This work focuses on a relatively rare phenomenon in the western Pacific: the monsoon gyre. These huge lower-tropospheric cyclonic disturbances occur roughly once every two years (Lander 1994). They are typically the locus of multiple tropical cyclone formations. In this work, we will describe the history of two major monsoon gyres, from 1988 and 1991. Of interest are (i) the conditions under which they form; (ii) their subsequent movement and evolution; and (iii) their eventual decay or merger into the monsoon trough. Preliminary work has confirmed the influence of middle latitude forcing (Lander 1994) in the development of these gyres. In particular, repeated penetration of mobile midlatitude troughs equatorward of 30°N preceded development of the gyres. Nevertheless, the mechanisms by which they form and grow remain uncertain. Both gyres continued to expand in size and grow in amplitude well after the midlatitude forcing had ceased. The composite vertical structure of the gyres and its evolution will be described.

Although the gyres are impossible to miss on individual maps, and can last for weeks, we have found that no real definition of what constitutes a monsoon gyre has been provided in the literature. Some monsoon gyres show extremely clearly in lowpass-filtered fields (OLR and vorticity), while others described as monsoon gyres do not. The definition of a monsoon gyre will also be addressed.