A Midlatitude Influence on Australian Monsoon Bursts

The initiation of northern Australian monsoon rainfall bursts is accompanied by an increase in cyclonic circulation in the monsoon region. This study shows that the change in circulation at the start of the composite rainfall burst is predominantly influenced by midlatitude frontlike features. By exploiting the relationship between circulation tendency and the convergence of absolute vorticity flux, the circulation changes accompanying the initiation of Australian monsoon bursts is investigated. Moisture flux convergence is found to be proportional to the circulation changes in the monsoon region. Using a composite analysis it is shown that absolute vorticity fluxes through the southern boundary are by far the most important influence on monsoon burst circulation changes, with only one-third of events more closely related to other influences including the Madden–Julian oscillation. This is shown to be true throughout the wet season.