Cut-off low pressure systems over southern Australia: climatology and case study

Cut-off low-pressure systems are among the most important weather systems that affect southern Australia. They can bring moderate to heavy rainfall over large areas and as such are extremely important to the Australian agricultural industry. There are two components of this study. First, a statistical analysis is carried out on 14 years of archived data (1983–1996) to produce a climatology. The emphasis in the climatology is on the frequency, duration and tracks (location and movement) of cut-off lows in southern Australia. Second is a case study of the cut-off low of late August 1997, in which the distinctive features of cut-off lows are highlighted. The results from the climatology reveal the following: there is a maximum in the frequency of cut-off lows in southern Australia during May–October; the most active area for the genesis of cut-off lows in southern Australia is the southwestern region; and the vast majority of cut-off lows move either eastward or southeastward. The case study is aimed at revealing details of the formation and structure of one particular ‘classical’ example of a cut-off low. It is found that there is strong baroclinicity in the mid-upper troposphere, favourable for the intensification of the low. The synoptic analysis also shows that the moisture supply for the precipitation was transported from the tropical ocean (the Coral Sea) to higher latitude regions by a low level jet (LLJ), forming a narrow moist tongue which interacted with the cut-off low and its associated cold front. The cut-off low was located at the right-front quadrant of an upper level jet (ULJ), which is a favourable position for low system development.