Investigation on the Physical Mechanisms and Variability of the Australian Summer Monsoon

In order to understand the physical mechanisms of the Australian summer monsoon,cyclostationary EOF analysis was conducted on the 23-year (1979-2002) Xie- Arkin pentad summer precipitation data and other key physical variables over the Australian monsoon region,including the tropical and southern Indian and Pacific Oceans. The first mode of precipitation represents the El Niño signal, the second mode is the seasonal cycle, the third mode describes thephase transition of ENSO between El Niño and La Niña, and the fourth and fifth modes represent separate components of the Madden-Julian oscillations. All five modes together explain about1/3 of the total variance of the pentad precipitation data. The physical mechanisms of the seasonal cycle, which explains the "mean" seasonal evolution of the Australian summermonsoon in the absence of other external physical mechanisms, are identified. The onset and termination mechanisms of the Australian summer monsoon are stronglyassociated with the surface temperature changes over the Australian continent and the resulting sea level pressure (SLP) changes. Almost a month prior to the monsoon onset, the surfacetemperature over northern Australia increases and consequently SLP decreases. This initiates an cyclonic circulation over Australia. The increased upward motion induced by the surfacewarming together with the anomalous cyclonic circulation results in the increased lower-level moisture convergence over the interior of Australia. This increases the amount of precipitationover northern Australia and eventually starts the monsoon. In conjunction with the anomalous cyclonic circulation, zonal wind anomalies over northern Australia change from easterly towesterly. During the termination stage, physical conditions over Australia are reversed. Variability of the Australian monsoon onset, termination, and strength was alsoinvestigated in terms of the first five major modes of monsoon precipitation found via CSEOF analysis. Whereas the seasonal cycle defines the fixed onset (~January 5 ± 5 days) and termination (~March 5 ± 5 days) dates, the presence of other modes alters these dates. The impact of each mode was investigated after each mode was added to or subtracted from the meanseasonal cycle, each having a strength corresponding to one standard deviation of its variability. It is shown that the contribution of each mode to the monsoon precipitation is geographicallycomplex and varies significantly throughout the monsoon period. One striking feature is that the impact of each mode is highly asymmetric with respect to its phase. The negative modes generally affect more significantly the onset, the termination, and the amount of monsoonprecipitation. Although each of the four modes make a unique and tangible contribution, the ENSO mode contributes most significantly to the overall mean and variance of the monsoonprecipitation variability. While the positive ENSO (El Niño) mode does not seriously alter the onset and termination times, the negative ENSO (La Niña) mode prolongs the duration of the Australian summer monsoon significantly. Although the contribution to the overall mean is small, the two MJO modes are the most dominating factor controlling the onset and terminationtimes of the Australian summer monsoon.