Fujiwhara effect: When 2 cyclones join forces to produce a supertyphoon

South_Korea_Asia_Typhoon_34377 — High waves crash onto a beach in Busan, South Korea, as Typhoon Hinnamnor approaches the Korean Peninsula. Image Credit: AP

For most of 2022, the world's vast oceans have been relatively pacified. Tropical cyclone Hinnamnor broke that.

Over the last few days, the super-typhoon hogged headlines as it spinned up quickly to Category 5 strength, hammering parts of Japan and South Korea.

It shut schools, halted flights, created severe flooding and blackouts after making landfall near the city of Geoje, South Korea, early on Tuesday, before turning away into waters east of the peninsula. At least seven drowned in flooded South Korean car park, and 10 more were reported missing.

The supertyphoon pummeled the Korean peninsula with ferocious winds, torrential rain and tornadoes Tuesday.

In the Atlantic, two hurricanes have been spotted overnight on Wednesday: codenamed Earl and Danielle.

What's going on?

Supertyphoon Hinnamnor was not only remarkably strong but also provided a textbook example of a relatively rare weather phenomenon, known as the “Fujiwhara effect”.

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People stand in front of a collapsed building after Typhoon Hinnamnor made landfall in Pohang, South Korea. Image Credit: Reuters

South Korean rescue workers rescue a man from a flooded riverside park in Ulsan. Image Credit: AFP

High waves crash onto a beach in Busan, South Korea, as Typhoon Hinnamnor approaches the Korean Peninsula. Image Credit: AP

Debris caused by Typhoon Hinnamnor are seen at a waterfront park in Busan, South Korea. Image Credit: AP

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What is the “Fujiwhara Effect”? Why is called such?

When two tropical cyclones get close to each enough other to generate a “shared” center, which then forces the two storms to whip around that common central point, it’s known as the Fujiwhara effect. Here’s the satellite footage of what happened.

How did it get its name?

It is named after Sakuhei Fujiwhara, a Japanese meteorologist. Dr. Fujiwhara performed experiments on water “vortices” from 1921 to 1923. He discovered interaction between two cyclonic vortices when they were close to each other. The vortices would rotate cyclonically about an axis connecting their centres.

VORTEX

Air that rotates around an axis (like a cyclone or tornado) is called a vortex (plural: vortices). They also form in stirred fluids — and may be observed in smoke rings — whirlpools in the wake of a boat, and the winds surrounding a tropical cyclone, tornado.

A vortex tube creates cold air and hot air by forcing compressed air through a generation chamber, which spins the air at a high rate of speed (1,000,000 rpm) into a vortex.

What happens then?

If the storms are somehow similar in strength, they can sometimes “merge”.

Occasionally, they can also “slingshot” around each other and continue on their separate ways. On rare occasions, the storms can combine into one larger storm.

Fujiwhara effect hinnamnor — If one of the storms is stronger than the other, the effect usually leads to the weaker one being “devoured” by the stronger one — as demonstrated when then-Super Typhoon Hinnamnor overpowered and absorbed the weaker tropical depression “Gardo” nearby, literally becoming a “two-nado”. Image Credit: JMA | Twitter

If one of the storms is stronger than the other, the effect usually leads to the weaker one being “devoured” by the stronger one — as demonstrated when then-Super Typhoon Hinnamnor overpowered and absorbed the weaker tropical depression “Gardo” nearby, literally becoming a “two-nado”.

How strong in Hinnamnor?

With winds of more than 241 km/h (150 mph), Hinnamnor was rated by the Japan Meteorological Agency (JMA) as a “very strong typhoon”.

This week, it shed off its super-typhoon status, but is still strong enough to wreak havoc in South Korea — packing up to 160 km/h (100 mph) winds and gusts close to 225 km/h (140 mph), thus making it a Category 2 storm on the Saffir-Simpson Hurricane Wind Scale.