A case study of orographic cyclogenesis over South America

Abstract

This study presents a synoptic and dynamic analysis of a cyclogenesis event that occurred between July 8 and 9, 1996, over Uruguay, in the region of maximum frequency of cyclogenesis as detected in previous studies. The study is based on isobaric and isentropic analysis of meteorological variables, and also, on potential vorticity perspective. The development of the cyclone occurred as a pre-existent midtropospheric cut-off low, over the Pacific Ocean, which propagated crossing over the Andes at the same time as a strong thermal advection was observed to the west of the mountain ridge. The synoptic analysis of this case revealed that this event can be “classified” as a lee cyclogenesis, since the existence of the mountain barrier was fundamental for the intensification of the surface baroclinicity, which, in turn, led to the formation of the extratropical cyclone. Also, the Andes defined the vertical tilt of the system to the west, favoring the cyclonic vorticity advection at higher levels above the warm thermal advection at low levels, and it defined the final baroclinic configuration that fed the system in its greater development stage. By using potential vorticity inversion, it was possible to evaluate quantitatively the relative contributions of upper- and low-level potential vorticity, as well as bottom temperature anomalies. It was observed that the bottom temperature anomaly was important in the initial development stages, contributing to almost 100% of the total circulation. As the surface cyclone deepened, the contributions of low- and upper- level anomalies became greater. In the stage of maximum development of the system, a phase-locking at low and upper levels seems to occur, where the upper-level advection of cyclonic vorticity, inducing northeasterly flow at surface, had the greater contribution.