When two storms move towards another, an uncommon phenomenon called the Fujiwhara Effect can happen. This phenomenon is also known as the Fujiwara effect, Fujiw(h)ara interaction, or binary interaction. The effect was first described in 1921 by Sakuhei Fujiwhara, a Japanese meteorologist.
When two cyclones or hurricanes near each other, the two storms can interact. The two storms will start to circulate around each other, drawing closer together. The stronger of the two storms will often eventually absorb the smaller storm. In other instances, two storms of equal strength may either merge or spin around a common center before moving off into separate paths.
This GOES-16 satellite imagery from 2017 shows the Fujiwhara Effect happening to hurricanes Hilary and Irwin in the east Pacific between July 25 and August 1. Hurricane Irwin is the storm on the left which collides with Hurricane Hilary on the right. The two storms merge briefly before separately dissipating out over the ocean.
More recently, the Fujiwhara Effect was observed off the coast of Western Australia between Tropical Cyclone Seroja and a weaker tropical low, Cyclone Odette. Between April 7 and 9, the two cyclones came within 1,400 km of each other and started circulating. On April 9, the two storms moved within 500 km of each other and Seroja absorbed the weaker storm.
The interaction of the two storms can be seen in this video of Himawari-8 IR10.4 at 20 minute intervals.
After absorbing the Cyclone Odette, Tropical Storm Seroja spun off southward in an unusual direction towards western Australia. The movement of storms of this intensity southward is rare, with researchers estimating that this has happened only about 26 times in the last 5,000 years.
Tropical Storm Seroja made landfall on April 11, 2021 as a Category 3 storm. The storm moved over 1,000 kilometers (600 miles) of land, destroying trees and knocking out power to 1,500 storms. Before hitting western Australia, Seroja had already hit Indonesia on April 5, killing people and destroying homes.
Fujiwhara, S. (1921). The natural tendency towards symmetry of motion and its application as a principle in meteorology. Quarterly Journal of the Royal Meteorological Society, 47(200), 287-292. https://doi.org/10.1002/qj.49704720010
Fujiwhara effect. (n.d.). National Weather Service. https://www.weather.gov/news/fujiwhara-effect
Roesli, H. (2021, April 12). Tropical cyclone Seroja undergoes Fujiwhara effect. EUMETSAT. https://www.eumetsat.int/tropical-cyclone-seroja-undergoes-fujiwhara-effect
Pater, K. (2021, April 12). Seroja slams Australia. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/148180/seroja-slams-australia