In April 2021, tropical cyclones Seroja and Odette merged with one another in the Indian Ocean, just north-west of Australia. The Australian Bureau of Meteorology (BOM) described this merger as rare. Two scientists from the University of Oldenburg, Germany have studied this phenomenon in detail to understand how these newer, more unpredictable weather patterns, driven by climate changes, are impacting us.
Category 1 tropical cyclone Seroja (meaning a lotus flower in the Malay language) first hit Timor Leste and Indonesia, causing torrential rainfall and floods. On April 6th, it came within approximately 1,600 kilometres of another, smaller Category 1 cyclone Odette (meaning wealth in French). Seroja initially caused Odette to stall (stay in a small area for a long time) and finally merged with it to form a larger Category 3 cyclone. This is called the Fujiwhara or binary interaction, named after a Japanese meteorologist who first described it.
After merging, Seroja abruptly changed its trajectory by 90⁰ and hit the mid-west region of Western Australia near Kalbarri. It caused structural damage and left thousands of people without power or network.
The merger of these two storm cells was not the only unusual phenomenon. Sea surface temperatures cooled by 3⁰C after the merger. This is because the swirling of the cyclones not only disturbed the atmospheric air, but it also churned the water. It pushed the cooler water from the depths of the ocean toward the surface, in a phenomenon called “upwelling”. Marine scientist, Jens Meyerjürgens, from the University of Oldenburg, said “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 depths of the ocean to the surface.”
The amount of cooling was also noteworthy; this depth of upwelling is usually only seen in Category 4 or 5 cyclones. Marine scientist Dr. Oliver Wurl is worried how these unusual weather patterns are going to impact weather prediction and said, “These events can cause abrupt course changes, complicating forecasts of cyclone behaviour.”
As the ocean waters churn during upwelling, they also absorb additional heat from the atmosphere. Through ocean currents, these warmer waters are then transported to higher latitudes (temperate zones), affecting climate patterns globally.
Scientists Oliver Wurl and Jens Meyerjürgens aim to further deepen their understanding of such phenomenon. They are journeying across the Mediterranean and subtropical Atlantic waters aboard the German research vessel, METEOR, in order to do so.
References:
https://cosmosmagazine.com/news/what-happens-tropical-cyclones-collide
https://www.weatherzone.com.au/news/seroja-and-odette-dancing-dangerously-close-to-australia/533874