Onset of the Australian summer monsoon during 1986/87 is examined using the gridded tropospheric winds and surface pressure data from the Australian Bureau of Meteorology Tropical Region Analyses. Dramatic and sudden circulation changes on the synoptic and larger scale are known to occur at onset of the ,monsoon. The enhanced observing network during AMEX 1987 afforded an excellent opportunity to study these changes as the onset occurred during the experiment. During November 1986 through early January 1987 anomalous convection and low-level westerly winds persisted in the central equatorial Pacific associated with an El Niño/Southern Oscillation event. This anomalous convection forced a large-scale sinking, dry southeasterly flow over the Australian tropics, inhibiting the onset of the monsoon. In early January the anomalous ENSO convection in the central Pacific diminished rapidly, apparently due to the passage of the downward branch or a 40–50 day oscillation event. Thus the attendant dry sink... Abstract Onset of the Australian summer monsoon during 1986/87 is examined using the gridded tropospheric winds and surface pressure data from the Australian Bureau of Meteorology Tropical Region Analyses. Dramatic and sudden circulation changes on the synoptic and larger scale are known to occur at onset of the ,monsoon. The enhanced observing network during AMEX 1987 afforded an excellent opportunity to study these changes as the onset occurred during the experiment. During November 1986 through early January 1987 anomalous convection and low-level westerly winds persisted in the central equatorial Pacific associated with an El Niño/Southern Oscillation event. This anomalous convection forced a large-scale sinking, dry southeasterly flow over the Australian tropics, inhibiting the onset of the monsoon. In early January the anomalous ENSO convection in the central Pacific diminished rapidly, apparently due to the passage of the downward branch or a 40–50 day oscillation event. Thus the attendant dry sink...