Multiple typhoons hurricanes can merge cyclones

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The Fujiwhara Effect

A meteorological peculiarity is looming in the southern hemisphere off the west coast of Australia. Two tropical cyclones are relatively close to each other and threaten to collide in the next few days.

Tropical cyclones in the Indian Ocean are called cyclones. In principle, these storms arise just like the hurricanes of the North Atlantic, which are more familiar to us.
Warm sea surfaces, low wind shear and the interplay of evaporation of sea water and condensation of water vapor create high-reaching cloud towers that develop into thunderstorms. Once they have reached a certain size, the cloud cluster begins to rotate due to the Coriolis force. In the southern hemisphere, a tropical cyclone rotates clockwise, exactly the opposite of the hurricanes in the northern hemisphere. Gradually, the storms can intensify and form a cloud-free area in the center - the eye. The high-reaching clouds directly around the eye are called the eye wall and this is where the highest wind speeds can be found. Depending on the wind speed, they are divided into five different categories. For comparison: A category 1 cyclone has mean wind speeds of 63 to 88 km / h, which corresponds to around 8 to 10 Bft on the Beaufort scale known to us.

There are currently two tropical depressions off Western Australia. One of them has already strengthened to a cyclone and was named "Seroja". Cyclone Seroja is a few hundred kilometers northwest of the city of Broome and moves southwest. The other tropical low, which will presumably reach cyclone strength from Thursday, is currently almost exactly in the path of cyclone Seroja.

This is where the so-called Fujiwhara effect comes into play. The effect describes the phenomenon when two tropical cyclones come very close and then start to rotate around each other. The cyclones influence the trajectory of the other and even a merging of the storms is possible. The prediction of the trajectories of both storms is made considerably more difficult because even the smallest changes can lead to large deviations in the predicted trajectory or intensity. The effect was named after the Japanese meteorologist Fujiwara, after he first described the effect in 1921.

However, a merger of the two cyclones is very rare and is not expected in these two storms. Presumably they will rotate around each other for a while, then separate again and develop their own trajectories. Since these trajectories have not yet been determined after the interaction, it is also possible that the coastal areas of Western Australia will be affected by the storms again, but this will have to be observed in the next few days. For now, this dance of storms takes place far enough from the coasts that the effects on the land are limited.

Wednesday April 7th 2021

Pila boss man

ARD weather editor