Western North Pacific Monsoon Depression Formation and Structure

The monsoon depression is one synoptic-scale feature favorable to tropical formation over the western North Pacific during the summer monsoon. Little is known about the processes that influence the horizontal and vertical structure of the monsoon depression. Numerical studies will be conducted to determine the conditions required for formation and the parameters that determine its structure.

Since detailed aircraft observations were made in the monsoon depression that preceded the formation of Typhoon Robyn (1993), this monsoon depression will be the prototypical case. However, a survey of the monsoon depressions that were associated with tropical cyclone formations during the past four years will be compared with the prototypical case to provide the range of environmental conditions that precede monsoon depression formation. The environmental conditions that existed prior to the pre-Typhoon Robyn monsoon depression included a region of equatorial westerlies approaching the formation region on the equatorward side, and a region of stronger tradewind easterlies approaching on the poleward side. Thus, a region of zonally-uniform tradewind easterlies and a region of equatorial westerlies will be separately simulated in the MM5, model to illustrate a monsoon trough-like effect. These cases are similar to the barotropic model simulations of the monsoon trough by Guinn and Schubert (1993) except with a baroclinic model and diabatic processes. Zonally-varying tradewind easterlies and equatorial westerlies will be simulated to illustrate the additional effect of zonal convergence. Then zonally-varying tradewind easterlies and equatorial westerlies at varying meridional separations and east-west distances between the maxima of the two flows will be simulated to illustrate the role each plays in the development and structure of the monsoon depression. The role of subsidence associated with the convection embedded in the flow maxima in leading to falling surface pressures and possible flow enhancement on the monsoon depression scale will be diagnosed. The relative roles of barotropic, baroclinic, and diabatic processes will be diagnosed with a potential vorticity budget. This sequence of simulations is anticipated to provide a description of the differences in structure from a monsoon trough environment to a monsoon depression environment for tropical cyclone formation.

Sensitivity studies based on the ranges of variables from the observational study will provide more insight on the physical processes of monsoon depression formation as well as structure variability. For those environmental conditions that lead to realistic monsoon depression structures, future studies can be conducted to determine the monsoon depression's role in tropical cyclone formation.