Climatology of tropical cyclone genesis in the Australian region

Statistics of tropical cyclone origin in the Australian region are presented for the period July 1959‐June 1979. Cyclone genesis is found to occur in preferred areas and to have a high incidence of coastline developments, often with the precursor system forming over land. There is an average of 1.3 occurrences per year of ‘regeneration’ whereby a cyclone develops from a pre‐existing cyclone which has lost intensity after crossing on to the continent. A synoptic analysis of individual developments over five years reveals that 84 per cent of the pre‐cyclone cloud clusters first appear on the gradient level monsoon shear line, whereas 97 per cent are on the shearline at the point of development. Statistical evidence is given for the enhancement of lower and upper level vorticity fields by mid‐latitude systems prior to cyclone genesis, but on a case by case basis the role of these systems is highly variable. The structure of the pre‐cyclone systems for individual cases and composites shows the upper level warm core and large scale cyclonic surrounding circulation in the middle and lower troposphere, known to exist for similar systems in other parts of the world. At upper levels the systems are surrounded by tangential winds in the anticyclonic sense beyond 6 degrees latitude radius.