An investigation of some unexpected frost day increases in southern Australia

Abstract

Understanding changes in frost events is vital given their impacts on crops. This study examines monthly frost day trends in southern Australia, using Australian homogeneous temperature stations. Century-scale annual frost day trends show decreases consistent with expectations in a warming climate. The past 32 years however exhibit more complex and seemingly contradictory changes. At some locations increases in annual number of frost days are observed, while other lo- cations show decreases more consistent with the longer term trend. Assessing monthly trends, we nd a consistent increase in frost days at all stations in south- east Australia in late autumn (May). The trends are downwards during the winter months (June, July and August) indicating a decrease in the number of frost days. In early spring (August and September), the sign of the trend varies substantially between stations. Similarly, southwest Western Australia has experienced in- creases in the monthly number of frost days. However, in contrast to southeast Australia, the increases are observed throughout the second half of the cold sea- son. We examine the relationship with the concurrent recent drying since the 1990s in southeast Australia and since the 1970s in southwest Western Australia. In dry periods during the winter half year, clear-sky conditions are expected to occur more frequently, leading to larger diurnal temperature ranges and colder night-time tem- peratures. This study provides further evidence that southeastern Australian frost day increases are linked to concurrent rainfall declines by analysing the relationship between number of frost days and number of precipitation days.