TheFujiwhara effect, sometimes referred to as theFujiwhara interaction orbinary interaction,[1] is a phenomenon that occurs when two nearbycyclonicvortices move around each other and close the distance between the circulations of their correspondinglow-pressure areas. The effect is named afterSakuhei Fujiwhara, the Japanese meteorologist who initially described the effect. Binary interaction of smaller circulations can cause the development of a larger cyclone or cause two cyclones to merge into one.Extratropical cyclones typically engage in binary interaction when within 2,000 kilometres (1,200 mi) of one another, whiletropical cyclones typically interact within 1,400 kilometres (870 mi) of each other. Fujiwhara described the effect in a 1921 paper which was based in an 1889 paper by a Japanese researcher and eventually gained popularity in theUnited States afterWorld War II.
When cyclones are in proximity of one another, their centers will circle each other cyclonically: counterclockwise in theNorthern Hemisphere and clockwise in theSouthern Hemisphere,[3] 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 thewind orvorticityadvection.[4] 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.[5] If the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will circle around it. The effect is named after Sakuhei Fujiwhara, the Japanese meteorologist who initially described it in a 1921 paper.[6][7] 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.[8]
The phenomenon happened in theEast Pacific Ocean in July 2017 whenHurricane Hilary andHurricane Irwin interacted; Hilary became stronger while Irwin became weaker. The interaction caused Irwin to change course northwest before dissipating.[5] The phenomenon is uncommon in theAtlantic Ocean; a notable instance was in 1995, whenHurricane Iris absorbedHurricane Humberto[9] and a more recent one happened in 2025 withHurricane Imelda andHurricane Humberto.[10] In theWest Pacific Ocean,Typhoon Parma andTyphoon Melor engaged in a Fujiwhara dance in 2009; the interaction caused Parma to stall near thePhilippines.[11] In the centralIndian Ocean, the phenomenon was undergone byCyclone Diamondra andCyclone Eunice in 2015, while in the eastern Indian Ocean,Cyclone Seroja andCyclone Odette experienced the effect in 2021.[12] Extratropical cyclones also undergo the Fujiwhara effect.[13]
Diro Kitao, a Japanese researcher, studied interactions between tropical cyclones in 1889 which served as the basis for the studies of Sakuhei Fujiwhara.[14] 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".[6][7] The interaction in the paper was researched through a series of water tank experiments.[15] During the 1920s, Fujiwhara published numerous other papers detailing the effect.[16]: 12 TheUnited States Army gained damage multiple times during World War II due to typhoons which led them to establish a center for typhoon tracking inGuam which provided warnings.Douglas MacArthur'sinvasion of Japan was postponed in 1945 whenTyphoon Susan andTyphoon Ruth approached the country while interacting, giving them an opportunity to analyze the interaction of these tropical cyclones.[14]
After this, the phenomenon was popularized in the United States from a research paper in 1951;[16]: 6 the first well-known instance of the phenomenon was in1964 whenTyphoon Marie andTyphoon Kathy merged.[12] The effect was examined in theMonthly Weather Review journal in an edition from 1 November 2003, which highlighted factors influencing the phenomenon.[8] According to a report by theNational Research Institute for Earth Science and Disaster Resilience (NIED), the definition changed drastically: the original definition was the merging of tropical cyclones while the modern definition is the general interaction between cyclones.[17]
Tropical cyclones canform when smaller circulations within theIntertropical Convergence Zone merge.[18] 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 1,400 kilometres (870 mi) of each other. Rotation rates within binary pairs accelerate when tropical cyclones close within 650 kilometres (400 mi) of each other.[1] Merger of the two systems (orshearing out of one of the pair) becomes realized when they are within 300 kilometres (190 mi) of one another.[8]
Binary interaction is seen between nearby extratropical cyclones when within 2,000 kilometres (1,200 mi) of each other, with significant acceleration occurring when the low-pressure areas are within 1,100 kilometres (680 mi) of one another. Interactions between their circulations at the 500 hPa level (5,500 metres or 18,000 feet abovesea level) behave more predictably than their surface circulations.[1] 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.[19] 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.[8]
