Circling Cyclones
Since we’re currently in hurricane season, it seems a goodtime to talk about hurricanes and cyclones. These types of weather systemsdon’t often find themselves in a traffic jam in Earth’s vast oceans, but whenthey do, there are consequences.
When two storms move too close together, they start toinfluence one another’s strength and track. This tropical tango is known as theFujiwhara effect. That scenario played out this week when Hurricanes Imelda andHumberto danced together off the southeastern US coast.
After devastating the coast of North Carolina, Humbertomoved out to sea, but as it did so, it tugged Imelda to go with it.
On Tuesday (9/30), they were both Category 1 hurricanes andthey were within 467 miles of each other, which is the closest that twoAtlantic hurricanes have come to each other in at least the past 60 years. Twohurricanes churning side-by-side is more common in the Pacific Ocean but rarelyhappens in the Atlantic.
The interactions during these events can vary greatly,which can make forecasting the tracks and intensity a challenge. Imelda’s trackforecast changed dramatically after she was pulled away from the US and towardBermuda.
Both hurricanes struggled in the face of the other’s windshear, with Humberto weakening and finally dissipating Wednesday morning. Afterthat, forecasters expect Imelda to hit Bermuda on Thursday as a Category 2.
In 1921, Japanese meteorologist Sakuhei Fujiwhara publisheda paper that said two storms spinning near each other could start rotatingtogether around a common center point. He was right.
How close the storms need to be to trigger the Fujiwharaeffect is dependent on the size of each storm. If two large storms—those thatspan hundreds of miles—get within about 850 miles of each other, they start todance. The distance shrinks to 350 miles for smaller storms.
Closely matched storms will orbit around a common point butthen go their separate ways. The circling tugs each storm off the path theywould have taken. In 2017, Hilary and Irwin had this experience in the eastPacific. They were well matched. Both systems interacted for so long, theyeventually dissipated around the same time.
But if one storm is much stronger than the other, thestronger one could consume the weaker one. In the west Pacific in 2022, thepowerful typhoon Hinnamnor was headed for Taiwan before it met a tropicaldepression that was trying to grow. Hinnamnor and the depression rotated untilthe typhoon devoured the depression. Hinnamnor slowed and weakened for a bit,but when it restrengthened, it took a nearly 90-degree turn from its originaltrack.
In a rare Fujiwhara scenario, two weak storms spinningtogether could merge and create a larger storm,
All of these potential interactions and outcomes pose anincredible forecasting challenge, even for computers. Any small changes in thestrength or size of each storm, or slight deviation from the anticipated trackthrows a model’s complex calculations into chaos.
