The monsoon trough is a persistent lower tropospheric feature in the western North Pacific during the Northern Hemisphere summer. The location of this feature can be important for the prediction of tropical cyclogenesis, as the monsoon trough represents a potential source of low level vorticity necessary for tropical cyclogenesis. Furthermore, Lander (1996) has shown that tropical cyclone tracks are significantly influenced by the strength and orientation of the monsoon trough. Considering the above, it seems reasonable that understanding the mechanisms controlling the evolution of the monsoon trough can be crucial in providing some insight to tropical cyclone formation and motion.

Towards this end, ECMWF gridded analyses have been utilized to examine the evolution and occasional reversal in orientation of the monsoon trough. A twenty-day low-pass filter has been applied to the relative vorticity fields, in order to highlight the location and strength of the monsoon trough. Outgoing long-wave radiation (OLR) data are used in conjunction with the ECMWF grids as an indicator of convective activity. Preliminary results from the examination of a few periods indicates that a reverse monsoon trough develops when mid-latitude upper-tropospheric troughs repeatedly penetrate into the subtropics. Tropical cyclogenesis is enhanced along the axis of the reverse monsoon trough at peak intensity. In the talk, the development and breakdown of the reverse monsoon trough, and the role of middle latitude forcing, will be examined.