Abstract
Systematic changes, since the beginning of the 20th century, in average and extreme Australian rainfall and temperatures indicate that Southern Australian climate has undergone regime transitions into a drier and warmer state. South-west Western Australia (SWWA) experienced the most dramatic drying trend with average streamflow into Perth dams, in the last decade, just 20 % of that before the 1960s and extreme, decile 10, rainfall reduced to near zero. In south-eastern Australia (SEA) systematic decreases in average and extreme cool season rainfall became evident in the late 1990s with a halving of the area experiencing average decile 10 rainfall in the early 21st century compared with that for the 20th century. The shift in annual surface temperatures over SWWA and SEA, and indeed for Australia as a whole, has occurred primarily over the last 20 years with the percentage area experiencing extreme maximum temperatures in decile 10 increasing to an average of more than 45 % since the start of the 21st century compared with less than 3 % for the 20th century mean. Average maximum temperatures have also increased by circa 1 °C for SWWA and SEA over the last 20 years. The climate changes are associated with atmospheric circulation shifts and are indicative of second order regime transitions, apart from extreme temperatures for which the dramatic increases are suggestive of first order transitions.
Highlights
Over the last seventy years, since the middle of the 20th century, aspects of Australian climate, rainfall and temperatures, have undergone significant changes (CSIRO and Bureau of Meteorology, 2015; Alexander and Arblaster, 2017; Dey et al, 2019; Bureau of Meteorology and CSIRO, 2020; Osbrough and Frederiksen, 2021, hereafter OF21, review the literature)
Our particular interest in this article is whether the changes that have occurred in Australian climate and climate extremes over the last seventy years are indicative of regime transitions in a noisy environment
We have focussed on the changes over Southern Australia in rainfall, temperatures and extremes, and associated circulation features since the early 20th century
Summary
Over the last seventy years, since the middle of the 20th century, aspects of Australian climate, rainfall and temperatures, have undergone significant changes (CSIRO and Bureau of Meteorology, 2015; Alexander and Arblaster, 2017; Dey et al, 2019; Bureau of Meteorology and CSIRO, 2020; Osbrough and Frederiksen, 2021, hereafter OF21, review the literature). Further developments in the role of regime transitions and tipping points in various aspects of the climate system, including under global warming, have been considered by Franzke et al (2015); Freitas et al (2015); Jones and Ricketts (2017); Dijkstra (2019); Lenton (2019); Kypke et al (2020); Yan et al (2020); Fabiano et al (2021); and Australian Academy of Science (2021). It is clear from all the studies mentioned in this Introduction that there are dynamical and thermodynamical processes of the climate system that can result in regime transitions.
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