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OUT OF THIS WORLD | Science Behind the Weather

Watch two tropical cyclones spin a rare dance in the Pacific


Scott Sutherland
Meteorologist/Science Writer

Thursday, August 3, 2017, 2:19 PM - Two tropical storms that were spinning in the eastern Pacific Ocean entered into a rare dance that saw them spin around each other, thanks to an interaction known as the Fujiwhara effect.

Originally published on July 25, 2017, this story has been updated to reflect the most current information.

In the last week of July, tropical cyclones Irwin and Hilary, which formed off the Pacific coast of Mexico and Central America, caught the attention of meteorologists.

This is because, over the next several days, these two storms were expected to draw close enough to one another that they would directly interact, bringing about a rare meteorological treat.

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NASA satellite imagery of the eastern north Pacific Ocean, from July 25 to August 2, 2017, taken by the polar-orbiting Suomi NPP satellite, show the interaction between Irwin and Hilary over several days. Credit: NASA Worldview/Scott Sutherland

When two tropical cyclones get close enough to one another, roughly 1,400 km apart, the winds from the two vortices begin to pull at one another, forcing the storms closer together, while sending them spinning around a common center-point.


NASA camera views of Earth, imaged from the DSCOVR satellite, 1.5 million km away, between the Earth and the Sun, from July 25 to August 1, 2017. Inset shows a close-up view of the two storms, Irwin and Hilary, interacting. Credit: NASA Epic Team/Scott Sutherland

In the animation above, Irwin and Hilary start out as hurricanes on July 25, and day-by-day, they draw closer and then rotate around a common center-point, weakening to tropical storms towards the end of July and then dissipating.

This binary interaction of two storms, in which they orbit about a common point between them, is known as the Fujiwhara effect.

Although the original paper describing this effect, written in 1921 by Japanese meteorologist Sakuhei Fujiwhara, described vortices in water, the effect works exactly the same way when it comes to storm vortices in the atmosphere.

When two storms of equal size interact in this way, they get locked into a circulation where they both orbit around a fixed point between them, in a counter-clockwise direction (cyclonically). This interaction causes them to pull closer together, speeding up in their mutual orbit until they eventually merge into one larger storm.

With storms of unequal size, like Irwin and Hilary, they will also begin to slowly orbit around a fixed point between them, but the larger of the two will cause the smaller one to orbit around it. This is how the interaction between Irwin and Hilary began, because Hilary was still the stronger of the two. As their dance continued, though, the pair moved over cooler waters, Hilary weakened (thus giving them a more equal interaction), and then the both dissipated.

Sources: National Hurricane Center | NASA Worldview | NASA EPIC

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