On the Relative Motion of Binary Tropical Cyclones

Abstract

The interaction between spatially proximate (binary) tropical cyclones is such that relative rotation in the counterclockwise sense and decreasing separation distance between the two storm centers can be expected. This is referred to as the Fujiwhara effect. This study analyzes this effect for 43 binary tropical cyclone systems which occurred over the western North Pacific, 1949–78. It is shown that most demonstrated mutual interaction according to Fujiwhara expectations. However, there were notable apparent exceptions. Further analysis of these exceptional cases shows that environmental currents in which the storms were embedded had a significant effect on relative motion and masked the Fujiwhara effect. Additionally, it was found that storms exhibiting behavior most in accordance with Fujiwhara expectations were located in or near the Intertropical Convergence Zone. The main conclusion of the study, in confirmation of earlier studies, is that forces relative to environmental steering must be determined and filtered before one can determine forces attributable to the Fujiwhara effect alone.