Fujiwhara Effect

The Fujiwhara effect describes the rotation of two storms around each other. It’s most common with tropical cyclones but also occurs in other cases. A good way to picture this is to think of two ice skaters who skate quickly towards each other, nearly on a collision course, grab hands as they are about to pass and spin vigorously around in one big circle with their joined hands at the center. To complete the effect, the entire system – the two storms and the central point between them – must move off in a single direction while the storms continue spinning about each other. It is thought to occur when storms get about 1500km apart.

The effect is named after Dr. Sakuhei Fujiwhara who was the Chief of the Central Meteorological Bureau in Tokyo, Japan, shortly after the First World War. In 1921 he wrote a paper describing the motions of “vortices” in water. Fujiwhara looked closely at many different types of vortices to see how they acted when they came close to each other. He noted that if two vortices were equal in size and strength and spun counter-clockwise, they would rotate around each other. If one of the vortices was larger than the other, they would begin spinning around each other for a short time with the larger one dominating. Eventually the smaller of the two vortices would get caught in the circulation of the larger one and be gobbled up. If the similar vortices spun in opposite directions (one clockwise and one counter-clockwise), they would push each other away if they got too close. (Source: USATODAY)

We witnessed this effect with Hurricane Earl and Tropical Storm Fiona earlier this month. As Earl was vastly stronger than Fiona, when they got close to each other Earl started to “eat” Fiona (the satellite image below shows a line of thunderstorms passed from Fiona to Earl). Had the Fujiwhara Effect continued, at the very least, Tropical Storm Fiona would have nudged Earl slightly further southwest (towards the US coastline), while Fiona would have been displaced closer to Bermuda.

It also happened on 23rd August 1995 with Hurricane Humberto and Tropical Storm Iris and 8 days later with Hurricane Iris and Tropical Storm Karen, with the latter being absorbed by Iris.

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