Tropical cyclones are infamous for their dramatic weather patterns, but their impact isn’t confined to the skies.
These powerful systems also churn the ocean, creating a complex interplay between air and sea – and the creatures that live within it. Now, new research from the University of Oldenburg in Germany sheds light on what happens when two cyclones collide – a rare phenomenon that can significantly amplify these kind of effects.
Marine scientists Dr Oliver Wurl and Dr Jens Meyerjürgens studied the encounter of two relatively weak tropical cyclones in the Indian Ocean in April 2021: Seroja and Odette.
Their findings, published in Tellus A: Dynamic Meteorology and Oceanography, reveal interactions between ocean and atmosphere previously associated only with much stronger storms. As climate change drives more frequent and intense tropical cyclones, these dramatic interactions could become more common, say the researchers.
A rare encounter
In April 2021, tropical cyclones Seroja and Odette converged in the southeastern Indian Ocean, off the coast of Australia, triggering a week-long spectacle.
By combining satellite observations, float measurements and numerical models, the researchers were able to analyse how this unusual meeting affected the ocean. They analysed parameters such as salinity, water temperature and vertical water movement – measuring depths of up to 2,000 metres.
The cyclones approached within 1,600 kilometres of one another on 6 April. “Seroja first of all stalled the smaller cyclone Odette and then merged with it three days later,” explains Wurl. This merging caused Seroja to suddenly veer 90 degrees on 9 April.
"This chain of events not only influenced weather patterns but also triggered a previously unobserved interaction with the ocean underneath," says Wurl.
Unprecedented ocean interactions
The analysis revealed that the merging of the cyclones caused sea-surface temperatures to drop by 3°C – a significant cooling effect linked to 'upwelling', where deep, cold water is drawn upwards to the surface. This effect, unusually intense given the relatively weak strength of the storms, saw cold water rising from depths of 200 metres at speeds of up to 30 metres per day – well above the usual 1 to 5 metres per day.
Surprisingly, these oceanic changes mirrored those typically observed in much stronger hurricanes of Categories 4 or 5, despite the cyclones’ maximum wind speeds reaching only Category 1.
Implications for a warming world
While such encounters between tropical cyclones are currently rare, climate models predict that rising global temperatures will lead to more frequent and intense storms. As a result, the likelihood of such cyclonic mergers – and their associated ocean-atmosphere extremes – may increase.
The merging of cyclones also presents challenges for weather prediction. “These events can cause abrupt course changes, complicating forecasts of cyclone behaviour,” notes Wurl.
Beyond the immediate impacts, these interactions have broader climatic consequences. The upwelling of cold water allows the ocean to absorb additional heat from the atmosphere, which it then transports to higher latitudes, influencing global climate patterns, say the researchers. Cyclones also convert thermal energy into mechanical energy, redistributing it across the planet.
Looking ahead, Wurl and Meyerjürgens aim to deepen their understanding of these processes. Their upcoming expedition aboard the research vessel METEOR will focus on studying ocean-atmosphere interactions in the Mediterranean and subtropical Atlantic, shedding light on how such phenomena contribute to extreme weather events worldwide.
Main image: The tropical cyclones Seroja and Odette came together in the Indian Ocean north-west of Australia in April 2021. After the two cyclones merged, Seroja abruptly changed course by 90 degrees/Japanese Space Agency (JAXA), JAXA P-Tree System
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