The Fujiwhara Effect

2017-09-19 15:22:06.000 – Taylor Regan, Weather Observer

A few weeks ago, I wrote a post about the formation of hurricanes, to usher in the start of the hurricane season. Since then, we’ve seen multiple tropical cyclones form, even concurrently! Sometimes, these systems appear to revolve around each other as they move across the oceans. This is known as the Fujiwhara effect, after Japanese Meteorologist Sakuhei Fujiwhara, who first observed the phenomenon in a paper in 1921 describing the motion of vortices in water.

As the paths of two tropical cyclones pass within proximity of each other, they begin to interact. Fujiwhara found that as tropical cyclones approach each other, they begin to rotate counter-clockwise about a central point, with the degree of interaction increasing as the distance between the two systems decreases. Further studies have shown that the degree of interaction is dependent not only on the distance between the storm centers but also the strength and size of each storm as well as the overlying environmental setup.

There are two main scenarios possible when tropical systems begin to interact. The first is depicted in Figure 1 below, where the two systems begin to orbit about a central point and then eventually break free.

Figure 1. Fujiwhara effect resulting in two orbiting cyclones that break away. Photo from Hong Kong Observatory

The second scenario typically involves storms of two separate strengths. In this case, the weaker storm will typically orbit around the stronger storm and eventually be absorbed into the bigger system. Figure 2 below, shows the effect of a smaller storm being wrapped around and absorbed into the larger circulation.

Figure 2. Fujiwhara effect resulting in merger of two tropical cyclones. Photo from Hong Kong Observatory.

There are several instances in recent memory of tropical systems engaging in this delicate “dance.” One such example is when tropical systems Hilary and Irwin pivoted around one another as they continued to move northwest prior to Hilary absorbing Irwin this past year.

Figure 3. GOES-16 over the Pacific July 25 – August 1 2017. Image from the Weather Channel.

Taylor Regan, Weather Observer

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