Conceptual Model of Western North Pacific Monsoon Depression Formation

The monsoon depression is one synoptic-scale feature favorable to tropical cyclone formation over the western North Pacific during the summer monsoon. Little is known about the processes that influence the formation and development of the monsoon depression. A conceptual model of formation was developed based on the formations of the pre-Typhoons Robyn (early August 1993) and Man-Yi (early July 2007).

Based on the initial observations of the formation of the pre-Typhoon Man-Yi monsoon depression, three possible mechanisms were hypothesized to contribute to its formation. The first mechanism was cross-equatorial flow (southerly surge), the second was approach of an easterly wave to the eastern end of the monsoon trough, and the third was Rossby wave dispersion from a South China Sea tropical cyclone to another monsoon depression upstream of the developing monsoon depression. Assessment of the relative contributions to formation via each hypothesized mechanism was by means of a wave activity flux study, specifically whether a wave activity flux convergence is occurring in the appropriate region, and if that convergence is associated with a spinup of cyclonic vorticity associated with monsoon depression formation.

A broader observational study is being carried out to test the generality of the conceptual model from the pre-Typhoon Man-Yi monsoon depression formation. This study consists of examining a number of monsoon depressions from 2006 using the GFS reanalysis fields. These monsoon depressions are being categorized based on the primary contributing mechanism of formation. These results will be presented at the conference.

Finally, idealized simulations of the primary mechanism of formation are being carried out to determine the sensitivity to various characteristics of the environment that have been found in the observational studies, and to the physical processes.