On a mechanism for orographic triggering of tropical cyclones in the Eastern North Pacific

It has been observed that the subtropical Eastern Pacific Ocean, just west of the Sierra Madre Mountains in central Mexico, is a favored location for the initiation of tropical cyclones, in spite of the fact that sea surface temperatures are not anomalous in the region. Currently accepted theories of tropical cyclogenesis require the presence of a pre-existing vortex located above a suitably warm sea surface. The distribution of tropical cyclogenesis off Mexico suggests that the initial disturbances may be orographically generated. We employ a two-dimensional, adiabatic, inviscid, Boussinesq, quasi-geostrophic model of flow over topography on a β-plane to examine the generation of regions of cyclonic vorticity in the subtropics by an incident flow which may approach the mountain at a specified angle. For incident wind configurations that correspond to the climatological northeast tradewinds, the orographic disturbance consists of an evanescent mountain anticyclone. For certain southeasterly flows, corresponding to the presence of large scale standing or slowly moving waves in the region, the orographic disturbance consists of a series of cyclonic and anticyclonic vorticity centres located in the southwest quadrant of the model atmosphere. These results are consistent with the observed behaviour of tropical cyclogenesis off of the coast of Mexico.