart on 21 May 2020 ( left panel ) with a frontal system that has developed into a low pressure system ( an ECL ), 24 hours later ( right panel ).