Tropical cyclone motion due to environmental interactions represented by empirical orthogonal functions of the vorticity fields.

Abstract

Tropical cyclone motion influenced by adjacent synoptic features is examined with relative vorticity fields at 700, 400 and 250 mb computed from operationally-analyzed wind fields. When an empirical orthogonal function (EOF) method is used to map the vorticity onto spatial grids, a smoother but more dependable depiction of the vorticity dynamics is provided than by the original fields. The effect of five processes that contribute to motion relative to the steering flow during periods of interaction with adjacent circulations are evaluated for their contribution to binary rotation and to changing the separation distance between interacting circulations. This vorticity dynamics approach indicates tropical cyclones interact with various features at different levels. The Fujiwhara effect is most evident with stronger interaction events. Only the convergence effect of one tropical cyclone circulation on another tropical cyclone can explain the cases with decreasing separations. By contrast, the advection by the tropical cyclone circulation of the vorticity gradient of the other cyclone could not explain the decreasing separations. The Beta effect depends on the orientation of two interacting systems of different sizes, but the environmental vorticity gradient is not just due to earth vorticity in these interacting cases. The effects of environmental shear strongly depend on the orientation of the ITCZ and geographical orientation of the two cyclones