Propagation of a Tropical Cyclone in a Meridionally Varying Zonal Flow: An Energetics Analysis

Abstract

Tropical cyclone propagation (the beta drift) is driven by a secondary circulation associated with axially asymmetric gyres (beta gyres) in the vicinity of the cyclone center. In the presence of the beta effect, the environmental flow may interact with the symmetric circulation and beta gyres of the cyclone, affecting the development of the gyres and thereby the cyclone propagation. An energetics analysis is carried out to elucidate the development mechanism of the beta gyres and to explain variations in propagation speed of a barotropic cyclone embedded in a meridionally varying zonal flow on a beta plane. Two types of zonal flows are considered: one with a constant meridional shear resembling those in the vicinity of a subtropical ridge or a monsoon trough, and the other with a constant relative vorticity gradient as in the vicinity of an easterly (westerly) jet. Zonal flow with a constant meridional shear changes the generation rate of the gyre kinetic energy through an exchange of energy directly with the gyres. The momentum flux associated with gyres acting on the meridional shear of zonal flow accounts for this energy conversion process. Zonal flow with an anticyclonic (cyclonic) shear feeds (extracts) kinetic energy to (from) the gyres. The magnitude of this energy conversion is proportional to the strength of the meridional shear and the gyre intensity. As a result, the gyres are stronger and the beta drift is faster near a subtropical ridge (anticyclonic shear) than within a monsoon trough (cyclonic shear). Zonal flow with a constant relative vorticity gradient affects gyre intensity via two processes that have opposing effects. A southward vorticity gradient, on the one hand, weakens the gyres by reducing the energy conversion rate from symmetric circulation to gyres; on the other hand, it enhances the gyres by indirectly feeding energy to the symmetric circulation, whose strengthening in turn accelerates the energy conversion from symmetric circulation to gyres. The effect of the second process tends to eventually become dominant.