A Diagnostic Analysis of Two Intense Monsoon Depressions over Australia

Abstract

Analyses of mean sea level pressure, wind, temperature and dewpoint am used to study the life cycles of two intense, heavy-rain-producing monsoon depressions over northern Australia. Two aspects are considered: (a) the large forcing, using both synoptic flow field changes and angular momentum budgets, and (b) the role of convective and stratiform clouds using kinematic and thermodynamic budgets. For each situation, the Northern Hemisphere circulation becomes favorable well prior to genesis. The short-term trigger for development is the strengthening of the Southern Hemisphere subtropical ridge at the surface and an amplifying upper-level trough and subtropical jetstreak to the southwest of the formation point. The outer region structure of these monsoon depressions is remarkably similar to that of a tropical cyclone, even though the systems develop over land. During development, maximum convective heating occurs at middle levels and within a region of already high cyclonic vorticity. Evidence suggests that the cloud population is mostly comprised of deep cumulonimbus clouds, middle-level stratiform cloud and shallow cumulus. The physical significance of these findings is discussed.