Alternative mechanisms of tropical cyclone formation in the Eastern North Pacific



It has been known for some time that the subtropical Eastern North Pacific Ocean, just west of the coast of Mexico experiences a high incidence of tropical cylone formation. One currently accepted mechanism for tropical cyclone initiation in the Eastern Pacific involves easterly waves that originate over Africa and propagate across the Atlantic. Recent investigations indicate that other mechanisms, such as the interaction of the large scale flow with the topography or the formation of mesoscale convective systems (MCSs), may also be important in initiating tropical cyclones in this region. Infrared satellite imagery is used to illustrate specific examples of the alternative mechanisms described above that occurred during the 1989 hurricane season. Hurricane Lorena and Tropical Storm Manuel appear to have originated from a single region of disturbed weather associated with flow over the Sierra Madres and Tropical Storm Priscilla appears to have formed from an MCS that first appeared over Mexico. Conventional sounding and wind data are used to demostrate that the incident flow present was favorable for the generation of orographic cyclones during the formation of Lorena and Manuel and that the environment was conditioned for MCS formation prior to the development of Priscilla.

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