This is what happens when tropical cyclones collide

In April 2021, tropical cyclones Seroja and Odette clashed in the southeastern Indian Ocean, just north-west of Australia, before finally merging completely.

The Australian Bureau of Meteorology describes this interaction as rare: “Seroja produc[ed] torrential rainfall and devastating floods in parts of Indonesia and Timor Leste. Later, it interacted with Tropical Cyclone Odette via the Fujiwara effect, a phenomenon rarely observed in the Australian region. Finally, it strengthened into a category 3 tropical cyclone producing a severe impact in the Mid-West region of Western Australia, unusually far south for a coastal crossing of a Severe Tropical Cyclone.”

These types of convergences are one of the most extreme interactions between the ocean and the atmosphere on Earth. But with the number and intensity of tropical cyclones increasing because of global warming, understanding their impacts has become more important than ever. 

“Seroja first of all stalled the smaller cyclone Odette and then merged with it 3 days later,” says Oliver Wurl of the University of Oldenburg in Germany. After the cyclones merged, Seroja abruptly changed course by 90 degrees.

The whole encounter lasted for about a week.

“This chain of events not only influenced weather patterns but also triggered a previously unobserved interaction with the ocean underneath,” says Wurl.

In a new report in in the journal Tellus A: Dynamic Meteorology and Oceanography, Wurl and colleague Jens Meyerjürgens analysed the encounter between the 2 relatively weak tropical cyclones and found effects that have only been observed with much stronger systems.

They did this by combining satellite data, measurements of the upper ocean, such as salinity and water temperatures obtained from ARGO floats and autonomous drifters, and numerical modelling.

The researchers found that sea-surface temperatures dropped by 3°C in the aftermath of the merge due to cold water upwelling towards the sea surface from depths of 200m.

Given cyclones’ intensity as a Category 1 on the Hurricane Scale, this cooling effect and depth of upwelling was “exceptionally high” – on the scale observed in Category 4 or 5 hurricanes.

“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,” explains Wurl.

The researchers conclude that the simultaneous formation and interaction between tropical cyclones could increase in the future with global warming and, with it, the extreme thermodynamic responses of the upper ocean.