E. Linacre and B. Geerts
It has long been known that dry spells as well as wet spells occur in the north Australian monsoon season (1). The wet spells usually last longer than the dry spells, which last from a few days to two weeks. Wet spells in northern Australia are associated with westerly winds at the surface, and easterly wind anomalies aloft. Many wet spells seem to originate as cold air outbreaks over Southeast Asia, where it is winter.
A synoptic analysis of the evolution leading up to a wet spell was performed for a large number of cases during 1980-1992 (2). The surges are initially evident over one of three regions: the South China Sea, the northwest Pacific (near Japan) and Indochina. Five days later there is an increase of northerly surface wind over Indonesia. The onset of a wet westerly monsoon wind over the Top End of Australia then occurs another 2-5 days later, usually leading to high rainfalls there.
Spectral analysis of rainfall in Darwin also reveals a 40-day cycle, probably related to the Madden and Julian Oscillation.
(1) McBride, J. 1988. The Australian summer monsoon. In: Reviews of monsoon meteorology (Ed. Krishnamurti), 203-231.
(2) Suppiah, R. and X. Wu 1998. Surges, cross-equatorial flows and their links with the Australian summer monsoon circulation and rainfall. Aust. Meteor. Mag. (in press).