Fujiwhara effect

The mutual rotation and interaction of two nearby cyclonic vortices around a shared center. Their interaction can alter their paths, intensify one another, or lead to merger.

Cyclonic circulation: Rotation around a low-pressure center in the same sense as a planet’s local Coriolis-driven cyclones. Each vortex contributes circulation that steers the other around their shared center.
Hurricanes Hilary and Irwin (2017): Two eastern Pacific hurricanes that interacted and rotated around one another in August 2017. Their close interaction is a well-documented modern example of mutual cyclone rotation.
Sakuhei Fujiwhara: A Japanese meteorologist who studied the interaction and motion of vortices in fluids. His experiments established the phenomenon now named for him.
Environmental steering: The movement of a tropical cyclone with the larger-scale atmospheric winds surrounding it. Unlike mutual steering, this motion comes from the surrounding flow rather than another cyclone.
Vorticity: A measure of local rotation in a fluid, defined mathematically as the curl of its velocity field. The vortices’ rotating flow is described and tracked through vorticity.
Hurricanes Fujiwhara and Kiyohara (1966): Two western North Pacific typhoons that interacted in August 1966. This historical pair is associated with the effect’s naming and early storm observations.
Vortex theory: The mathematical study of rotating regions of fluid and their interactions. Fujiwhara’s cyclone observations grew from broader work on fluid vortices.
Tropical cyclone binary interaction: The broader set of dynamical interactions between two nearby tropical cyclones. Fujiwhara rotation is one recognizable form within this wider class of paired-storm interactions.
Vortex merger: The process in which nearby vortices combine into one larger coherent vortex. Close, similarly sized cyclones can rotate together until their circulations consolidate.
Hurricanes Iris and Humberto (1995): Two Atlantic hurricanes that interacted near the Caribbean in August 1995. Their encounter illustrates how one cyclone can alter the motion and structure of another.

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