A small area of the eastern Pacific, to the west of the Sierra Madre Mountains of Mexico, is a favored location for tropical cy- clogenesis. Currently accepted theories of tropical cyclone form- ation involve the preexistence of a cyclonic disturbance in the prevailing easterly tropical flow, over an adequately warm sea sur- face, as a necessary (but not sufficent) condition for the incep- tion of hurricanes. Zehnder and Gall propose to continue to explore their hypothesis that flow over the mountains of central Mexico provides, under suitable angles of attack, cyclonic distortion of the flow downstream of the orographic obstacle . Their previous research has shown that certain standing and slowly traveling waves, interacting with the topography, are more active in years with above average numbers of tropical cyclones in this region. They have also demonstrated theoretically that such planetary-scale waves can generate a steady lee-side trough of realistically small scale and trains of secondary traveling synoptic-scale waves which may serve as the initial locations of tropical cyclones. They propose to employ numerical model simulations as well as further analyses of observational data sets, including one to be obtained in the area during the summer of 1991, in their continuation of the research. The question as to why tropical cyclones like to form off the west coast of Mexico is not only interesting for its own sake. Its answer should also shed light on tropical cyclogenesis in general, and could lead to improvements in hurricane forecasting.
