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 sinking motion over the Australian tropics diminished, and monsoon onset subsequently occurred in the second week of January.
Onset in the Australian region appears to have been triggered by the arrival of a westward moving equatorial wave disturbance. This synoptic scale disturbance appears to have originated in the ENSO Westerlies in the central Pacific just before their temporary demise in early January. It propagated at least 5000 km across the Pacific into the Australian region. Synoptic-scale low-level moist northerlies due to the wave drastically increased the low level humidity while the wave's convergence appears to have initiated organized convection over the Australian region. Subsequently, in a matter of one to two days, monsoon convection and low-level westerlies developed over a longitudinal span of ∼40°. The synoptic-scale wave disturbances also appear to be associated with formation of tropical cyclones in the region soon after onset. The subsequent expansion of the monsoon westerlies after onset results from a combination of an amplification of the synoptic-scale wave disturbance in the Doppler-shifted group velocity direction (eastward) and by the passage of a 40-50 day oscillation event
From a thermodynamic viewpoint, substantial but unrealizable conditional instability existed over the Australian tropics prior to the monsoon onset. Because of the lack of low-level saturation (due to the dry sinking circulation produced by the ENSO anomaly) substantial lifting was required to achieve buoyancy prior to onset. The onset process appears to be one where initial development of convection induced by synoptic convergence and moisture advection cools and further moistens the lower troposphere and thereby reduces the degree of lifting required to achieve buoyancy. Onset occurs as a dramatic blowup of convection over a large longitudinal span (∼40°) when the lifting required for buoyancy achieves a minimum but prior to the loss of deep conditional instability due to the vertical transport of heat and moisture by the monsoon convection.
