On April 6^th, 2021, a rare event began to unfold, the interaction and merging of two tropical cyclones (TCs) in the Indian Ocean off the northwest tip of Australia. TCs Odette and Seroja combined fully on April 9^th. The remaining Seroja abruptly changed direction by roughly 90° on the 10^th and headed towards the Western Australian coast.

The interactions of cyclones is known as the Fujiwhara effect. It is named after Japanese meteorologist Sakuhei Fujiwhara. The Fujiwhara effect doesn’t just apply to TCs; it describes the behaviour of any vortex of liquids or gases. In 1992, D. G. Dritschel and D. W. Waugh found five forms of interactions between vortices: elastic interaction, partial straining out, complete straining out, partial merger, and complete merger.

However, the effects of the cyclonic interactions on the ocean were unknown. A research paper by Oliver Wurl and Jens Meyerjürgens from the University of Oldenberg in Germany, published in August 2024, focused for the first time on the impact merging TCs have on the ocean. “This chain of events not only influenced weather patterns but also triggered a previously unobserved interaction with the ocean underneath,” says Wurl.

All tropical cyclones leave the ocean surface cooler as they pass over it. The TC’s strong winds create upwellings in the ocean. Upwellings are colder water from the deep being brought up to mix with the warmer water of the upper ocean. TCs also draw thermal energy from the warm surface water and convert it to mechanical energy, additionally reducing temperatures.

After the union of the two cyclones, the resulting storm was at Category 1. What was unusual about this event was the influence it had on the ocean below. In some places, the water was 3°C cooler and the upward flow of the ocean was six times stronger than normal, after Odette and Seroja collided. These consequences are generally seen during Category 4 or 5 TCs. Wurl said, “As a result of the interactions of a cyclone with the ocean and the upwelling of cold, deep water, the ocean absorbs additional heat from the air and then transports it to higher latitudes – a crucial process that influences the climate worldwide.”

Oliver Wurl and Jens Meyerjürgens sourced their data from ARGO floats, drifters, radiometers, geothermal satellites, and various meteorology agencies.

Sources:

https://a.tellusjournals.se/articles/10.16993/tellusa.4083#main-text-B46

https://journals.ametsoc.org/view/journals/mwre/131/11/1520-0493_2003_131_2656_acobtc_2.0.co_2.xml

https://www.ncei.noaa.gov/products/international-best-track-archive?name=browse

https://www.weather.gov/news/fujiwhara-effect

https://media.bom.gov.au/social/blog/2571/when-cyclones-collide-the-fujiwhara-effect

https://cosmosmagazine.com/earth/climate/explainer-cyclones-earlier-stronger-lingering-longer

https://cosmosmagazine.com/earth/climate/explainer-how-a-cyclone-spins-to-life

https://phys.org/news/2024-12-ocean-cyclones-collide.html

https://www.discoverwildlife.com/environment/tropical-cyclone-collision-indian-ocean

https://pubs.aip.org/aip/pof/article-abstract/4/8/1737/402721/Quantification-of-the-inelastic-interaction-of?redirectedFrom=fulltext

https://media.bom.gov.au/social/blog/1381/tropical-cyclones-at-sea-the-ocean-below-the-storm/#:~:text=Ocean%20cooling,also%20helps%20reduce%20ocean%20temperature.