The synoptics of cold events in Melbourne and Perth, Australia

Days of unseasonably low daily maximum temperatures (expressed in terms of deviation from the relevant monthly mean) in Melbourne and Perth, Australia have been studied for the period January 1972 - June 1991. One of our aims was to determine whether the synoptic situation which set up cold events were significantly different for the two cities, given that they are located on opposite sides of the continent.

The ten coldest summer and ten coldest winter days for the two cities were chosen for analysis. Using back trajectories analysis it is found that air of recent Antarctic origin is not a feature common to the majority of the cold events (CEs) studied. Synoptic analysis led us identify a number of synoptic types which were associated with CEs. The most common of these was the 'classic' pattern which is associated with the passage of a strong surface front and the influx higher latitude air behind the front. For Melbourne 4 of the CEs in summer were associated with this type, while 6 were so-connected in winter. For Perth the representation of the 'classic' type was greater, accounting for 8 and 7 events in the two seasons. Part of the reason for these differences between the cities is due to the differing natures of the regional surface pressure climatologies upon which the synoptic patters are superimposed. Having said this, the structure of the 'classic' patterns for the two cities (when longitudinal translation was performed) were remarkably similar.