Variability of the Australian Summer Monsoon at Darwin: 1957–1992

The variability of the Australian summer monsoon is reexamined using data covering 35 monsoon seasons. A new, easily applied objective definition of active and break phases of the monsoon, based solely on the zonal wind at Darwin, is proposed. Attempts to define “wet westerly” onsets are shown to be misleading, since no clear relationship is found between westerly winds and rainfall, on the timescales associated with the transition between active and break phases. The resulting dates of monsoon onset at Darwin differ from those reported in some recent studies, resulting in significantly different relationships with the Southern Oscillation. In particular, the date of monsoon onset is shown to be significantly related to, and hence predicted from, prior values of the Southern Oscillation index. Also in contrast to a number of recent studies that have highlighted the so-called 40–50-day oscillation in the Australian summer monsoon, no dominant timescales are found in the length of the active periods or in the recurrence time between active phases.