On the dynamics of vortex propagation and associated asymmetric gyres

Abstract

Dynamics of vortex propagation and associated asymmetric gyres for a vortex being embedded in a resting environment and environmental flows on a beta-plane is investigated with a FSU regional model and a shallow-water model. In a quiescent environment vortices with different initial horizontal structures follow substantially different tracks, which are associated with the evolution of the beta effect-induced counter-rotating gyres (beta-gyres). The beta-gyres are characterized by intensity variation, azimuthal rotation and outward movement. Kinetic energy for development of beta-gyres primarily comes from symmetric circulation through planetary vorticity advection process (beta-conversion). An analysis of the beta-conversion reveals that (1) beta-gyres develop only when an anticyclonic gyre is located to the east of the cyclonic vortex center in the Northern Hemisphere, and (2) the rate of asymmetric kinetic energy generation increases with increasing relative angular momentum of the symmetric circulation. The counterclockwise rotation of inner beta-gyres which are located near the maximum wind radius is caused by the advection of beta-gyre vorticity by symmetric cyclonic flow. However, the clockwise rotation of outer beta-gyres which are located near the periphery of the cyclonic azimuthal wind is determined by concurrent intensification in mutual advection of the asymmetric and symmetric circulations and weakening in the advection of Earth's vorticity by symmetric circulation. The outward movement of the outer beta-gyres is mainly caused by the advection of symmetric vorticity by beta-gyres relative to the vortex drifting velocity. The presence of environmental flows may change the intensity and orientation of asymmetric gyres whose flow over the vortex center advects symmetric vorticity and determine vortex propagation. The kinetic energy exchange between environmental flow and gyres is a key process responsible for the intensity difference of gyres between constant anticyclonic- and cyclonic-shear cases. The kinetic energy conversion from the environmental flow to gyres results in stronger gyres in constant anticyclonic-shear case, while that from the gyres to environmental flow causes weaker gyres in constant cyclonic-shear case. For parabolic-jet cases in which environmental absolute vorticity gradient is zero or 2β, the small and larger rates of kinetic energy conversion from symmetric flow to gyres create, respectively, initial stronger and weaker gyres in the former and latter cases. The kinetic energy conversion between environmental and symmetric flows is a key process for the intensity evolution of gyres. The environmental flow feeds energy to symmetric flow in the former case causing dramatic development of symmetric vortex. As a result, the rate of asymmetric kinetic energy generation becomes larger causing stronger development of the gyres.