In April 2021 two tropical cyclones underwent the phenomena known as the Fujiwhara effect, when TC Seroja consumed an unnamed tropical storm.
19 April 2021
12 April 2021
By HansPeter Roesli (Switzerland)
Between 1 and 11 April Tropical Cyclone Seroja moved from the Savu Sea (between Flores and Timor) to the West Australian coast. Figure 1 shows the track annotated on Himawari-8 infrared imagery.
On the way, between 7 and 9 April, Seroja had a close encounter with a weaker unnamed tropical cyclone. This tropical cyclone, initially positioned south of Seroja, moved cyclonically around Seroja and approached Seroja's centre down to a distance of well below 1,400 km.
During the day on 9 April when the distance fell to ~500 km, both tropical cyclones slowed down. But overnight Seroja grew more vigorous and slowly engulfed the unnamed tropical cyclone in the local night — known as the Fujiwhara effect.
Although, tropical cylones passing each other isn't uncommon, see the Lekima-Krosa case study where the two tropical cyclones passed each other practically undisturbed at a distance above 1,400 km. However, in this case the unnamed tropical cyclone crossed under the 1,400 km limit and, thus, underwent the Fujiwhara effect.
The Fujiwhara effect, sometimes referred to as 'Fujiwhara interaction', says that "binary interaction of smaller circulations can cause the development of a larger cyclone, or cause two cyclones to merge into one".
The effect is named after Sakuhei Fujiwhara, the Japanese meteorologist who initially described the effect. According to Wikipedia "Extratropical cyclones typically engage in binary interaction when within 2,000 kilometres of one another, while tropical cyclones typically interact within 1,400 kilometres of each other".
The couple's life cycle can be seen in Himawari-8 IR10.4 animated imagery at 20-minute intervals (Figure 2).
Seroja off the Australian west coast, Sentinel-3 OLCI True Colour, 10 April 01:38 UTC
Cyclone Seroja (CIMSS Satellite Blog)
The rare weather phenomenon which sent Cyclone Seroja down south (ABC News)
Cyclone Seroja: Storm makes landfall in Western Australia (BBC News)
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