A broad-scale Australian monsoon index (AUSMI) describing multi-time scale variations is defined by using 850 hPa zonal wind averaged over the area (5 S–15 S, 110 E–130 E). This circulation index reflects monsoonal rainfall variability over Northern Australia and maritime continent. The index can be used to depict the seasonal cycle (for instance the onset) and measure the intra-seasonal, inter-annual, and inter-decadal variations of the Australian monsoon. The interannual variation of the Australian monsoon onset determined by the AUSMI agrees well with that derived from the rainfall and winds at Darwin in the previous studies. We found a significant anti-correlation between the monsoon onset date and the seasonal (DJF) mean AUSMI anomalies; ie, an early onset is accompanied by a strong Australian summer monsoon and vice versa. These inter-annual variations are also strongly associated with El Nino-Southern Oscillation (ENSO). In contrast, the retreat dates are not significantly different between the strong and weak Australian summer monsoon years. The AUSMI is useful in monitoring the weather and climate variations of the Australian monsoon and validating the performance of climate models. Copyright 2009 Royal Meteorological Society