When cyclones collide: the Fujiwhara effect
14 April 2021
The winds in our atmosphere are constantly on the move. Contending with friction, temperature, and the rotation of the Earth, winds are twisted into a myriad of spirals, curves and whirls. So, what happens when some of the windiest systems on Earth, tropical cyclones, do battle with each other?
Tropical cyclones – spinning storms
Tropical cyclones are deep low pressure systems that form over warm, tropical waters. A product of their warm and moist environment, they grow and deepen, encouraging even more winds into their domain. They can persist in ideal environments over the open ocean for many days, even weeks, and may follow quite erratic paths. They're usually solitary, but every once in a while another tropical system comes along and the two interact.
What is the Fujiwhara effect?
If two cyclones move into each other's domain, things become interesting. The natural, almost magnetic attraction results in an interaction known as the Fujiwhara effect.
In this situation, the two cyclones begin to orbit around a common centre, almost like a dance. Cyclones spin clockwise in the southern hemisphere, and the two cyclones will also rotate around each other in a clockwise fashion.
It's usually not an equal playing field, with a bigger cyclone appearing to push the smaller one around. If they come close enough, they can change direction significantly from their previous paths – by as much as 90 degrees – as they wander towards each other.
Eventually they can merge and form a bigger cyclone if they move close enough, within 300 km of each other. Or, if each cyclone moves too far away from each other's influence, they can drift apart and regain their independence.
Animation: Watch this short animation to understand how the Fujiwhara effect can influence tropical cyclones
Fujiwhara effects are rare. You need two tropical cyclones to be in the same neighbourhood at the same time. The interaction starts to happen when they're about 1400 km apart.
The Fujiwhara effect was named for Japanese meteorologist Sakuhei Fujiwhara. He first described the interaction between whirling masses of fluid or air in 1921.
Where else does the Fujiwhara effect happen?
The Fujiwhara effect isn't limited to cyclones. In fact, Fujiwhara’s original studies also looked at spinning water in tanks and canals.
In the tropical atmosphere the Fujiwhara effect influences cyclones. But it also affects how storms behave in the frigid polar regions. It can even affect eddies in your local creek or bathtub.
Have you ever looked at a close-up picture of Jupiter, with its colourful array of bands and spirals? Some of them are undergoing the same process. Even our Milky Way galaxy, in its distant future, will have a similar interaction with a much larger Andromeda galaxy.
What does the Fujiwhara effect mean for cyclone forecasts?
The Fujiwhara effect can make forecasting cyclone track and intensity more challenging. It adds to the number of scenarios can occur as the systems move and develop. The 'tug of war' these cyclones play out is something our meteorologists factor into our cyclone forecasts.
Video: This satellite footage shows the Fujiwhara effect in action as tropical cyclones Seroja and Odette interact above the ocean off Western Australia in April 2021
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Small image at top left credit: MODIS Land Rapid Response Team, NASA GSFC