Abstract
Most of the rainfall in southern Australia is associated with cyclones, cold fronts, and thunderstorms, and cases when these weather systems co-occur are particularly likely to cause extreme rainfall. Rainfall declines in some parts of southern Australia during the cool half of the year in recent decades have previously been attributed to decreases in the rainfall from fronts and/or cyclones, while thunderstorm-related rainfall has been observed to increase, particularly in the warm half of the year. However, the co-occurrence of these systems, particularly the co-occurrence of cyclones or fronts with thunderstorms, can be very important for rainfall in some areas, particularly heavy rainfall, and changes in the frequency of these combined weather systems have not been previously assessed. In this paper we show that the majority of the observed cool season rainfall decline between 1979–1996 and 1997–2015 in southeast Australia is associated with a decrease in the frequency of fronts and cyclones that produce rainfall, while there has simultaneously been an increase in the frequency of cold fronts and thunderstorms that do not produce rainfall in some regions. Thunderstorm rainfall has increased in much of southern Australia, particularly during the warm half of the year, including an increase in rainfall where a thunderstorm environment occurs at the same time as a cyclone or front.
Highlights
Southern Australia is located at the equatorward edge of the midlatitude storm tracks, where annual rainfall is dominated by the passage of rain-bearing fronts and cut off lows, during the cooler months of the year (Pook et al 2011, 2014; Risbey et al 2009)
This paper focuses on results for Australia south of 25° S, as the methods used for identifying cyclones and fronts are more effective at identifying midlatitude weather systems than they are in the tropics (Pepler et al 2020)
As well as showing spatial changes, we show changes for four key regions (Fig. 1): southwestern Western Australia (SWWA) and southeast Australia (SEA), which have experienced long-term cool season rainfall declines (Hope et al 2006; Rauniyar and Power 2020; Timbal et al 2006, 2010); the eastern seaboard (ESB), where rainfall influences and trends differ from elsewhere in SEA (Dowdy et al 2015; Pepler et al 2014; Timbal 2010), and Tasmania, which is located in the main midlatitude storm tracks with distinctly different projected future changes (Hope et al 2015)
Summary
Southern Australia is located at the equatorward edge of the midlatitude storm tracks, where annual rainfall is dominated by the passage of rain-bearing fronts and cut off lows, during the cooler months of the year (Pook et al 2011, 2014; Risbey et al 2009). Are no statistically significant trends in frontal frequency in either western Australia (Hope et al 2014) or the Southern Hemisphere (Rudeva and Simmonds 2015) While these studies have provided useful insights into the synoptic systems influencing rainfall change, they typically classified events into either a front or a cut-off low, and have not considered cases when these systems co-occur, which can lead to more significant impacts (Dowdy and Catto 2017). The differing role of weather systems in southern Australian rainfall between 1979–1996 and These four datasets are combined to create a 6-hourly, 0.75° resolution grid across Australia that classifies each point into one of eight categories (Table 1), with a focus on the intersections between cyclones, fronts and thunderstorms following the approach of Dowdy and Catto (2017). The gridded rainfall data is considered adequate for examining moderate rainfall extremes of this nature (King et al 2013)
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