Fujiwhara

The Fujiwhara effect refers to the mutual interaction between two nearby cyclonic vortices, most commonly tropical cyclones, which causes them to orbit around a common center of mass. The phenomenon is named after Sakuhei Fujiwhara, a Japanese meteorologist who described the interaction in the early 20th century. It occurs when the centers of the vortices are close enough that their circulations influence each other, typically within a few hundred to about a thousand kilometers.

In such cases, each cyclone’s wind field and vorticity alter the other's trajectory, leading to orbital motion around a shared barycenter. Depending on the relative sizes and intensities, several outcomes are possible. The storms may rotate around each other for a period and then merge into a single larger cyclone, or one system may be absorbed by the other. Alternatively, the vortices may repel or diverge after a brief interaction, leaving both storms weakened or reoriented. The exact evolution is sensitive to the environmental flow, moisture, and the internal structure of the vortices, making forecast of Fujiwhara interactions challenging.

The effect is primarily observed with tropical cyclones, including typhoons and hurricanes, but similar vortex interactions can occur with extratropical cyclones or other mesoscale vortices. In practice, Fujiwhara interactions can complicate track forecasts, leading to nontrivial motion and intensity changes that differ from those of solitary storms. Modern understanding relies on theoretical fluid dynamics and numerical weather prediction to simulate and anticipate the interactions under varying atmospheric conditions.