Lee vorticity Production by Large-Scale Tropical Mountain Ranges. Part I: Eastern North Pacific Tropical Cyclogenesis

Abstract

Numerical simulations using the Penn State University/NCAR MM4 model are performed to examine a dry, stably stratified, zonal easterly flow past a large-scale three-dimensional mountain range in a rotating, initially barotropic, atmosphere. Upstream blocking by the mountain range diverts the flow primarily to the south and around the mountain. Conservation of potential vorticity results in the formation of a horizontal jet at low levels south of the mountain. This jet is barotropically unstable and leads to a continuous production of synoptic-scale vorticity maxima, which separate from the mountain and propagate downstream. Simulations using an idealized mountain representative of the Sierra Madre in Mexico imply that this mechanism may be an important contributor to tropical cyclogenesis in the eastern North Pacific Ocean. The wave train produced in the simulations corresponds to waves with 3?7 day periods, which have been identified observationally in the eastern North Pacific region. The sensitivity of this effect to the stability of the basic state and the upstream wind speed is investigated. A simulation using realistic topography is also presented and indicates that the Isthmus of Te-huantepec provides a suitable location for jet formation.