The Interaction of Planetary-Scale Tropical Easterly Waves with Topography: A Mechanism for the Initiation of Tropical Cyclones

Abstract

The interaction of a basic state flow consisting of a planetary-scale easterly wave superimposed on a uniform easterly zonal wind with an isolated topographic feature is examined by numerical integration of the shallow water equations in an equatorial beta-plane channel. The topographic parameters are chosen to represent the Sierra Madre Mountains of Mexico. The zonal wavelength and phase speeds of the basic state easterly waves are chosen to correspond with previously published observations. The topographic modification of a slow Rossby mode in which the basic state vorticity maximum is located near 15° latitude is characterized by the generation of a cyclonic vorticity maximum in the lee of the mountain, which extends the entire length of the ridge, and a secondary maximum which propagates downstream. A fast mixed Rossby?gravity mode in which the vorticity maximum is located at the equator interacts less strongly with the mountain, with a lee vorticity maximum that is confined near the southern portion of the ridge and an absence of a secondary vorticity maximum downstream. The relative vorticity maxima generated by the interaction of nonzonal easterly flow may serve as the locations for tropical cyclogenesis and help explain the tropical cyclone outbreaks that are common off the coast of Mexico.