Orographic Influence on the Synoptic-Scale Circulations Associated with the Genesis of Hurricane Guillermo (1991)

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Abstract
The early stages of tropical cyclogenesis in the eastern Pacific Ocean are investigated in this case study, which is focused on the development of the initial circulation that eventually intensified into Hurricane Guillermo (1991). The authors document the synoptic and mesoscale winds from upper-air soundings, satellite imagery, and a gridded analysis. In addition, flight-level observations taken during the Tropical Experiment in Mexico are used. These winds reveal that, prior to the formation of the tropical cyclone, an easterly wave moved over the Caribbean Sea and that the initial circulation developed while the easterly wave was located over the central Caribbean, east of the mountains in Central America. Numerical simulations with the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model are used to examine the details of the structure of the flow that resulted in the formation of the eastern Pacific circulation. In these simulations, the model is initialized with the European Centre for Medium-Range Weather Forecasts analyses. The simulated fields indicate that the low-level flow associated with the easterly wave is blocked, resulting in an easterly jet south of the mountains of Central America. In addition, a northeasterly jet and accompanying vorticity maximum formed over the Gulf of Tehuantepec. The combination of these two jets along with the intertropical convergence zone produces a closed circulation. The position, structure, and intensity of this circulation are in agreement with the observations of the circulation that intensified into Hurricane Guillermo.

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