Tropical Cyclone Motion: Environmental Interaction Plus a Beta Effect

The dynamics of tropical cyclone motion are investigated by solving for instantaneous motion tendencies using the divergent barotropic vorticity equation on a beta plane. Two methods of solution are presented a direct analytic solution for a constant basic current, and a simple numerical solution for more general conditions. These solutions indicate that cyclone motion can be accurately prescribed by a nonlinear combination of two processes. 1) an interaction between the cyclone and its basic current (the well known steering concept), and 2) an interaction with the Earth's vorticity field which causes a westward deviation from the pure steering flow. The nonlinear manner in which these two processes combine with the effect of asymmetries in the steering current raise some interesting questions on the way in which cyclones of different characteristics interact with their environment, and has implications for tropical cyclone forecasting and the manner in which forecasting techniques are derived.