Last December, two German marine scientists published a groundbreaking study on the merger of Tropical Cyclones Seroja and Odette in the Indian Ocean, which resulted in an unexpected cooling effect on ocean temperatures. This rare event demonstrated that even relatively weak tropical cyclones can significantly alter ocean conditions, potentially reshaping climate forecasting.
The merger of two cyclones led to unexpected changes in storm intensity and ocean conditions, challenging existing cyclone behavior models. Dr. Oliver Wurl and Dr. Jens Meyerjürgens from the University of Oldenburg in Germany found that when the initially weak Category 1 storms interacted, Seroja intensified to Category 3 and made an unexpected 90° change in the trajectory. This rare event triggered significant ocean cooling, with sea-surface temperatures dropping 3°C and cold water upwelling from 200 meters deep at an unprecedented rate of 30 meters per day. This phenomenon is typically seen only in much more severe Category 4 or 5 Cyclones.
The rare convergence of two cyclones is explained by the Fujiwhara effect, first identified a century ago. Named after Japanese meteorologist Sakuhei Fujiwhara in 1921, it describes how two tropical cyclones within about 1,400 kilometres of each other can orbit, merge, or repel. Although the Fujiwhara effect has been studied for decades, its impact on ocean temperatures remains largely unexplored. Indeed, while past storms – such as Hurricanes Iris and Humberto (1995) in the Atlantic and Typhoons Parma and Melor (2009) in the Pacific – have provided some insights, previous research primarily focused on storm trajectories and intensity, until the study this past December.
“This chain of events not only influenced weather patterns but also triggered a previously unobserved interaction with the ocean underneath.” Dr. Wurl explained. Using advanced satellite data, upper-ocean measurements and climate modelling, the scientists observed that even weaker cyclones can dramatically impact ocean thermal dynamics, potentially influencing global climate patterns and complicating storm forecasting. With global warming, the frequency of simultaneous tropical cyclone formation and interactions is expected to rise, leading to more extreme ocean response. “The ocean absorbs additional heat from the air and then transports it to higher latitudes. This is a crucial process that influences the climate worldwide,” Dr. Wurl observed. These findings highlight why constant monitoring of cyclone interactions is vital for improving storm intensity and path predictions.
The unexpected behaviour in relatively weak cyclones has broader implications for climate forecasting and disaster preparedness. The Australian Bureau of Meteorology reported that “Seroja produc[ed] torrential rainfall and devastating floods in parts of Indonesia and Timor Leste,” illustrating the real-world impacts of such storms.
Looking ahead, the research plans to expand their research by investigating similar interactions in the Atlantic Ocean using the METEOR research vessel. “Our goal is to better understand the ocean-atmosphere interactions and how they contribute to climate regulation,” added Dr. Wurl. As extreme weather events become more frequent, such studies will be instrumental in shaping future climate resilience strategies.