Recent trends in extratropical lows and their rainfall over Australia

Low pressure systems are an important source of rainfall in southern Australia, particularly deep lows that extend from the surface to at least 500 hPa. This paper uses multiple reanalyses to assess long-term trends in lows over the period 1959-2023, and identifies statistically significant decreasing trends in the number of surface low pressure systems near southern Australia during May-October, linked to a decrease in cyclogenesis near south-western Western Australia. Trends in lows at 500 hPa are also negative but weaker than at the surface, and are less consistent between reanalyses owing to less consistent observations through time. The spatial pattern of observed declines during the cool season is consistent with trends using eight CMIP6 models, but global climate models systematically underestimate the magnitude of the observed decline in surface lows. Trends in rainfall associated with lows are also shown, including assessing the sensitivity of trends to the specific years used. Despite well above average numbers of lows and enhanced rainfall during recent La Ni & ntilde;a years 2020-2022, total rainfall from low pressure systems is declining during the cool season in south-east Australia. Trends in rainfall from lows are largest on the east coast, where they explain more than 70% of observed rainfall changes since the 1960s. Extratropical lows and east coast lows cause a large proportion of rainfall in southern Australia during the months May-October. Their frequency varies from year to year, but new data show that they have been becoming less common in recent decades compared to the 1960s. This is contributing to decreases in cool season rainfall in southern Australia. (Image credit: Acacia Pepler.)