Sentinel-3 OLCI True Colour 10 April 2021

Tropical cyclone Seroja undergoes Fujiwhara effect

1-11 April 2021

Sentinel-3 OLCI True Colour 10 April 2021
Sentinel-3 OLCI True Colour 10 April 2021

In April 2021 two tropical cyclones underwent the phenomena known as the Fujiwhara effect, when TC Seroja consumed an unnamed tropical storm.

Last Updated

10 June 2022

Published on

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 Australian west coast. Figure 1 shows the track annotated on Himawari-8 infrared imagery.

Himawari-8 IR10.4
Figure 1: Himawari-8 infrared image with Seroja's track overlaid.

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 below 1,400km.

During the day on 9 April when the distance fell to around 500km, 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 1,400km. However, in this case the unnamed tropical cyclone crossed under the 1,400km 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).

Figure 2: Himawari-8 IR10.4, 7 April 00:20 UTC-9 April 23:40 UTC

Additional content

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)