A Composite Study of Onset of the Australian Summer Monsoon

Onset of the Australian summer monsoon is identified each year (1957–87) using the wind and rainfall record at Darwin. Onset is defined as the first occurrence of wet, 850 mb westerly winds. Composites of atmospheric fields at stations in and about the Australian tropics are constructed relative to the onset date at Darwin. The composite onset is accompanied by the development of a convectively driven, baroclinic circulation over northern Australia. Upper tropospheric easterlies expand about the equator and the subtropical jet shifts poleward at onset. This behavior is interpreted as a transient southerly shift of the local Hadley circulation concurrent with the development of an upper level anticyclone over northern Australia. The composite onset coincides with the arrival of an eastward propagating convective anomaly. The anomaly originates in the southern Indian Ocean, propagates eastward at 5 m s−1 and is detectable as far east as the date line. An eastward propagating zonal wind anomaly also is detectable at tropical stations east and west of Darwin. These features are indicative of the 40–50 day oscillation and thus the composite onset is concluded to coincide with the traversal of the oscillation across northern Australia. The composite onset is further shown to coincide with the first occurrence of the convectively active 40–50 day oscillation during each southern summer.