In April 2021, two tropical cyclones, Seroja and Odette, collided in the Indian Ocean northwest of Australia. Researchers from the University of Oldenburg have studied how this rare phenomenon affected the ocean. According to their case study, the rendezvous caused unusual cooling of surface waters and an abrupt change in the direction of the combined storm. With the frequency and intensity of tropical cyclones increasing due to global warming, the researchers suggest that such encounters—and consequently more extreme air-sea interactions—could become more common in the future.
Intensified ocean-atmosphere interactions during cyclone collisions
Tropical cyclones (TCs) not only disturb the air masses in the atmosphere, but also stir up water masses in the ocean. When two cyclones collide and merge, the interactions between the ocean and the atmosphere can intensify significantly, as Prof. Dr. Oliver Wurl and Dr. Jens Meyerjürgens from the University of Oldenburg report in a paper published in Tellus A: Dynamic Meteorology and Oceanography. They analyzed the interaction between the relatively weak tropical cyclones Seroja and Odette in 2021, finding effects that are typically only seen with much stronger cyclones. They conclude that the frequency of such interactions is likely to increase as global warming continues.
The two cyclones came together north-west of Australia in April 2021. To investigate the effects on the ocean, Wurl and Meyerjürgens combined satellite data with measurements from ARGO floats, autonomous drifters, and numerical models. The data provided insight into factors like salinity, water temperatures from the sea surface to 2,000 meters deep, and vertical flow velocities.
The encounter lasted for about a week, beginning on 6 April, when the cyclones came within 1,600 kilometers of one another.
„Seroja first stalled the smaller cyclone Odette and then merged with it three days later,“ says Wurl. After merging, Seroja abruptly changed course by 90 degrees on 9 April. „This chain of events not only influenced weather patterns but also triggered a previously unobserved interaction with the ocean beneath,“ he explains.
Unexpected cooling and upwelling effects
The analysis revealed that sea-surface temperatures dropped by three degrees Celsius after the cyclones merged, with deep, cold water rising from 200 meters in a process called „upwelling.“ The cooling effect was „exceptionally high“ for the cyclones‘ intensity. The highest wind speeds of about 130 kilometers per hour reached Category 1 on the Hurricane Scale, while the observed cooling and depth of upwelling were typically associated with Category 4 or 5 hurricanes.
The researchers were particularly surprised by the strength of the upwelling. At times, deep-water masses rose at speeds of up to 30 meters per day, far exceeding the typical ocean velocity of one to five meters per day.
„Thanks to satellite technology and autonomous deep-sea ARGO floats, we were able to demonstrate how the rotation of the cyclones transports cold water from the ocean depths to the surface,“ says Meyerjürgens.
While encounters between tropical cyclones have been rare, climate models predict that the number and intensity of cyclones will rise due to global warming. This could lead to more frequent and powerful collisions of hurricanes, resulting in „the most extreme interactions between the ocean and the atmosphere,“ the authors write. The merging of two cyclones can also cause sudden changes in their paths, making future behavior more difficult to predict.
Wurl also emphasizes another crucial consequence: „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.“
Cyclones convert thermal energy into mechanical energy, which is then carried to higher latitudes as they progress. Wurl and Meyerjürgens will join an expedition aboard the research vessel METEOR next year in the Mediterranean and subtropical Atlantic to further investigate these interactions and their connection to extreme weather events.
Original publication: Oliver Wurl, Jens Meyerjürgens: „Intense cooling of the upper ocean with the merging of tropical cyclones: a case study in the southeastern Indian Ocean.“ Tellus A: Dynamic Meteorology and Oceanography, doi.org/10.16993/tellusa.4083
