The Role of Equatorial Rossby Waves in Tropical Cyclogenesis. Part II: Idealized Simulations in a Monsoon Trough Environment

Abstract

Abstract This is the second of two papers examining the role of equatorial Rossby (ER) waves in tropical cyclone (TC) genesis. Based on results from Part I, it is hypothesized that genesis resulting from the circulation of an ER wave alone is uncommon and that the majority of ER wave–related genesis events occur when a sufficiently intense ER wave interacts with a favorable background flow environment. This paper examines this contention by performing a series of simulations in which ER waves are imposed upon idealized background flows. The background flows are designed to resemble a region of a monsoon trough (MT), a flow feature observed at certain times of the year in all of the TC basins, and most dramatically, in the western North Pacific basin. It is believed that ER wave interactions with the MT may speed up the internal breakdown genesis mechanism of the MT, or even result in genesis when the MT is too weak to breakdown from in situ processes alone. The latter scenario is examined here. When just the MT is simulated without the ER wave anomaly fields, the MT remains quasi-steady and TC genesis does not occur. It is only when the ER wave is imposed on the MT that TC genesis is initiated. The results imply that the ER wave–MT interactions produce more TCs than would otherwise occur if no such interactions took place. Results demonstrate that wave breaking of the ER wave is a mechanism by which vorticity is organized on the scale of a TC. This process features a decrease in the initial horizontal scale of the cyclonic gyre of the ER wave to a scale comparable with a TC. This genesis mechanism is sensitive to the magnitude of the background cyclonic vorticity of the MT, as TC genesis is only initiated when the value of the 850-mb relative vorticity of the MT is larger than 2 × 10−5 s−1. This genesis pathway provides a unique interpretation of TC genesis and is compared with previous theories on TC genesis.