thumb|right|Hurricanes [[Hurricane Imelda|Imelda (left) and Humberto (right) displaying the fujiwhara effect during the 2025 Atlantic hurricane season.]]
The Fujiwhara effect, sometimes referred to as the Fujiwhara interaction or binary interaction,]]
When cyclones are in proximity of one another, their centers will circle each other cyclonically: counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, about a point between the two systems due to their cyclonic wind circulations. The two vortices will be attracted to each other and eventually spiral into the center point and merge. It has not been agreed upon whether this is due to the divergent portion of the wind or vorticity advection. If the two vortices are of equal size, they partake in a "dance" where the vortices orbit the center point; they can also deflect each other to the other direction. Numerous factors affect the phenomenon, namely the separation distance, the relative size of the vortices, and their intensity. For instance: when two vortices are close, they have a high chance of merging.thumb|[[Typhoon Parma (left) and Melor (right) interacting with each other in the Philippine Sea on October 6, 2009.]]
The phenomenon happened in the East Pacific Ocean in July 2017 when Hurricane Hilary and Hurricane Irwin interacted; Hilary became stronger while Irwin became weaker. The interaction caused Irwin to change course northwest before dissipating. The phenomenon is uncommon in the Atlantic Ocean; a notable instance was in 1995, when Hurricane Iris absorbed Hurricane Humberto and a more recent one happened in 2025 with Hurricane Imelda and Hurricane Humberto. In the West Pacific Ocean, Typhoon Parma and Typhoon Melor engaged in a Fujiwhara dance in 2009; the interaction caused Parma to stall near the Philippines. In the central Indian Ocean, the phenomenon was undergone by Cyclone Diamondra and Cyclone Eunice in 2015, while in the eastern Indian Ocean, Cyclone Seroja and Cyclone Odette experienced the effect in 2021. Extratropical cyclones also undergo the Fujiwhara effect.
History
left|thumb|241x241px|Sakuhei Fujiwhara, the meteorologist who discovered the phenomenon.
Diro Kitao, a Japanese researcher, studied interactions between tropical cyclones in 1889 which served as the basis for the studies of Sakuhei Fujiwhara. The effect was first noticed when Fujiwhara described it in a 1921 paper about the motion of vortices in water titled "The natural tendency towards symmetry of motion and its application as a principle in meteorology". During the 1920s, Fujiwhara published numerous other papers detailing the effect. The United States Army gained damage multiple times during World War II due to typhoons which led them to establish a center for typhoon tracking in Guam which provided warnings. Douglas MacArthur's invasion of Japan was postponed in 1945 when Typhoon Susan and Typhoon Ruth approached the country while interacting, giving them an opportunity to analyze the interaction of these tropical cyclones.
In cyclones
Tropical cyclones
thumb|[[2020–21 Australian region cyclone season#Tropical Cyclone Odette|Odette (left) and Seroja (right) engaged in a Fujiwhara interaction whilst intensifying between 7–9 April 2021.]]
Tropical cyclones can form when smaller circulations within the Intertropical Convergence Zone merge. The effect is often mentioned in relation to the motion of tropical cyclones, although the final merging of the two storms is uncommon. The effect becomes noticeable when they approach within of each other. Rotation rates within binary pairs accelerate when tropical cyclones close within of each other. This most often results in a merging of the two low-pressure systems into a single extratropical cyclone, or can less commonly result in a change of direction of one or both of the cyclones. The precise results of such interactions depend on factors such as the size of the two cyclones, their distance from each other, and the prevailing atmospheric conditions around them.