The Interaction of Easterly Waves, Orography, and the Intertropical Convergence Zone in the Genesis of Eastern Pacific Tropical Cyclones

The interaction of an idealized easterly wave with an orographic feature representing the Sierra Madre of Mexico and a large-scale, meridionally sheared zonal flow representing the ITCZ in the eastern North Pacific is examined through numerical integration of the shallow water equations on an equatorial beta-plane channel. The model simulations show that as a wave approaches the orography from the east, the relative vorticity in the lee increases due to the wave being modified by the orography. This relative vorticity maximum increases the northward advection of the ITCZ in the lee of the mountain that occurs due to the influence of the mountain anticyclone. The result of this interaction is the development of a localized region of enhanced cyclonic vorticity that moves parallel to the contours of the model orography. There is a minimal increase in the amplitude of the vorticity occuring during interactions between the wave and orography, ITCZ and orography, and wave and ITCZ. The position of the incident wave upstream of the orography at the time that the disturbance in the lee begins to develop and the subsequent motion of the leeside vorticity maximum are in agreement with an observed case of eastern Pacific tropical cyclogenesis, which is also presented.