Recent seasonal and long-term changes in southern Australian frost occurrence

As part of part of a special issue on natural hazards, this paper explores recent changes in Australian minimum temperature extremes. Using minimum temperature data from the 112 observation locations making up the Australian Climate Observations Reference Network – Surface Air Temperature (ACORN-SAT) data set, as well as the Scientific Information for Land Owners (SILO) minimum temperature gridded data surface, we analyse and map trends in extreme minimum temperature indices across southern Australia at seasonal, annual and multi-decadal timeframes since 1960. Our analyses highlights that across southern Australia, despite a warming trend of 0.17 °C per decade since 1960 in the mean annual minimum temperature, there exist regions of localised cooling as well as a much broader spatially-coherent pattern of increasing “frost season” length. Our analysis identifies that the “frost season length” has, across the whole southern portion of Australia, increased on average by 26 days (at 2014) compared with the 1960 to 1990 long term mean. Some areas of south eastern Australia now experience their last frost an average four weeks later than in the 1960s (i.e. mean date of last frost for the period 1960 to 1970 was 19 September versus 23 October for the period 2000 to 2014). Over isolated portions of southern Australia (i.e. northern Victoria and southern New South Wales), the annual frequency of frost events occurring after August has increased by as much as 4 events per year over the last decade, with localised increases in the occurrence of consecutive frost days also observed. This analysis builds upon earlier more localised trend analyses work by these authors (Crimp et al. 2015), as well as a growing body of international research, highlighting a complex spatio-temporal pattern of temperature change despite a general pattern of annual warming in minimum temperatures.