Introduction To The Fujiwhara Effect

The Fujiwhara Effect also known as the Fujiwara interaction or binary interaction is a phenomenon where two nearby cyclonic vortices can move around one another and close the distance between their respective low-pressure circulations.

Binary interaction between smaller circulations can lead to the formation of a more significant storm or cause two cyclones to merge into one. Binary interaction is common for extratropical cyclones when they are within 2,000 km (1,200 mi).

Tropical cyclones usually interact within 1,400 km (870 mi) of each other. Sakuhei Fujiwhara is a Japanese meteorologist who first described the phenomenon.


Cyclones that are located in close proximity to one another will have their centres circle the other cyclically (counterclockwise in Northern Hemisphere, clockwise in Southern Hemisphere).

This is due to the cyclonic wind circulations. Both vortices will attract one another and eventually merge at the centre point. The reason for this divergent wind portion or vorticity-advection has yet to be determined. If the vortices of different sizes interact, the larger vortex will dominate, while the smaller vortex will circle around.

Sakuhei Fujiwhara is a Japanese meteorologist who first described the effect in a 1921 paper on the motion of vortices within water.

Fujiwhara effect depicted in the gfs weather model in the coral sea
Fujiwhara effect depicted in the gfs weather model in the coral sea

Tropical Cyclones

When smaller circulations in the Intertropical Convergence Zone combine, tropical cyclones may form. Although the merging of two storms is rare, this effect is frequently mentioned in relation to tropical cyclone motion.

Hurricane igor and tropical storm julia showing fujiwhara effect interaction
Hurricane igor and tropical storm julia show fujiwhara effect interaction. Credit noaa nesdis environmental visualization laboratory.

It becomes apparent when the two storms approach within 1400 km (870 mi) of one another. When tropical cyclones are within 650 km (400 mi) of one another, rotation rates in binary pairs accelerate. When they are within 300 km (190 mi) of each other, the merger (or shearing away of one of them) is realized.

Extratropical Cyclones

Binary interaction can be seen between extratropical cyclones located within 2,000 km (1,200 mi). Significant acceleration occurs when low-pressure areas are less than 1,100 km (680 mi) from one another.

Their circulations at 500 hPa (18,000 feet above sea level) are more predictable than their surface circulations. This often results in the merging of two low-pressure systems to form an extratropical storm.

However, it can also result in the change in the direction of either one or both of them. These interactions are dependent on many factors, including the size and distance of the cyclones from one another and the atmospheric conditions.

Further Reading

Have two hurricanes ever merged? And what was the result?


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